FOUS30 KWBC 170053
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Jul 16 2024

Day 1
Valid 01Z Wed Jul 17 2024 - 12Z Wed Jul 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL HIGH PLAINS TO THE OHIO VALLEY...

...01Z Update...
Was able to trim the Slight Risk area out of much of the Ohio
Valley as convection in that part of the country weakened with the
loss of daytime heating. The Slight risk area remained in place
from parts of Arkansas and southern Missouri westward to the
Central High Plains as additional convection rolls off the higher
terrain and propagates along and immediately north of a quasi-
stationary boundary...with increased chances of heavy rainfall as
storms encounter a strengthening low level jet. Still lingering
convection in northern New England and the Southwest US that should
gradually weaken (in both areas) later this evening.

Bann

...16Z Update...

Adjustments this forecast cycle were to expand the Slight Risk area
over the Central Plains farther south to include more of the
northern TX Panhandle, northern OK, and northern AR. Latest
guidance has shown the nearby stationary front inching a little
more south compared to overnight guidance, which would allow for
segments of storms tonight being farther south than initially
progged over northern OK and even as far west as the TX Panhandle.
Closer to northern AR, strong surface based heating that further
destabilizes the atmosphere ahead of the front may lead to strong
storms by itself given the 89F convective temp via the 12Z LZK
sounding. Then there is the concern for outflows from thunderstorm
activity over northern OK that may approach and act as another
trigger tonight over the Ozarks.

In the Northeast, the soils are more primed in parts of NY's North
Country and around the Finger Lakes following yesterday's stormy
afternoon. Another MCV approaching from the west may lead to a near
carbon-copy setup for today, but storm motions should remain
progressive enough to limit the areal extent of flash flooding to
be more localized. One wildcard to factor in would be areas where
more extensive tree damage occurred that could be more prone to
flash flooding. The 12Z HREF also showed moderate-to-high
probabilities (50-70%) for 3-hr QPF surpassing 3-hr FFGs just east
of Lake Ontario and along the Tug Hill. Still, the progressive
nature of the storms kept any Slight Risk from being issued for the
time being. One adjustment was to expand the Marginal Risk across
most of northern New England as the soils (particularly in northern
ME) are a little more saturated (NASA SPoRT-LIS sows >80% soil
moisture percentiles). Given their added sensitivities and PWATs
that are topping 1.5" (>90th climatological percentile according ot
ECMWF SATs), the Marginal Risk extension was decided upon this
forecast cycle.

Mullinax

---Previous Discussion---

...Mid-Mississippi and Ohio Valleys...
Strengthening southwesterly flow ahead a shortwave moving across
Kansas will support deepening moisture ahead of a cold front
sliding southeast into the mid Mississippi Valley this morning and
the lower Ohio Valley later today. Persistent inflow into this
slow-moving boundary along with favorable forcing aloft is
expected to support periods of training storms, with areas of heavy
rainfall likely, especially from central and southern Missouri
eastward into the lower Ohio Valley. Excessive rainfall is
especially a concern from central and southern Missouri through
southern Illinois to the western Indiana and Kentucky border, where
the 00Z HREF is showing high neighborhood probabilities for
accumulations of 3 inches or more through the period.

...Central Plains...
Moist upslope flow behind a cold front settling south through the
central Plains will support storms developing over the high terrain
before these storms spread east into the Plains during the
evening. Merging storms along with increasing organization ahead of
a shortwave digging southeast through the Plains is expected to
support locally heavy amounts from southeastern Colorado through
southwestern Kansas and northwestern Oklahoma, where the HREF is
also showing some notable probabilities for 3 inches or more.

...Upper Ohio Valley to the Northeast...
Ample moisture ahead of an amplifying upper trough over the Great
Lakes and its associated cold front will fuel additional storms
later today. Overall, storms that develop are expected to be
progressive in nature. However, intense rainfall rates may produce
isolated runoff concerns.

...Southwest...
An upper high sitting over the Four Corners will allow moisture to
funnel across the region, with an expansion of convective activity
expected today, bringing the risk for isolated heavy amounts and
flash flooding to a greater portion of Arizona and New Mexico.

Pereira


Day 2
Valid 12Z Wed Jul 17 2024 - 12Z Thu Jul 18 2024

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS
COLORADO AND NEW MEXICO, FROM THE ARKANSAS AND RED RIVER BASINS TO
MID-SOUTH, AND FOR THE NORTHERN MID-ATLANTIC I-95 CORRIDOR...

...2030Z Update...

...Northern Mid-Atlantic I-95 Corridor...

As daytime heating unfolds, the atmosphere will rapidly destabilize
as high temperatures rise into the mid 90s and dew points average
around 70F. Latest CAMs guidance shows thunderstorms firing
initially along a pre-frontal trough Wednesday afternoon with
potential redevelopment Wednesday evening as the cold front
traverses the region. PWs are expected to range between 2.0-2.2",
which is between the 90-97.5th climatological percentile for the
time of year. Soundings in the region also show no shortage of
MUCAPE (at least 2,000 J/kg) and hodographs that are not only
supportive of organized clusters of storms, but potentially capable
of containing organized mesocyclones (which are more efficient
rainfall producers). The storms will have a rather progressive
motion and soils can most definitely use the rain. However, the
potential for >2"/hr rainfall rates along a highly urbanized
corridor that contains no shortage of hydrophobic surfaces is a
recipe for potential flash flooding. The potential for additional
storms in the evening in wake of the initial pre-frontal storms may
also make some soils more sensitive for the final round of storms
along the approaching cold front. Given these reasons, and in
collaboration with LWX and PHI, a Slight Risk was introduced for
Wednesday.

Otherwise, did adjust the Slight Risk in the Lower Mississippi
Valley and ArkLaTex a little farther south given the slightly
farther south progression with the cold front. The rationale behind
the threat area remains unchanged. Made just minor adjustments to
the Marginal and Slight in the Southwest to account for recent QPF
changes and 12Z HREF guidance.

Mullinax

---Previous Discussion---

....Southwest to the Central to southern Rockies and High
Plains...
As the previously noted front continues to slide to the
south through the Plains, increasing moisture afforded by low-
level easterly flow will support an increasing threat for locally
heavy rainfall and flash flooding concerns. Ample moisture along
with increasing lift will support storms capable of producing heavy
rainfall rates, that will develop and drop south across the
region. The greater threat continues to center from the Colorado
Sangre de Cristos into the northern and central New Mexico ranges,
where models show some of the better low level inflow and greater
PW anomalies.

Elsewhere, a more typical monsoon pattern will continue, with
diurnal convection generating isolated heavy rainfall and flash
flooding concerns across much of the Southwest.

...Southern Plains to the Northeast...
An upper trough will continue to amplify over the Great Lakes,
pushing a cold front that will extend from the Northeast to the
southern Plains this period further south and east. While locally
heavy rainfall and an isolated threat for heavy rainfall cannot be
ruled out for any area along the front, deeper moisture and
stronger forcing are expected to elevate the threat for some areas.
This includes portions of the Red River and Arkansas basins into
the Mid-South. Deepening moisture and increasing ascent ahead of a
shortwave approaching from the northeast is expected to support
organized heavy rainfall across the region. With plenty of typical
difference in the details, several of the overnight deterministic
models indicate locally heavy amounts of 2-3 inches within the
Slight Risk area.

Pereira
---
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FOUS30 KWBC 181004
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
604 AM EDT Thu Jul 18 2024

Day 1
Valid 12Z Thu Jul 18 2024 - 12Z Fri Jul 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, THE EASTERN MID-ATLANTIC REGION, & THE
SABINE RIVER VALLEY...

...Southern Mid-Atlantic & South...
A cold front will advance southeastward into the Mid-Atlantic
region today while the western portion slows across the southern
Plains. Along and ahead of this boundary will remain a focus for
deep moisture (PWATs > 2 inches in some places of the Mid-
Atlantic, Southeast and Gulf Coast states. The greatest threat for
heavy rainfall appears to be with and ahead of the front along
pre-frontal convergence axes from southeastern Virginia into
eastern North Carolina. In addition to the anomalous moisture and
low level convergent axes, upper levels will be favorable within
the right entrance region of a jet positioned east of an upper
trough axis advancing eastward through the Great Lakes, with an
area of upper level diffluence forecast over the eastern Mid-
Atlantic region due to the presence of a ridge off of the Southeast
coast. For the 24 hour period ending 12Z Friday, 00Z HREF guidance
supports EAS probabilities for 3 inches of 30 percent between
Albemarle and Pamlico Sounds along with standard probabilities for
8 inches of 30-40 percent in the same location. While this is
typically a less flash-flood prone region, these probability values
are not super common and could be highlighting the potential for
locally significant rainfall totals somewhere from southeastern
Virginia into eastern North Carolina.

...Eastern Texas into Western Louisiana...
A second and relatively small Slight Risk was maintained for
locations near the Arklatex and southward along the Sabine River to
account for locally greater potential for heavy rain. Upper level
diffluence will be maximized here and greater coverage of
thunderstorms are during the afternoon/overnight near/south of an
850 mb low located near the Arklatex. Individual members of the 00Z
HREF and RRFS indicate localized QPF maxima of 4 to 6+ inches.

...Southwest to the Southern Rockies...
Another day of monsoonal convection is expected for the southern
Rockies into the Southwest as the center of the mid-level ridge
shifts slightly west from the Four Corners region through Friday
morning. The overall pattern, available moisture and instability
are expected to remain largely unchanged from Wednesday, resulting
in similar threats for heavy rainfall and flash flooding across
the region. Rainfall accumulations of 1-2 inches in less than an
hour and localized maxima of 2-3 inches are expected with a focus
across the Rockies where low level moisture flux will be maximized.

Otto/Roth


Day 2
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES AND EASTERN CAROLINAS...

...Carolinas to the Lower Mississippi Valley...
A front will linger across the region, focusing deep moisture and
another day of unsettled weather. The greatest threat for
organized heavy rainfall is expected to focus along the eastern
extent of the boundary ahead of a mid-level shortwave forecast to
track northeastward from Mississippi/Alabama. After settling a bit
further south across the eastern Carolinas, guidance shows a wave
developing along the front bringing deeper moisture back to the
north. Some portions of eastern North Carolina could see back-to-back
days of heavy rain, raising flash flooding concerns across the
region, though it appears the main axis of rainfall will be
generally south for Friday to Saturday compared to the Thursday to
Friday period.

...Southwest to the Southern Rockies...
No significant changes in moisture are expected for Friday into
Saturday compared to the previous day. The mid-level ridge will
begin to show better movement toward the Great Basin but diurnal
convection is expected to again raise at least isolated flash
flooding concerns for many of the same areas as the Thursday to
Friday period. Therefore, the Slight and Marginal Risks remain
similar to the previous day with potential for rainfall
accumulations of 1-2 inches in less than an hour and localized
maxima of 2-3 inches.

...Eastern Nebraska and Adjacent Locations...
A progressive front is expected to bring precipitable water values
over 1.5 inches across eastern Nebraska Friday afternoon/evening
with 850 mb winds of 20-30 kt ahead of a developing low. Sufficient instability, shear and moisture will be in place within the region
of maximized low level moisture transport to support areas of
organized convection. Given the above, hourly rain totals to 2"
with local amounts to 4" are possible. As the front is progressive,
any flash flooding should be limited in scope and a Marginal Risk
was maintained from continuity to cover the possibility.

Otto/Roth


Day 3
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
SOUTHEAST AND FROM THE CENTRAL/SOUTHERN PLAINS INTO THE
CENTRAL/SOUTHERN ROCKIES AND SOUTHWEST...

...Mid-Atlantic to Central Gulf Coast...
A slow moving front will continue to extend from the Mid-Atlantic
coast into portions of the Tennessee Valley with anomalous moisture
(1 to 2 standardized anomalies of precipitable water) in place in
the vicinity of the front to the Gulf and Southeast coasts.
Slightly stronger southwesterly low level winds are expected across
the Carolinas which could support areas of training and/or slow
movement of thunderstorm clusters containing localized rainfall
rates of 2-3 in/hr. Southwesterly flow will be in place to the east
of a longwave trough axis centered near the Mississippi River and
upper level jet support will likely focus convective activity over
southern Virginia into North Carolina ahead of a shortwave to move
across the Mid- Atlantic states during the day on Saturday. A
Marginal Risk remains in place where the highest probabilities for
2+ inches of rain exist in the latest ensemble guidance. There
wasn't enough agreement in the latest deterministic guidance for
anything higher than a Marginal Risk at this time, but upgrades may
be warranted with future forecast cycles.

...Central Plains to Texas Panhandle...
While some minor differences exist in the models, there is broad
agreement that the central to southern Plains will be situated in
northwest flow aloft with a possible closed mid-level center in the
vicinity of Iowa. A weak frontal boundary may exist across northern
Texas but the focus for potentially excessive rainfall will exist
ahead of an 850 mb low/trough extending from near the Mid-Missouri
River Valley into the Texas Panhandle with weakly anomalous
moisture from northwestern Texas into the Missouri River Valley.
Current thinking is for some 2-4 inch totals, which may fall over a
short period of time supporting localized areas of flash flooding.

...Central/Southern Rockies into the Desert Southwest...
Mid-level ridging is expected to migrate westward into the Great
Basin from Saturday to Sunday morning. The translation of the ridge
and possible shortwave trough advancing southeast across the
southern Plains will increase the easterly component of the low to
mid-level flow across New Mexico into eastern Arizona. Moisture is
forecast to be seasonable to above average, with the best
potential for above average moisture setting up from West Texas
into southern New Mexico and southern Arizona. In terms of upper
level forcing and mean steering flow however, nothing stands out as
anything greater than a Marginal Risk of flash flooding at this
time with typical monsoonal convection.

Otto
---
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FOUS30 KWBC 190828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Jul 19 2024

Day 1
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES AND EASTERN CAROLINAS...

...Carolinas...

The cold front from the past few days will become quasi-stationary
across the Carolinas with a strong low-level convergence signature
focused from Northeast NC down through Central SC in the Piedmont.
Areal PWATs are sufficient for heavy rainfall potential with
anomalies nestled between 1-1.5 deviations above normal with values
between 1.8-2.2" focused over the above area. Current mean QPF is
right around 1-1.5" with some smaller maxima located across
multiple portions of the Carolinas which is reflective of the
overall convective environment. The primary signature of note for
the potential is heavily based in the latest probability fields
with EAS signatures between 25-40% for at least 1" present through
much of the Eastern Carolinas with the max confined to the eastern
extent of the Piedmont down to around Columbia, an urban area more
prone for flash flooding historically. Neighborhood probabilities
for at least 3" show multiple areas between 50-70% with another max
located within the escarpment of SC/NC/TN where models are
depicting some terrain focused convection later this afternoon.
There are some minor signals for at least 5" totals within the
confines of the Eastern Carolinas as well signaling a general
maximum of up to 6" in any cells. Area hodographs indicate a weak
steering pattern which would lead to either slow moving, or even
stationary storms that could enhance the local threat further as
the storm motions would only be accelerated through outflow
propagation. The SLGT risk from the previous forecast was
maintained with some expansion over the northern and southern
fringes to match the recent trends within the probability fields.

...Southern Rockies...

General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel as noted within
the convective QPF footprint. The area will see another round of
diurnally driven convection with more mid-level perturbations
rounding the eastern flank of the ridge, moving overhead during the
peak of diurnal heating. This will create an enhancement within the
large scale ascent pattern along with the already favorable
instability field with the theta-E ridge bisecting much of Central
and Northern NM. Local totals of 1-2" with a few spots seeing
higher amounts are forecast across the terrain down into the
northern extent of the Bootheel near Gila National Forest.
Probabilities for at least 1" are relatively higher when gauging
the neighborhood probability output with some scattered signals for
at least 2" in portions of Central NM. The combination of multi-day
impact within complex terrain and remnant burn scars exacerbating
the flash flood concerns was plenty of reason to extend the SLGT
risk maintain general continuity.

Kleebauer


Day 2
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES THROUGH MUCH OF NEW MEXICO...

...Southern Rockies...

A pattern of persistence within the mid and upper-levels will lead
to an ongoing threat of convective development in-of the Southern
Rockies and adjacent terrain of NM due to primed environmental
destabilization coinciding with a progression of mid-level
vortices streaming down the eastern flank of the ridge across the
Western U.S. Theta-E ridge is forecast to shift focus across NM
with the tongue of elevated instability generally confined to the
terrain extending from the Bootheel up through the Sangre de
Cristos. Scattered to numerous thunderstorms will be capable of
locally heavy rainfall that will interact with complex terrain and
the burn scars littered across the Southern Rockies down to the
Sacramento Mountains. Areal QPF average between 0.75-1.5" is
forecast with a few locations potentially seeing upwards of 2"
which would easily cause issues no matter where they occur given
the lower FFGs situated across the state. Considering the ongoing
impacts from prior periods and the convective coverage anticipated,
maintained the previous SLGT risk with some broadening of the risk
area to reflect the concerns even outside the terrain focused
areas.

...Southeast to Southern Mid Atlantic...

Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.

Kleebauer


Day 3
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES THROUGH MUCH OF NEW MEXICO...

...Southern Rockies...

Mid-level ridge axis will begin sliding eastward with building
heights centered over the Lower Colorado River Basin into the
southern portion of the Great Basin. A multitude of mid-level
vortices will continue to plunge down the eastern flank of the
ridge and interact with ample surface destabilization located
within the Southern Rockies. With steep low to mid-level lapse
rates and prominent moisture anomalies running close to 1-2
deviations above normal,AND SOUTHWEST convective threats will
induce periods of moderate to heavy rainfall within the general
confines of the Southern Rockies with emphasis on the Sangre de
Cristos and the Northern fringe of the Sacramentos. There will also
be an extension of convective concerns down closer to the NM
Bootheel within the Gila National Forest just to the north of
Silver City. Areal QPF within the means are consistent with
0.75-1.25" with some bias corrected maxes exceeding 2" in spots,
mainly over Northern NM. With the successive period of heavy
rainfall potential within the terrain and remnant burn scars, the
threat remains well within the SLGT risk range inherited from prior
forecast. Thus, outside some adjustment within the fringes to
reflect QPF trends, the SLGT risk was majority continuity.


...Mid-Atlantic to Central Gulf Coast...

A mid-level trough will begin to carve out across the Central
Plains with a lingering stationary front still bisecting much of
the Southeast into the Southern Mid Atlantic. Broad extent of
thunderstorm chances will remain stagnant in terms of placement and
potential impact with greater effects generally relegated to focal
points near higher surface convergence patterns and deep layer
moisture field present in any given area. Scattered to numerous
thunderstorms are forecast across TX to the Central Gulf Coast
during the afternoon on Sunday with locally heavy rainfall
plausible in any cells that develop. General 1-2" are possible in
any convective signal, however the area over the Central Gulf has
the best chance for heavier returns due to a deeper moisture field
located along the Gulf. These areas do have a higher FFG however,
so the threat is on the lower end of MRGL during the period, along
with the area across TX. Continued threat of heavy rainfall is
forecast across the Southern Mid Atlantic where multiple shortwaves
will enter the region and enhance regional ascent within the
convergence corridor near the stationary front. The main area of
impact is subject to some variability at range due to the forecast
frontal placement being a major part of where training convection
could develop. The MRGL risk across the area was broad but worthy
of the current location given the setup. An upgrade is possible
pending the convective evolution over the previous period.

Kleebauer
---
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FOUS30 KWBC 200818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Sat Jul 20 2024

Day 1
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST, SOUTHERN ROCKIES, AND SOUTHEASTERN VIRGINIA...

...Southeast Virginia...

A well-defined shortwave analyzed across the AL/GA line will advect
northeast through the early portion of the period, eventually
closing off at 85H as it progresses into Southern VA by the early
afternoon period. At the surface, a quasi-stationary front will
lift northward towards the vicinity of the Hampton Roads corridor
with a surface reflection developing over the NC/VA border and
riding along the confines of the boundary. Increasingly backed flow
within the boundary layer will promote the advection of deeper low
to mid-level moisture that will align with the frontal boundary as
the surface wave approaches with generally more favorable support
within the 850-700mb height field. The combination of the above
variables will allow for a stronger low-level convergence zone
aligned within close proximity to the wavy frontal positioning
transposed over the region. The latest HREF and correlating CAMs
output have all increased the signal within the QPF footprint with
the ensemble means generally depicting an axis of heavier precip
from I-85 to the east towards the I-64 corridor and adjacent
coastal areas. QPF maxima in exceedance of 5" is common within the deterministic output along with the HREF blended mean signifying a
stripe of 2-3+" within the confines of Williamsburg down close to
Norfolk. HREF neighborhood probabilities were also fairly stout
when it comes to the upper quartile of the threat as the >3" probs
sit at a widespread 40-70% over the entire stretch of Southeast VA
from I-85 to I-64 with a bullseye of 80% located along I-64. The

5" probability field is less robust out towards the I-85 area up

to Richmond, but there's still a significant corridor of 30-60%
probabilities situated across that I-64 area down close to Hampton
Roads.

The signals were pronounced enough to warrant a targeted upgrade
across the aforementioned area with the alignment of the risk
comprising of the higher forecast probabilities for at least 3"
with the best threat likely over the I-64 area southeast of
Richmond. The general proxy of the front and the aided surface wave
bisecting the area will likely target an area under or just to the
north of the general circulation. The track of any 850mb low
development will only add to the prospects of increasing low-level
convergence signatures that would spawn an axis of very heavy
rainfall (>2"/hr) within the northern periphery of that circulation
as well. A SLGT risk was added to account for the evolving setup.

...Southwest and Southern Rockies...

General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel over into
Southeast AZ as noted within the convective QPF footprint. The
area will see another round of diurnally driven convection with
more mid-level perturbations rounding the eastern flank of the
ridge, moving overhead during the peak of diurnal heating. This
will create an enhancement within the large scale ascent pattern
along with the already favorable instability field with the theta-E
ridge bisecting much of Central and Northern NM. Local totals of
1-2" with some locally enhanced amounts are forecast across the
terrain down into the northern extent of the Bootheel near Gila
National Forest. HREF neighborhood probabilities for at least 1"
are very high with an areal coverage exceeding 70% that encompasses
much of north-central NM, the Sacramento Mountains, and the
Huachucas in Southeastern AZ. The 2" probabilities are also higher
than previous days with an expanse of 15-30% outputs with a few
bullseyes over 30% within the aforementioned zones above. The
combination of multi- day impact within complex terrain and remnant
burn scars exacerbating the flash flood concerns was plenty of
reason to extend the SLGT risk and maintain general continuity.

Kleebauer


Day 2
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND SOUTHWEST...

...Southwest and Southern Rockies...

A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The ensemble mean QPF was
not as robust compared even with the addition of the elevated
deterministic signals leading to less confidence of higher impacts
that would necessitate an expansion of a higher risk across parts
of Central AZ. The previous update does have the eastern most
confines of the Mogollon Rim within the inherited SLGT risk which
does jive with the latest NBM probability fields signaling the best
threat further east away from the ridge center. QPF mean around
0.75-1.25+" was the target precip depiction for where to outline
the SLGT risk which aligns with the lower FFG exceedance markers
across the region. The highest probabilities for those totals
within the NBM are mainly in NM with only some of AZ seeing those
types of better outputs. This solidified the SLGT risk from
previous forecast with some minor modifications based on the QPF
footprint. The primary areas of interest within the current SLGT
will reside within the complex terrain and areas surrounding the
remnant burn scars located within the Sacramentos and Sangre de
Cristos. SBCAPE between 800-1200 J/kg is forecast within the
ensemble means with some max of 1500+ J/kg showing up within the
terrain of NM and AZ. Expect a general continuity for the next
succession of updates with perhaps that westward expansion of the
SLGT into the Mogollon Rim if the short range guidance trends
upward with the heavy rain signal.

...Southeast to Southern Mid Atlantic...

Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.

...Southern Plains...

The eastern extent of the broad ridge axis across the Western U.S
will help send down multiple mid-level perturbations that will
allow for convective development upstream over the Eastern NM High
Plains, eventually propagating south and east along expected
outflow generation and mean steering flow out of the north-
northwest. There's a growing signal for organized multi-cell
clusters to move off the Caprock and enter portions of West TX and
the nearby Upper Trans Pecos. Deterministic output is subject to
variability in the location and magnitude of any convective cluster
that materialized and enters the area. The signal for potential is
present and was enough to expand the western MRGL risk area further
southeast to account for the threat of more organized heavy
rainfall. This is within the lower end of the risk threshold for
now, but will be subject to expansion or removal based on later
trends within numerical output, especially in the hi-res window.

Further to the southeast across Central and Eastern TX, an
approaching front will become a focal point for diurnally driven
convective development in-of the boundary itself as the environment
it moves into becomes increasingly favorable for cell generation.
The convergence signal is not nearly as impressive as areas further
to the east, but the ensemble mean QPF footprint dictates a
smattering of smaller QPF maxima that would indicate some locally
enhanced rainfall potential. Mean QPF is only around 0.75-1.25" in
spatial coverage, but some deterministic output is much more robust
with 2-4+" bullseyes in portions of Hill Country, Concho Valley,
out into the I-35 corridor. This is a lower end MRGL risk
opportunity within the region, but the upper quartile of outcomes
could spell for some local flash flood concerns if everything
breaks right (or wrong depending on perspective).

Kleebauer
---
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FOUS30 KWBC 201551
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Sat Jul 20 2024

Day 1
Valid 16Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST, SOUTHERN ROCKIES, AND SOUTHEASTERN VIRGINIA...

16Z Update...
Overall...changes to the on-going Excessive Rainfall Outlook were
pretty minor. Broad area of deep moisture and resulting instability
remained in place across the Southeast US with weak and difficult-
to-time shortwaves embedded within the mean flow. Flow looks to
remain confluent...which will help support cell development and
fuel the potential for isolated to widely scattered downpours
throughout the day and evening. There were some signals from the
12Z HREF probabilities for 1 to 2 inch per hour rates over portions
of northern Iowa into far southern Minnesota. The previously issued
Marginal did not reach quite that far north...so extended the area
a bit to cover the potential.

Over the western US...models still tended to support the previously
issued Marginal risk area as well as the embedded Slight Risk area
extending from southeast Arizona across much of New Mexico into
southern Colorado. Except for a few nudges...the area remained
unchanged from the initial issuance.

Bann

...Southeast Virginia...

A well-defined shortwave analyzed across the AL/GA line will advect
northeast through the early portion of the period, eventually
closing off at 85H as it progresses into Southern VA by the early
afternoon period. At the surface, a quasi-stationary front will
lift northward towards the vicinity of the Hampton Roads corridor
with a surface reflection developing over the NC/VA border and
riding along the confines of the boundary. Increasingly backed flow
within the boundary layer will promote the advection of deeper low
to mid-level moisture that will align with the frontal boundary as
the surface wave approaches with generally more favorable support
within the 850-700mb height field. The combination of the above
variables will allow for a stronger low-level convergence zone
aligned within close proximity to the wavy frontal positioning
transposed over the region. The latest HREF and correlating CAMs
output have all increased the signal within the QPF footprint with
the ensemble means generally depicting an axis of heavier precip
from I-85 to the east towards the I-64 corridor and adjacent
coastal areas. QPF maxima in exceedance of 5" is common within the deterministic output along with the HREF blended mean signifying a
stripe of 2-3+" within the confines of Williamsburg down close to
Norfolk. HREF neighborhood probabilities were also fairly stout
when it comes to the upper quartile of the threat as the >3" probs
sit at a widespread 40-70% over the entire stretch of Southeast VA
from I-85 to I-64 with a bullseye of 80% located along I-64. The

5" probability field is less robust out towards the I-85 area up

to Richmond, but there's still a significant corridor of 30-60%
probabilities situated across that I-64 area down close to Hampton
Roads.

The signals were pronounced enough to warrant a targeted upgrade
across the aforementioned area with the alignment of the risk
comprising of the higher forecast probabilities for at least 3"
with the best threat likely over the I-64 area southeast of
Richmond. The general proxy of the front and the aided surface wave
bisecting the area will likely target an area under or just to the
north of the general circulation. The track of any 850mb low
development will only add to the prospects of increasing low-level
convergence signatures that would spawn an axis of very heavy
rainfall (>2"/hr) within the northern periphery of that circulation
as well. A SLGT risk was added to account for the evolving setup.

...Southwest and Southern Rockies...

General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel over into
Southeast AZ as noted within the convective QPF footprint. The
area will see another round of diurnally driven convection with
more mid-level perturbations rounding the eastern flank of the
ridge, moving overhead during the peak of diurnal heating. This
will create an enhancement within the large scale ascent pattern
along with the already favorable instability field with the theta-E
ridge bisecting much of Central and Northern NM. Local totals of
1-2" with some locally enhanced amounts are forecast across the
terrain down into the northern extent of the Bootheel near Gila
National Forest. HREF neighborhood probabilities for at least 1"
are very high with an areal coverage exceeding 70% that encompasses
much of north-central NM, the Sacramento Mountains, and the
Huachucas in Southeastern AZ. The 2" probabilities are also higher
than previous days with an expanse of 15-30% outputs with a few
bullseyes over 30% within the aforementioned zones above. The
combination of multi- day impact within complex terrain and remnant
burn scars exacerbating the flash flood concerns was plenty of
reason to extend the SLGT risk and maintain general continuity.

Kleebauer


Day 2
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND SOUTHWEST...

...Southwest and Southern Rockies...

A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The ensemble mean QPF was
not as robust compared even with the addition of the elevated
deterministic signals leading to less confidence of higher impacts
that would necessitate an expansion of a higher risk across parts
of Central AZ. The previous update does have the eastern most
confines of the Mogollon Rim within the inherited SLGT risk which
does jive with the latest NBM probability fields signaling the best
threat further east away from the ridge center. QPF mean around
0.75-1.25+" was the target precip depiction for where to outline
the SLGT risk which aligns with the lower FFG exceedance markers
across the region. The highest probabilities for those totals
within the NBM are mainly in NM with only some of AZ seeing those
types of better outputs. This solidified the SLGT risk from
previous forecast with some minor modifications based on the QPF
footprint. The primary areas of interest within the current SLGT
will reside within the complex terrain and areas surrounding the
remnant burn scars located within the Sacramentos and Sangre de
Cristos. SBCAPE between 800-1200 J/kg is forecast within the
ensemble means with some max of 1500+ J/kg showing up within the
terrain of NM and AZ. Expect a general continuity for the next
succession of updates with perhaps that westward expansion of the
SLGT into the Mogollon Rim if the short range guidance trends
upward with the heavy rain signal.

...Southeast to Southern Mid Atlantic...

Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.

...Southern Plains...

The eastern extent of the broad ridge axis across the Western U.S
will help send down multiple mid-level perturbations that will
allow for convective development upstream over the Eastern NM High
Plains, eventually propagating south and east along expected
outflow generation and mean steering flow out of the north-
northwest. There's a growing signal for organized multi-cell
clusters to move off the Caprock and enter portions of West TX and
the nearby Upper Trans Pecos. Deterministic output is subject to
variability in the location and magnitude of any convective cluster
that materialized and enters the area. The signal for potential is
present and was enough to expand the western MRGL risk area further
southeast to account for the threat of more organized heavy
rainfall. This is within the lower end of the risk threshold for
now, but will be subject to expansion or removal based on later
trends within numerical output, especially in the hi-res window.

Further to the southeast across Central and Eastern TX, an
approaching front will become a focal point for diurnally driven
convective development in-of the boundary itself as the environment
it moves into becomes increasingly favorable for cell generation.
The convergence signal is not nearly as impressive as areas further
to the east, but the ensemble mean QPF footprint dictates a
smattering of smaller QPF maxima that would indicate some locally
enhanced rainfall potential. Mean QPF is only around 0.75-1.25" in
spatial coverage, but some deterministic output is much more robust
with 2-4+" bullseyes in portions of Hill Country, Concho Valley,
out into the I-35 corridor. This is a lower end MRGL risk
opportunity within the region, but the upper quartile of outcomes
could spell for some local flash flood concerns if everything
breaks right (or wrong depending on perspective).

Kleebauer
---
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FOUS30 KWBC 211555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sun Jul 21 2024

Day 1 Valid 16Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST AND THE WESTERN CAROLINAS...

...16Z Excessive Rainfall Discussion...
Few changes needed. There was a southeastward extension of the
Slight Risk a bit farther into Texas than before given a subtle
increase in QPF shown by the NBM and reflected by the HREF
probabilities. Otherwise...the 21/12Z suite of guidance largely
supported the on- going ERO/ERD.

Bann

...Western Carolina's...

General pattern persistence across the Southern Mid Atlantic will
yield another round of scattered to widespread convection in-of
the Carolinas with some locations setup to see a greater emphasis
for heavy rainfall. A few weak mid-level perturbations will advect
northeast within the mean flow, encroaching on the Piedmont of the
Carolina's by later this morning. The coupling of increased upper
support with a strong diurnal destabilization pattern will promote
the threat of stronger cell cores with heavy rain potential due to
anomalous moisture lingering along and south of the quasi-
stationary front to the north. Recent trends within the HREF and
associated CAMs have been for an increase in heavy rainfall for
points along and west of I-85 where the mid-level ascent will be
maximized along with the favored instability. Convection will
likely fire along the terrain of the escarpment up through the
Appalachians of NC before drifting to the east and northeast away
from the terrain. Probabilities for locally enhanced rainfall
exceeding 2" is very high (>70%) for much of Western NC down into
the far Upstate portion SC to the north of GSP. There's some hints
of upwards of 5" within the probability fields and individual CAMs
members along the escarpment up through areas like Boone points
east towards Greensboro. This area has seen its fair share of heavy
rain in the past 24-48 hrs at times, so the latest FFGs are lower
than climo. In coordination with the surrounding WFOs that bridge
coverage in the Western Carolina's, have introduced a SLGT risk
across the area encompassing much of the area west of the I-85
corridor.

...Southwest and Southern Rockies...

A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The primary areas of
interest within the current SLGT will reside within the complex
terrain and areas surrounding the remnant burn scars located within
the Sacramento's and Sangre de Cristos. Secondary areas of focus
include the NM Bootheel, Southeast AZ terrain around the Huachucas,
as well as the Eastern NM High Plains into the Northwest Permian
Basin of Texas.

The latter of the aforementioned areas is a newer development
within the latest ensemble means with a focus along a remnant
outflow that bisect the Caprock down into the Permian Basin,
outlined by a marginal theta-E gradient in place from Clines
Corner, NM down close to I-20 around the Midland/Odessa corridor. A
shortwave currently analyzed over CO will continue to make headway
to the south around the eastern flank of the ridge eventually
aiding in convective initiation across east-central NM by the
afternoon. Multi-cell cluster of thunderstorms will congeal by
prevailing cold pools and migrate to the southeast, riding right
along the theta-E gradient that will maintain presence through the
afternoon. There's a growing consensus that storms will be able to
hold together and impact a large area encompassing the Caprock of
Eastern NM down through the Northwest Permian Basin before
potentially collapsing in the evening. There is a chance this holds
together to the I-20 corridor and provides some heavier rain
within the confines of the Midland/Odessa area, but the probability
is lower compared to the Northwestern areas up across Southeastern
NM up through the Caprock along the TX/NM state lines. HREF
probabilities are highest for at least 2" within the confines
above, including some >70% output being displayed within the
Northwest Permian up through Lea and Roosevelt Counties in NM.
Despite a very dry signal within the soil moisture availability
over the region, rainfall rates in excess of 1.5"/hr will be
plausible given the elevated moisture presence as noted within the
latest NAEFS PWAT anomalies approaching 1-1.5 deviations above
normal across much of Southeast NM extending southeastward into the quasi-stationary front aligned near I-10.

The previous SLGT risk was maintained, but did allow for an eastern
extension to account for the increasing organized convective threat
aimed for portions of Southeastern NM down into West Texas.

...Oklahoma...

A compact but well-defined surface wave will linger within the
base of the mean trough carved out across the Mid-Mississippi
Valley back down into the northern fringes of the Southern Plains
in OK. Current IR satellite indicates a blossoming of convection
over Northeastern OK this evening that will lead to some locally
heavy rainfall close to Tulsa and surrounding locales. Hi-res
deterministic is consistent on the surface reflection sticking
around through the first half of the period with a short term
degradation of the convective field after 12z, but comes back in
earnest due to the addition of the diurnal destabilization along a
surface trough extending near and south of the surface low. Signals
for increasing low-level convergence within the confines of low
are present within several CAMs outputs later this morning and
afternoon leading to a narrow corridor of heavy rain that could see
totals breach 4" within a short period of time. The main threat is
confined within the small circulation with the northern fringes of
the low being the prime focus for where modest training could
occur. There's a small footprint within the ensemble means of 2+"
just south of the Tulsa metro with accompanying neighborhood
probabilities of >5" up between 25-40% in the same area. Whether
that's the exact location or not, the areal extent of flash flood
concerns is small due to the compact nature of the setup. A MRGL
risk was maintained from previous forecast, but want to make
mention the threat could trend towards more locally significant
impacts where the training convective pattern establishes itself.
Look for future MPD's on the threat as we move towards the late
morning and early afternoon hours.

Kleebauer


Day 2 Valid 12Z Mon Jul 22 2024 - 12Z Tue Jul 23 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES, TEXAS THROUGH THE ARKLATEX...

...Southern Plains...

A quasi-stationary front will bisect much of the state of Texas
with a westward extension into the Southwest TX terrain back
through the Big Bend, Edwards Plateau, Hill Country, all the way
out to the ArklaTex. Mid-level energy from the northwest will
navigate southeast towards the higher terrain south of I-20,
migrating slowly eastward within the confines of the front. The
added upper forcing in conjunction with the increasing low-level
convergence along the stationary front will induce a swath of
stronger thunderstorms capable of significant rainfall within any
cell core. Latest NAEFS PWAT anomalies signals a solid +2
deviations from climo for the moisture field present across much of
the central portions of TX back into the Stockton Plateau. Cell
initiation across the higher terrain out west will lead to
convective clustering with outflow generation stemming from areal
thunderstorm coverage, eventually propagating to the east into the
I-35 corridor in Central TX by later Monday afternoon and evening.
Precip totals within the initial cell development are generally
between 1-2", but some higher totals exceeding 4" will be possible
across the Lower Trans Pecos through the Southern Edwards Plateau.
The heaviest core of precip will lie within the eastern extent of
the Edwards Plateau across into Hill Country until it reaches the
I-35 corridor near Austin/San Antonio. This is where ensemble mean
QPF has been the most consistent for totals exceeding 2" with
indications of up to 3" in areal average QPF within Hill Country
and individual deterministic output exceeding 6" in places hit with
repeated cells. Mean storm motions within the confines of the
boundary are weak meaning slow moving convective clusters with
rainfall rates between 2-3"/hr are possible along that frontal
boundary. The SLGT risk from previous forecast was maintained but
expanded to the west to include more of the Stockton Plateau given
the latest trends within the mean QPF and favorable environment
over the area leading into the evolving event.

Further to the northeast through Northeast TX into the ArklaTex,
energy from the southwest will eventually advect northeastward with
convective generation during the afternoon and evening hours
becoming more organized with the additional upper support. Some
cell clusters will be capable of locally heavy rainfall extending
along the stationary front with some convective training plausible
due to similar conditions from upstream. Totals are not as prolific
within the means across the above region, but some totals of 2-4"
are not out of the question, so felt there was no reason to make
significant adjustments to the previously inherited SLGT risk.

...Southwest and Southern Rockies...

Scattered thunderstorms will develop once again within the confines
of the Sangre de Cristos with highest impacts situated over the
complex terrain and any burn scars within the mountain chain.
Totals are not expected to be as prolific compared to recent days
with less of a favorable mid-level pattern and lower SBCAPE
forecast. Regardless, considering the expected convective
development and the very sensitive nature of the flash flooding
potential within the burn scars, there was enough merit to continue
the focused SLGT risk across the Mountains with an extension down
into the Sacramento's due to the ongoing issues caused by the burn
scarring near Ruidoso. This is a lower end SLGT risk threshold with
impact based reasoning for the risk continuity.

Across the rest of the Southwestern U.S, the upper pattern will
shift to have less ridge potency leading to an expansion of the
convective risks a bit further west to include the Lower Colorado
River valley between CA/AZ/NV. The best threat will still be across
the Mogollon Rim where scattered thunderstorm coverage will likely
spawn some totals exceeding 1" within a short period of time along
the terrain of central AZ. The coverage and moisture anomalies were
not primed enough to warrant an upgrade at this time, but the
threat is still a mid to higher end MRGL risk, on the cusp of a
potential upgrade if the setup becomes more pronounced within the
means.

...Southeast to Southern Mid Atlantic...

Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. The one area of note for a potentially more
impactful setup is across the Central Mid Atlantic where a stronger
shortwave will advect northeast out of the Carolina's with
increased upper forcing traversing the DC/Baltimore metro area
during the middle of peak diurnal instability. Recent deterministic
output is a bit more robust compared to recent forecasts and is in
agreement with the ML output based within the GFS Graphcast and
ECMWF AIFS. It will be interesting to see the trends as we move
into the CAMs window because there could be a targeted SLGT risk
within the urban corridor if the signal holds. A MRGL risk is in
effect, but will be monitoring closely.

Kleebauer
---
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FOUS30 KWBC 220822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Mon Jul 22 2024

Day 1 Valid 12Z Mon Jul 22 2024 - 12Z Tue Jul 23 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, SOUTHERN PLAINS, MID ATLANTIC, AND THE CAROLINAS...

...Southern Plains...

A quasi-stationary front will bisect much of TX during the period
with a succession of mid-level disturbances entering the region
beginning later this morning, carrying through the rest of the
period as they pivot south-southeast around the western flank of a
mean trough to the north. A deep moisture presence will be situated
across Southwest TX through points east with a stronger low-level
convergence pattern and favorable anomalies tied to the lingering
front. Scattered convection will form across the Davis Mountains
and Stockton Plateau in the early afternoon time frame with cells
across the Edwards Plateau initiating not long after. Considering
the moist anomalies and relatively slow storm motions off the
terrain, some cells will be able to produce locally enhanced
rainfall with rates between 1-2"/hr on average with some of the
stronger cores capable of breaching 2"/hr anywhere across the
aforementioned area(s). Numerous multi-cell clusters will develop
by mid-afternoon with outflow generation likely considering the
higher DCAPE environment forecast across the western half of TX.
Outflow propagation will make progress to the east with more cell
initiation forming in the favorable environment away from the cold
pools.

Storms should develop initially over Hill Country to the
I-35 corridor, but more organized convective clusters will enter
the picture by early evening leading to more widespread heavy rain
chances and flash flood concerns as rates can sufficiently hit
2-3"/hr as reflected in the HREF hourly rate probability fields.
There's a very high spatial coverage of higher probabilities for at
least 1" of rainfall in the HREF EAS fields (30-50%) with the
neighborhood probabilities for at least 3" and 5" depicting a
formidable areal extent of 60-80% and 25-40% respectively for each
threshold. There's even some low-end >8" probabilities reflected in
the HREF output as well, indicative of a locally significant impact
potential across portions of Western and Central TX. The best
probabilities lie within the Edwards Plateau through Hill Country
with a small bullseye showing up in the means to the I-35 corridor.
This is well within the bounds of a SLGT risk with higher end SLGT
risk wording necessary for the current forecast. There is a non-
zero chance for an upgrade somewhere across the above region, but
there is less of a true, organized heavy rain risk to pinpoint, but
the area that will be impacted will see those locally significant
flash flood concerns arise.

...Mid Atlantic and Carolinas...

Persistent troughing to the northwest across the Great Lakes will
edge eastward with southwesterly flow and increasing diffluence for
much of the Mid Atlantic and Carolinas, moving into the Northeast
U.S by the back end of the period. At the surface, a lingering
quasi-stationary front will settle over the VA Tidewater down
through the Carolina's and points west with a stronger low-level
convergence footprint tied to the boundary. PWAT anomalies of 1-2
deviations above normal will spread north as we see a bit more
backing of the flow within the Mid Atlantic with the strongest ties
across the Southern Mid Atlantic thanks to a surface wave riding up
the front once again, reaching the VA/NC coastal areas by the late
afternoon hours.

The area(s) of highest interest are located across the VA Tidewater
and adjacent Northeast NC, the eastern side of the Carolina
Piedmont, and up into the Susquehanna Valley of PA where the HREF
neighborhood probability fields are all lit up for the potential
of seeing >3" in spots with even a strong >5" signature located
along the I-64 corridor in Southeast VA (50-60%). The increased
upper forcing due to better aligned mid-level diffluence and some right-entrance region of a developing upper jet streak just off to
the north will allow for a blossoming of scattered convection
across portions of the Mid Atlantic through Central PA. The primary
initiation points will likely be tied to the terrain at first, but
as cells propagate off the terrain, they will experience some
modestly favorable shear that would help sustain updrafts that
could lead to stronger cell cores and locally heavy rainfall. PWAT
anomalies are most favorable for heavy rain along and east of the
I-99 and US-15 corridors leading to a lot of CAMs members
ratchetingup the potential with isolated pockets of 2-4+" totals
within the deterministic output, and reflected in the HREF blended
mean data set showing the expanse of higher totals and increasing
flash flood concerns. The threat will also reside within the urban
corridor from DC to Philadelphia, but there is some disagreement
within the CAMs on the potential due to a small cull in the precip
field with initiation away from the major metro areas. The UFVS ML
First Guess Field is still insistent on a widespread SLGT risk
through the urban areas extending all the way up to the PA/NY
border. The QPF footprint within ML output is also insistent on
potential maxima within the metro areas, so the best course of
action was have that area within the new SLGT risk proposal with
areas to the north into PA outlined as well given the better
signals and agreement in the First Guess Fields.

Further south into Southeast VA and the Carolinas, the threat is
more defined thanks to the presence of the stationary front leading
to ample low-level convergence potential coinciding with fairly
stout theta-E indices located along and south of I-64. The highest
threat will reside over the Hampton Roads area up to Williamsburg
where several days of heavy rainfall have degraded the FFG indices
considerably leading into today. This signal of degraded indices is
documented down through the NC Piedmont all the way into Columbia,
SC where back-to-back days of significant convective impact have
led to flash flooding reports all across southern and central SC.
The threat for today extends into those areas again thanks to weak
mid-level perturbations ejecting northeast within the mean flow
correlating with the stationary front to provide another focused
area of heavy thunderstorms from basically the GA/SC border and
points northeast. As a result, the SLGT risk addition was also
extended to include part of the Delmarva down through Southeast VA
into portions of the Central and Eastern Carolinas.

...Southwest and Southern Rockies...

Continued Monsoonal convective pattern will impact the Desert
Southwest this period with focus along the terrain in AZ and NM,
including the hotter spots of the Mogollon Rim over into the Sangre
de Cristos and the Sacramento Mountain chains. Probabilities
continue to be modest for 1-2" maxima with some higher end
potential in stronger cores that linger within the terrain. The
continued mid and upper ridging in place has led to slower storm
motions with some training concerns under the weak mean steering
flow. Considering the onslaught of scattered to widespread
convection for the past several periods leading to many Flash Flood
Warning issuances by the WFOs across the Southwest, another SLGT
risk was maintained within the Southern Rockies where the impact
potential is highest thanks to remnant burn scars and persistent
flooding over the past week. A MRGL extended to much of the
Southwestern U.S with the Mogollon Rim as the secondary candidate
for higher totals and impacts for localized flash flooding
concerns. The threat probabilistically is still on the middle to
high end of MRGL for the aforementioned area, but a short term
upgrade is plausible if coverage ends up more sufficient than
currently forecast.

...Upper Midwest...

Shortwave trough across Manitoba will pivot southeast into the
Arrowhead of MN with increasing large scale ascent and accompanying
positive vorticity advection (PVA) within the confines of the area.
Relatively steep mid-level lapse rates within a corridor of modest
instability will result in scattered thunderstorm initiation once
the disturbance crosses the border into the U.S. Progressive storm
motions will limit the threat of flash flooding to more isolated
signals, however some heavier cores with rates between 1-2"/hr will
be plausible given some of the low-end probabilities within the
latest HREF output. Totals are generally within the 0.5-1" in areal
coverage, but the matched mean does indicate a few cells capable of
dropping 2-3" near the northern shores of Lake Superior, including
near DLH where urban flooding is more probable. A MRGL risk was
introduced in coordination with the local Duluth WFO for low-end
flash flooding concerns, but enough to warrant the targeted risk.

Kleebauer


Day 2 Valid 12Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL AND SOUTHEAST TEXAS...

...Texas...

Quasi-stationary front will push north and eventually wash out
across the Southern Plains as surface ridging across the Gulf
begins flexing more northwest allowing for deeper moisture presence
to advect into Southeast TX. Scattered convection will develop from
the Lower Trans Pecos through all of Central TX by the afternoon
due to diurnal destabilization with lingering outflow boundaries
from previous periods convection becoming a potential focal point
for heavy rainfall and training over much of the area. Areal QPF
averages are between 0.5-1" across West-Central and Central TX with
some of the upper quartile outcomes closer to 3-4" within different deterministic forecasts. This seems to be towards the upper
threshold of the potential across the region, but after what is
expected today, there's expected to be a lingering flash flood
threat after much of the area becoming primed from previous
rainfall. This was sufficient for a maintenance of the previous
SLGT risk.

Further to the southeast, the additional surge of low-level
moisture out of the Gulf will be accompanied by a weak mid-level
disturbance that will advect north out of the Bay of Campeche,
already causing some convective flare ups down that way when
assessing the latest IR satellite. Ensemble means are becoming more
bullish on the threat of heavier rainfall tied to the Middle and
Upper TX coast with some deterministic outputs pushing 3-5" between
Corpus to the far Upper TX coast, including coastal Houston.
The threat is gaining favorable within the ensemble probability
fields as well with the NBM now depicting a 25-40% chance of >2"
within the zone encompassing Matagorda up towards Port Arthur.
Considering this is working off the mean QPF of the blend, that is
impressive at 2-day leads. The extension of the SLGT was made to
encompass that area of the coast from CRP up to the Southwestern
corner of LA.

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.

...Midwest...

Shortwave trough over the Northern Great Lakes will dip further
southeast with trailing mid-level vorticity pivoting around the
base of the trough situated over Ontario. Scattered cells in the
more will move southeast, but lose fervor as they enter into MI. A
secondary pulse of convection is expected later Tuesday with a more
organized area of thunderstorms expected to form upstream near Lake
Superior, dropping southeast through WI and the western portions of
the UP. The storms will remain on the progressive side, but the
threat of rates up to 2"/hr could cause some isolated flash flood
concerns in more urbanized zones over Central and Eastern WI. This
includes places like Green Bay, Northern Milwaukee, Oshkosh, and
Sheboygan. The previous MRGL risk was maintained with some minor
adjustments on the western flank of the risk area to account for
QPF shifts in the ensemble means.

...Southeast through the Northeast U.S...

Scattered thunderstorms will continue across the Southeast all the
way up into the Northeastern U.S thanks to persistent synoptic
scale forcing and a relatively modest thermodynamic environment in
place. Smaller mid-level perturbations will be entrenched in the
mean flow and will help trigger some smaller, organized cell
clusters capable of isolated flash flooding basically extending
from the Deep South into the Mid Atlantic. Any cell generation
in the deep, moist environment will be capable of flash flooding
concerns with the highest threat likely within the Carolina's due
to the antecedent wet conditions in place over much of the eastern
2/3's of the region. A broad MRGL risk is in place extending from
the Southeast through the portions of the Northeastern U.S given
the threat.

Kleebauer
---
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FOUS30 KWBC 230815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Tue Jul 23 2024

Day 1 Valid 12Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY, AND THE SOUTHERN MID ATLANTIC...

...Southern Plains and Lower Mississippi Valley...

Two distinct areas of interest for the D1 period with different
synoptic and mesoscale evolutions to impact each area. The first
area will be across Central TX within the Edwards Plateau, Hill
Country, to the I-35 corridor. A persistent shortwave embedded
within the base of the trough located over the Southern Plains will
enhance regional upper forcing through the first half of the
forecast period, interacting with a weak quasi-stationary boundary
bisecting the aforementioned area. As high pressure noses in from
the north, a tightening theta-E gradient will transpire within
the confines of the shortwave impulse. Enough favorable large scale
forcing within a weakly capped environment will allow for the
initiation of a line of convection located within the proxy of the
front. Models are in agreement on the development and overall
impacts expected within the line of convection promoting locally
heavy rainfall within areas that were impacted over the previous 24
hrs. Area FFGs are much lower within the Edwards Plateau out
through Hill Country after yesterdays storms leading to a lower
threshold for impacts from flash flooding as the top layer of soil
moisture now sits between 60-80% according to the latest NASA SPoRT
viewer. Neighborhood probabilities for at least 3" are up around
50-80% within a large portion of the above area, extending
eastward to the I-35 corridor from Killeen down through New
Braunfels. >5" probabilities are also present, but more scattered
in the presentation with percentages closer to 15-25% leading to a
somewhat capped upper potential. This was sufficient for the
current SLGT risk continuity with minimal change from the previous
forecast.

Further southeast into the Texas coastal plain and the adjacent
Lower Mississippi Valley, the pattern will yield a much different
impact scenario thanks to the evolving upper levels leading to a
persistent Gulf moisture advection regime within a developing
coastal trough. Small impulses will round the western periphery of
a mid-level ridge extending through the Gulf, making their presence
known as they enter the proximity of the middle and upper TX
coasts. Bands of heavier rain with tropical origin in the moisture
field will promote efficient rain makers as they work their way
onshore. Just away from the coast, adequate surface buoyancy with
deep moisture presence will allow for the generation of more
thunderstorms just away from the coast leading to a larger QPF
footprint between the two convective initiation standpoints. As of
now, the prospects for flash flooding are highest along the Upper
TX coast between Houston to the southwest corner of LA as all
deterministic, ML, and associated ensemble means have overlapped
with the best focus of heavy rainfall around that corridor in
question. Areal average of 2-4" will be forecast within that span
of the coastal plain with QPF maximum upwards of 7-8" possible if
one particular area sees extended training. Probability fields
indicating at least 5" are running between 25-40% across an area
between Matagorda through Port Arthur right along the coast. The
probabilities are a bit less for the >5" totals away from the
immediate coast, but still generally between 10-20% for places like
Houston and Beaumont. This is bordering on a higher risk category,
but the protrusion of heaviest rainfall being mainly at the coast
leaves this on the fence, but well within the upper bound of the
SLGT risk. Will be monitoring the progression of the setup closely
as a targeted MDT is possible, especially if conditions allow for
the population centers within Houston/Galveston up towards Port
Arthur and Beaumont.

Some of the moisture entrainment within the northwest flank of the
ridge in the Gulf will be pulled inland to the northeast over LA
and MS leading to an expected band of heavy thunderstorms capable
of rates between 2-3"/hr at peak and totals pushing 2-5" locally
extending from Lake Charles up through Jackson, MS. The prospects
for localized flooding within urban corridors has grown from recent
updates allowing for an extension of the SLGT risk over TX to be
pulled more northeast to account for the threat.

...Southern Mid Atlantic...

Another day of scattered to widespread convective impact
anticipated across the Southern Mid Atlantic with the heavy rain
threat continuing within Central and Northeast NC up through the VA
Tidewater. Heavy rain is likely within any cell development thanks
to a persistent +2 deviation PWAT anomaly situated south of the quasi-stationary front bisecting the Central Mid Atlantic. Multiple
smaller impulses will advect from the southwest with the mean flow
aloft along with entering into the RER of an upper jet streak
forming to the north. This will correlate to cell initiation
within more favorable upper dynamics and modestly buoyant
environment in place over the Southeast VA through the Carolinas
creating another threat of heavy rainfall and repeated impact of
cells along the corridor from the SC Piedmont and points northeast.
HREF EAS probabilities between 25-40% for at least 1" covers a
large expanse of the Southern Mid Atlantic, a traditional symbol
for agreement within the CAMs on a widespread convective heavy
rain threat and within the threshold for a SLGT risk upgrade. Based
on the setup, repeated threat continuing of heavy rain and flash
flood concerns, and the probability fields insinuating more 3+"
totals locally in Southern VA and through NC, a SLGT risk was
issued over the aforementioned area(s).

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of the southern half of NV leading to
more coverage of the MRGL risk across those areas. QPF means are
generally light, but some of the higher end outputs within the
deterministic suite are upwards of 1-2", especially across the
Mogollon Rim. This is a classic Monsoonal setup with the favored
terrain being the primary focus for the period. The MRGL risk
inherited was maintained with full continuity as ensemble mean QPF
did not change much in terms of magnitude and precip placement from
previous forecast.

...Midwest...

Shortwave trough across the Great Lakes will continue to push
southeastward with modest upper forcing under the trough axis.
Modest instability will linger across the Midwest heading into
today with some trailing vorticity maxima entering Central Midwest
by late morning and afternoon. Convection will develop upstream
over the Upper Midwest and pivot south into Central and Southern
WI, IA, Northern IL, and eventually spread eastward into parts of
Michigan. Isolated heavy rain signals within the CAMs backed by
some modest probabilities for 1-2" and locally up to 3" promote a
general continuation of the MRGL risk in place. Only some minor
adjustments were made to the previous forecast given the relatively
stable ensemble QPF footprint.

Kleebauer


Day 2 Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS, SOUTHWEST LOUISIANA, AS WELL AS THE SOUTHEASTERN U.S...

...Texas and Louisiana...

A continuation of the pattern from the previous period will lead to
more shortwave impulses exiting out of the western Gulf as they
navigate northward between a broad upper trough to the northwest
and the persistent ridge off the southeast in the Gulf and Western
Caribbean. A coastal trough will strengthen at the surface right
along the mid and upper TX coast leading to a more defined axis of
surface convergence in-of the coastal plain. Moisture anomalies
will be poking closer to +3 deviations with area PWATs generally
running between 2.2-2.5 along the TX coast all the way into
Southwest LA. There is still some modest uncertainty as to where
the best axis of heavy rainfall will preside for the D2 period, but
the consensus is slowly merging towards the Upper TX coast with a
grazing of the Mid TX coast for the heaviest rain potential. Recent
ensemble QPF has an axis of 2-4" located right along the immediate
coast beginning near Matagorda, expanding to the northeast to as
far north as Lake Charles. Ensemble probabilities within both the
GEFS and ECENS are targeting the Middle TX coastal areas to the
southwest of Houston with the best probabilities for upwards of 2"
while the NBM has a split maxima of one overlapping the area the
GEFS and ECENS are targeting along with one to the north over Port
Arthur into Lake Charles. The split comes for a discrepancy in the
handling of a more robust shortwave that ejects to the north later
in the period. Due to that discrepancy, a potential for an upgrade
was passed on to take more time for guidance to come into agreement
on where the focal point for the highest totals will occur. There
is enough merit given the environment that if any area is favored
for at least 4" within the mean, and probabilities favor the threat
of over 5-6+", there could be a targeted MDT risk in future
updates. For now, a broad higher end SLGT has been maintained with
emphasis on the Middle and Upper TX coast, including near Houston
proper.

...Southeast...

A stronger mid-level impulse will eject northeast out of LA into
through the Deep South within a corridor of moderate instability
and deep moist access. The favorable upper forcing and accompanying thermodynamic presence has allowed for guidance to generate a
widespread area of higher QPF along the confines of the impulses
path. This is consistent within the ensemble bias corrected QPF
footprint with 1.5-2.5" totals in the ensemble positioned from
east-central AL through Central GA/SC into extreme southern NC.
Considering the prospects for 2-3+"/hr rates within the convective
zone Wednesday afternoon, a SLGT risk addition was warranted to
cover for the potential.

...Southwest and Southern Rockies...

Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.

...Great Lakes and Ohio Valley...

Surface low across MI will push northeast into Canada with a
trailing cold front progressing east and southeast over the course
of Wednesday. Isolated to scattered thunderstorms will develop
along and ahead of the cold front with some modest cell cores
plausible given the increased surface and upper forcing pattern.
QPF footprint is overall on the weaker side compared to what you
would want to see regarding more considerable flash flood threats,
but the environment is capable for some totals of 2-3" in a very
short time which if it falls within any of the urban corridors
around the Lakes could spell some isolated flash flood concerns.
The threat overall is low-end within the MRGL threshold, but enough
to warrant a continuation from previous forecast.

Kleebauer
---
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FOUS30 KWBC 240752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Jul 24 2024

Day 1 Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS THE UPPER
TEXAS COAST AND SOUTHWEST LOUISIANA...

...Texas and Louisiana Coast...

After coordination with the local Lake Charles and Houston WFO's, a
targeted Moderate Risk was issued across the Upper Texas coast into Southwestern LA, including Galveston Island. At the surface, a
coastal trough is positioned just off the Upper TX coastal plain
with a deep moist axis aimed orthogonal to the coast between
Galveston up through the Lower Sabine Valley. The entire area
within the coastal plain is positioned within a very anomalous axis
of elevated moisture with PWAT deviations running between +3-4 from
CRP up through all of Southeastern TX into LA. 00z soundings out of
KLCH depict a very tropical airmass with a deep moisture presence
through the column, basically running from the surface to the
tropopause. Warm cloud layer depth is running around 15.5k feet, a
classic signature of heavy rain potential with a greater propensity
to exude very efficient rainfall rates from warm rain processes.
Even within any deep layer convection, this would spell trouble for
local rates breaching 2"/hr with the sampled environment generally
capable for upwards of 4"/hr within the stronger cell cores. The
latest HREF hourly rate probabilities are indicative of just that
with a corridor of elevated probability signatures between 25-40%
for rates exceeding 3" within the confines of the coast stretching
from Galveston up into places like Port Arthur/Beaumont as the
frictional convergence regime begins towards dawn this morning and
maintains prominence through the early afternoon before
dissipating. The probability fields for total rainfall were the
biggest signal for the risk upgrades with the HREF EAS outputs
considerably bullish within the 2" (45-60%) and 3" (15-30%)
markers. Considering the conservative nature of the EAS due to the
process which it's calculated, this is a significant output that is
typically reserved for those higher end potential events.

Assuming the factors involved and the presence of deep, tropical
moisture advection off the Western Gulf, there was enough of a
signature for a MDT risk upgrade in the location with the highest
probabilities and mean QPF footprint indicating a solid 3-6" with
locally as high as 9" possible, indicated with the LPMM HREF.

...Southeast to Southern Mid Atlantic...

The same quasi-stationary front that has been stuck in the Southern
Mid Atlantic down through the Southeastern U.S will continue to be
a boon for the the region as another round of heavy thunderstorms
will develop within an axis of elevated theta-E's located along the
boundary from LA to Southeastern VA. Several small impulses also
within the stagnant upper pattern will aid in the necessary upper
forcing to promote small cell clusters of storms capable of a
larger areal extent of heavy rainfall that would promote flash
flood concerns within more urbanized corridors across the South,
along with any slower moving convection that gets anchored to
lingering cold pools that are running rampant from the previous
days of convection. A deeper moisture flux will be entering into
the Deep South with an embedded stronger mid-level vorticity maxima
pivoting northeastward after it exits the Gulf. This will spur a
more organized convective cluster downstream over places like
MS/AL/GA. This is causing a well defined bullseye of higher precip
within that corridor and is represented within the probability
fields with the >3" neighborhood output hovering between 40-70%
extending from Southern MS across into the SC Low Country. The
previous SLGT risk was expanded further west to account for the
convective signatures all the way into LA.

...Northeast...

A tongue of elevated theta-E's will be advected north on the lee
side of the upper trough pivoting through the Great Lakes. To the
southwest, a blossoming jet streak will develop over the Southern
Ohio Valley and begin nosing into the interior Northeast by later
this afternoon. At the surface, a cold front will be sweeping
eastward out of Ontario through Western NY and PA leading to
increasing low-level convergence within the axis of greater
instability. All this to say that scattered thunderstorms with
rainfall rates between 1-2"/hr will be plausible across the
interior with the highest chance for flash flood concerns within
Central and Upstate NY where the greatest merger of upper forcing
and repeated convective impacts will occur later this afternoon and
evening before the front kicks the threat downstream. HREF
neighborhood probabilities are generally high for at least 2"
across the Adirondacks back towards the eastern Finger Lakes. The
probabilities for at least 3" are still fairly modest within the
neighborhood output with a bullseye of 40-50% located over the
Adirondacks. The signals is scattered for the highest totals
within the deterministic outputs, so the setup is right on the
higher end of the MRGL and on the cusp of a small SLGT risk area.
For now, maintained continuity but will note the threat of a
targeted upgrade if the trends continue upwards in the next update.

...Midwest and Ohio Valley...

Surface low located over the Great Lakes will begin to occlude and
track northeast through Ontario with a trailing cold front swinging
through the Midwest and Ohio Valley later this morning and beyond.
A general consensus within the CAMs for a round of convection to
develop in-of IA and IL later this morning, spreading southeast
along the confines of the front. Flow will be modestly convergent
along the leading edge of the boundary with some favorable large
scale ascent caused by the northern half of the area sitting within
the left exit region (LER) of a blossoming jet max situated over
KY. Observing the latest forecast soundings off some of the CAMs
indicate a signature for locally heavy rainfall with some potential
training as the flow becomes a more parallel to the front as we
move into the evening hrs. The threat will not persistent however,
as the front will be on the move through the entirety of the
forecast. This has limited the overall potential of the setup to
mainly 1-2" maxes with perhaps as high as 3" as per the latest
HREF neighborhood probabilities of >3" settling between 15-25%
across portions of Central IL with lighter 5-10% markers further
east in IN/OH. This was enough for a continuation of the inherited
MRGL risk with only some minor adjustments necessary to reflect the
latest QPF and probability fields.

...Southwest...

Scattered Monsoonal convection will continue for another period
across the Southwestern U.S with the primary heavy rain footprint
aligned within the Mogollon Rim up through the Great Basin. Areal
SBCAPE values will range from 500-1500 J/kg through a large extent
of the west with PWAT anomalies generally +1 deviations across AZ
and NM with +2 and +3 deviations aligned from Northern CA across
into NV. Highest moisture anomalies will advect eastward through
the period as a shortwave trough across the PAC NW begins to
flatten the northern extent of the ridge sending the more prominent
moisture anomalies into Northern NV, Southern ID, and Western UT.
Current QPF signature within guidance is relatively scattered when
it comes to the higher values, but some 1-2+" totals are likely
within the current setup with the best chance focused across the
Mogollon Rim and the terrain west of I-19 in Southern AZ based on
the latest ensemble QPF output and probability fields. The MRGL
risk from previous forecast was maintained with only a few small
modifications to reflect the instability gradient and forecast QPF
footprint.

Kleebauer


Day 2 Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA, AS WELL AS PORTIONS OF THE
SOUTHEAST AND SOUTHERN MID ATLANTIC...

...Texas and Louisiana...

A continuation of the pattern from the previous period will lead to
more shortwave impulses exiting out of the western Gulf as they
navigate northward between a broad upper trough to the northwest
and the persistent ridge off the southeast in the Gulf and Western
Caribbean. Unlike today, the pattern has less definition to the
heavy rain threat with a more inland protrusion of the heavy rain
footprint when assessing the deterministic globals and ensemble
based QPF. There will likely be an area between the Middle TX coast
up through the Sabine River Valley that will experience some
significant rainfall potential so long as the upper pattern remains
favorable based in the model output. Verbatim, the threat is likely
to impact areas a bit further down the TX coast with the area from
Matagorda up through Houston the primary target initially with the
QPF maximum generally aimed within that corridor on most
deterministic and ML output. Current forecast for 1-3" areal
average is a good base to start prior to having some benefit from
the future CAMs that will be able to exhibit better defined
convective characteristics given the favorable mesoscale cores and
resolution. Expectation is for another round of locally heavy rain
with potentially some significant flash flood concerns within the
hardest hit zones as local maximum could climb upwards of 6" if the
pattern holds. The PWAT anomalies between +2-3 deviations will
still linger across all of Southeast TX through much of LA,
continuing the prominent warm rain processes that will allow for
efficient rainfall accumulations. The previous SLGT risk was
maintained with only some minor adjustments to the norther fringes
to align with the trends of a further inland protrusion of the
heavier precip signals. The risk is a higher end SLGT within the
coastal plain just north of CRP up into Southwestern LA.

...Southeast to Southern Mid Atlantic...

A cold front will migrate to the southeast pressing through the
southern half of the Mid Atlantic while the at the upper levels a
130kt jet streak will be passing over New England leading to better
large scale forcing within the RER of the jet across the
aforementioned area. Ample pre-frontal instability within anomalous
deep layer moisture will also contribute to the favorable
environment for heavy rain potential, especially as the front
approaches and enhances low-level convergence within the confines
of the boundary. After destabilization will aid in priming the
environment prior to the fronts approach with the initial area of
interest lying over Southeastern VA, eventually shifting focus into
the Carolina's as the front progresses south. This is the area of
greatest significance given the best surface to upper level forcing
presence working in tandem to create a period of widespread heavy
rainfall. Ensemble QPF between 2-3" is common within the confines
of the front from the Hampton Roads area down through Eastern and
Central NC/SC with much of the rain occurring within a short time
frame as the front approaches. This is a signature for heavy hourly
rates that will enhance the flood risk over the region. For that,
the SLGT risk was maintained from the previous forecast.

Across the Southeast, deep moisture presence with several mid-level perturbations entering the area through the period will assist in
maintaining a scattered thunderstorm signature with heavy rain
threats continuing thanks to the persistent elevated moisture
anomalies and ample instability. The threat is not as pronounced as
what will occur further to the north, but the setup is sufficient
for any cell producing flash flooding, especially over GA/SC where
the best instability is located.

...Southwest and Southern Rockies...

Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. The area
of interest remains across the Mogollon Rim down into Central AZ
where Locally 1-2" with upwards of 3" is possible currently in
that zone due to a stronger axis of instability under which could
end of being near a 600dm ridge on Thursday afternoon. Across the
north, a more prominent moisture advection pattern will continue
through the Interior Mountain West as a shortwave trough over the
PAC NW continues to squash the northern extent of the ridge and
funnel the moisture further into the interior after it began
navigating out of the CA/NV. More widespread convective coverage is
forecast across UT/Southern ID/Western WY as a result leading to
an expansion of flash flood concerns within those areas. The MRGL
risk was generally maintained from previous forecast but did
expand the risk slightly on the northern periphery to account for
the moisture anomalies and accompanying forecast instability across
those northern zones.

Kleebauer
---
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FOUS30 KWBC 250818
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Thu Jul 25 2024

Day 1 Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE MIDDLE
AND UPPER TEXAS COAST THROUGH SOUTHWEST LOUISIANA...

...Texas and Louisiana...

Current upper air and surface analysis across South TX indicates
the development of a weak surface reflection with a closed 85H
(850mb) low present as of the latest observations. This will play
a significant role in the upstream convective potential through the
period, especially along the immediate TX coast from CRP up
through the Southwest coast of LA. There's a near unanimous
consensus on the latest 00z CAMs of a targeted area of significant
rainfall within the region spanning from Matagorda Bay up to just
south of Galveston during the first half of the forecast period.
850mb low across South TX will slowly migrate to the northeast
with a focused LLJ on the eastern flank of the circulation. The
enhanced jet and increasing low level convergence under the
surface and 850mb low(s) will generate a skinny axis of heavy
rainfall with embedded convection given marginally sufficient
MUCAPE located near the coast. 00z sounding out of KCRP was
indicative of deep tropical moisture entrenched within the column
as PWATs nestled just under 2.4", a solid 3 deviations above normal
and within the 99th percentile climatologically, just falling
short of the daily maximum of 2.42". Regardless of a record or not,
the environment is indicative of a moist, tropical airmass that
will exude efficient warm rain processes through any convective
development and the evolving pattern this evening through the
morning is one that will very likely promote intense rainfall
locally within the confines of the surface and closed 850mb
reflections. This setup will slowly migrate to the northeast during
the morning, but the setup for later this afternoon and the
following evening still has the "jury out" on exactly how the 850mb
low behaves.

As of now, the mean QPF output via the HREF and associated
ensembles are depicting a narrow axis of significant rainfall
totals extending from just northeast of Port Aransas up through
Galveston Bay and eventually hitting the Upper Texas coast near the
Sabine River, the same place that was just impacted recently from
significant rainfall yesterday. The probability fields for >3" of
rain are pronounced within that corridor, but the upper quartile
outcomes and associated neighborhood probabilities for at least 5"
(50-80%) and 8" (20-40%) are much higher further south along the
Middle TX coast with the bullseye situated around Port Lavaca up
through Matagorda with the former being the focal point for the
heaviest rainfall as of the latest model output. Further away from
the coast remains a question on the inland extent of the heavy rain
as the QPF from many of the CAMs and even some global deterministic
are all shying away from a larger protrusion inland with the focus
still remaining primarily at the coast. So much of this is
contingent on the behavior of the 850mb low as a longer
maintenance of the low across the coast would likely enhance a
secondary push of convection inland by nightfall thanks to a
redevelopment of the LLJ aimed orthogonal to the coast after 00z
this evening. Models that do have this outcome are more bullish in
a larger impact footprint to the north as the low drifts that
direction and allows for a greater inland focus to arise as we step
into the back end of the forecast period. These solutions also
give credence to higher potential impacts for greater Houston, so
that will be something to monitor in time where there could be
adjustments to the current MDT risk forecast.

For now, the main threat is tied closer to the coastal areas with
the previous MDT risk generally maintained by expanded to the south
to the northeast of CRP, mainly from Matagorda Bay on northeast.
The northeastern extent of the MDT was trimmed back across LA with
guidance becoming less enthusiastic with the push of deeper
moisture inland near Lake Charles and points north and maintaining
a more confined coastal heavy rain threat, similar to where the
impacts were felt this past forecast period.

...Southeast into the Southern Mid Atlantic...

An intensifying upper jet across the Ohio Valley will extend into
the Northeastern U.S with a strengthening core upwards of 130kts
reflecting some fairly significant large scale ascent within the
right entrance region (RER) of the jet. At the surface, a cold
front will translate east and southeastward with progression
through the Mid Atlantic and the Southern Ohio Valley through the
period leading to some modest surface convergence along the front
with scattered to widespread convection out ahead of the boundary
thanks to the present large scale ascent from the aforementioned
jet. Sufficient low-level buoyancy located across Southern VA
through the Carolinas into the Deep South will promote an
opportunity of heavy thunderstorms with the highest probability
located over the Carolinas into Eastern Ga due to the presence of a quasi-stationary front still bisecting the area. This will become
a focal point for low-level convergence and will provide an anchor
for storms to develop and slowly drift within the confines of the
boundary. Rainfall rates of 1-2"/hr will be most common in any
storm development, but a max of 3"/hr is still plausible given the
low probabilistic output of 10-20% over a large portion of the
Carolinas into far Eastern GA for those 3"/hr potentials. Mean QPF
output via the HREF was solidly between 1-2" spatially with the
individual CAMs output signaling some much higher local totals that
could exceed 4" in some cases, a much greater threat for flash
flooding potential.

Considering the incessant nature of the pattern the past week with
copious heavy rainfall episodes contributing to flash flooding
across the Carolinas, a SLGT risk from previous forecast was
maintained with a small shift on the western extent of the risk
area where the probabilities dropped off significantly for the
threat of flooding rainfall, carrying the signal closer to the
SC/GA line than previous forecasts.

...West...

Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. More
widespread convective coverage is forecast across UT/Southern
ID/Western WY as a result leading to an expansion of flash flood
concerns within those areas. Neighborhood probabilities for the
higher convective totals >1" are still most prominent across AZ
with the focus areas being over the Mogollon Rim down through the
Southern terrain near the border where the probs are running
between 60-80% for the threat of at least 1". Secondary maxima are
popping up across the Inner Mountains west within Utah and Western
Colorado, but the threat is a little more subdued due to the lower
instability outputs that favor further south under the 500mb ridge.
Still, the moisture anomalies are forecasted to be best to the
north, so there could be some sneaky prospects for locally higher
impacts from convection, especially within the terrain and
adjacent valleys. The MRGL risk was generally maintained from
previous forecast but did expand the risk slightly on the northern
periphery to account for the moisture anomalies and accompanying
forecast instability across those northern zones.

Kleebauer


Day 2 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE CAROLINAS...

...Carolinas...

The cold front from the previous period will finally make progress
off the eastern seaboard across the northern half of NC while
making slow progress to the south through the rest of NC/SC region.
Scattered thunderstorms within the persistent deep moist and
unstable environment will allow for a period of heavy rain
potential, especially along and ahead of the cold front as it
migrates southward. The heaviest rain will align over Eastern NC
into the South Carolina Low Country, areas where FFGs are a bit
higher and potentially more difficult to flood. Previous rainfall
has lowered the FFG indices, especially as you work from Columbia,
SC to points northeast which plays a role in the current SLGT risk
maintenance. Ensemble QPF output is currently between 1-2" across
Southeast NC, including the Wilmington area extending down through
the Low Country and coastal sections of SC. Individual
deterministic output is as expected with some local maxima around
4" within any particular global member leading to ensemble bias
corrected means closer to 2-2.25" over the hardest hit areas. Did
trim back on the northern extent of the previous SLGT with
maintenance further south and southeast across central and eastern
SC up through Southeast NC where the front will slow in forward
progression and become a focal point for Friday afternoon
convection. Best threat for flash flooding will likely occur over
more urbanized zones due to the sandier soils present in the
confines of the current risk area which lead to the continuity,
and not expecting any upgrades in the current setup.

...Southwest through the Inner Mountain West...

Scattered thunderstorms within a favorable moist axis will linger
through the Southwestern U.S with the northern half of the
convective threat shifting further inland into the Interior before
hitting the proverbial wall at the Central Rockies. Any storm
within the period could exude heavy rainfall with rates generally
topping at 1"/hr, an intensity that could easily cause issues
within the complex terrain out west. The increased moisture
advection back over NM will also open the door for more convective
threats within the remnant burn scars in place over the Sangre de
Cristos, so the MRGL risk was promptly positioned to cover for the
threat. General rainfall maxima is expected to be between 1-2" but
a small chance for locally higher remains, especially from any
cells that get anchored to the terrain. The previous MRGL risk was
kept to general continuity, but did pull the MRGL risk a little
further west into NV due to some linger convective potential from a
trailing shortwave migrating through the region during peak diurnal instability, aligned with a further west QPF footprint as of the
latest NBM and ensemble bias corrected outputs.

...East Texas through the Lower Mississippi Valley...

Remnant mid-level energy will linger across Eastern TX into the
Lower Mississippi Valley within a persistent anomalous moisture
presence situated over the region. Sufficient buoyancy and
attendant large scale forcing will likely generate periods of
scattered convection capable of producing locally heavy rainfall
over the aforementioned areas. There are some indications that a
much more defined 500mb low will develop over the Southern Plains
which would increase the general forcing pattern allowing for a
more robust convective initiation over the eastern flank of the
mid-level circulation. This was noted via some of the 00z global
deterministic with the ECMWF being the most aggressive with the
solution. There are some hints of the threat becoming more
pronounced across East TX into parts of LA/AR given the recent GFS
Graphcast and ECMWF AIFS ML output, so a targeted upgrade may occur
over the above areas if the consensus grows and remains stable
in the upcoming model iterations. For now, a MRGL was maintained
from previous forecast.

Kleebauer
---
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FOUS30 KWBC 260828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Jul 26 2024

Day 1 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHEAST TEXAS AND ACROSS PORTIONS OF THE SOUTHEAST US COAST...

...Portions of Southeast Texas...
Radar was showing increasing coverage of showers in a region of
confluent flown off the Gulf of Mexico in the early morning
hours...and the expectation is that convection will be persisting
beyond 12Z. Given some overlap with a region of lower Flash Flood
Guidance values and potential for 1 to 2 inch per hour rates
redeveloping later in the day as shown by the HREF 40 km
neighborhood probability guidance, maintained a Slight Risk from
coastal Texas northeastward along the axis of highest precipitable water/instability. There is some upper support in the form of a
trof axis between 200 mb and 300 mb with a 50 kt speed max rotating
around the east side of the trof axis that looks help draw the
moisture towards the Southern Mississippi valley late in the
period,

...Portions of the Southeast US Coast...
Surface low pressure will linger along the Carolina coastline for
much of the period with an associated frontal boundary helping
provide some focus for convection as mid-level height falls
approach from the north and west. Much of the operational guidance
showed the heaviest rainfall along or immediately off the Carolina coastline...although the NAM maintained some threat of 2 to 5 inch
amounts falling inland. Between that and the fact that the 26/00Z
HREF showed 40 to 60 pct neighborhood probabilities of 1- and
3-hour QPF exceeding flash flood guidance along the immediate
coastline at time of maximum daytime heating...maintained the
Slight risk but continued to shrink the area somewhat from the placement
of the inherited Slight Risk area.

Surrounding the two Slight Risk areas was an expansive Marginals
Risk area. Models depict abundant moisture and instability in this
area but weak forcing mechanism and meager steering flow. Thus
cells that form will be slow moving and efficient rainfall
producers.

...Southwest...

Enough moisture and instability will still be in place over
portions of the Southwest and the Intermountain basin to support
the development of convection in the afternoon and which persists
into the evening...although the best focus will have shifted
eastward since Thursday. There is still concern that any storm
which forms will have the potential for 1+ inch per hour rates and
isolated storm total amounts in the 1 to 2 inch range...enough to
result in flooding and run off problems with the greatest risk
being over recent burn scars and in normally dry washes. Some minor
adjustments were made to the previous MRGL risk area but overall
there was a fair degree of continuity.

Bann


Day 2 Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A LARGE
PORTION OF THE SOUTHERN/SOUTHEASTERN US AND INTERMOUNTAIN REGION...

...Southern and Southeastern United States...
The 26/00Z suite of numerical guidance showed deeper moisture
getting drawn north and eastward from the Texas coast towards the
western part of the Tennessee Valley given persistent south to
southeasterly flow on the east side of an upper level
trough/closed low. With mid-level height rising from the western
Great Lakes to eastern Tennessee Valley, opted tom limit the
eastern extent of the Marginal. Elsewhere...enough instability and
sufficiently deep moisture will be in place for almost any storms
that develop to produce isolated flash flooding due to slow moving
downpours.

...Southwest United States...
Confidence remains below average in the potential and placement of
flash flooding across the West...with the areal coverage shrunk yet
again from the previous issuance. Given the terrain and lingering
deep moisture in much of the West, it's going to be really tough
to completely eliminate the flash flooding potential.

Bann


Day 3 Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN/WESTERN UNITED STATES AND UPPER MIDWEST...

...Gulf coast to parts of the Tennessee Valley...
A closed mid- and upper-level low that started to take shape late
Saturday night/early Sunday morning should begin to lift northward
during the day on Sunday and then become an open wave again by
Sunday evening. Lowering mid-level heights should help increase the
coverage of convection within an an atmosphere sufficiently moist
to support isolated downpours in a corridor from the middle Gulf
coast to the Tennessee valley during the afternoon and evening,

...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.

...Upper Midwest...
Scattered convection should develop along a cold front extending
into the northern tier of the US from a system in Canada. With
increasing precipitable water and instability values increasing as
a result of southerly flow ahead of the front...isolated downpours
could result in isolated flooding or run off problems in regions of
poor drainage.

Bann
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FOUS30 KWBC 270828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024

Day 1
Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TEXAS GULF COAST...

...Texas Coast to Southeast US...

Maintained the Slight Risk area with onshore southerly flow of
deep tropical moisture off the Gulf set to bring another day of
scattered convection capable of producing heavy rain over an area
where flash flood guidance has been lowered by several days of
moderate to heavy rainfall...with the expectation that the
convective organization should be less today compared with previous
days.

Elsewhere, only modest adjustments were made to the expansive
Marginal Risks surrounding the Slight Risk area and eastward to the mid-Mississippi Valley given persistent south to southeasterly
flow on the east side of an upper level trough/closed low. The
trough/upper low could lead to enough upper support to focus and
sustain storms that produce localized downpours...but even
convective initiation along local outflow boundaries will be
forming in an environment supportive of downpours from the
Mississippi Valley into parts of the Southeast U.S...so isolated
instances of multi- inch rainfall totals are quite possible.

...Southwest United States...
Signals from the 27/00Z suite of numerical guidance continues to
show decreasing threat of excessive rainfall as mid level flow
increases across the Intermountain region. The exception remains
over parts of Arizona and New Mexico where enough
moisture/instability linger in a region with meager southwesterly
steering flow...so will be maintain at Marginal risk area.

...Upper Midwest...
Maintained the Marginal Risk area mainly over northern Minnesota as
given the presence of increasing southwesterly low level flow
feeding an airmass with precipitable water values locally in
excess of 1.75 inches ahead of a cold front making its way across
the area. Given spread in forward speed of the front...expanded the
Marginal Risk area but largely maintained continuity in terms of
the area covered.

Bann


Day 2
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST...

...Northern Minnesota...

Rain associated with a cold front moving across the northern tier
of the country should be ongoing across northern Minnesota. While
there is likely to diminish in intensity for a period early in the period...there should be a renewed risk of excessive rainfall later
in the day and evening. With precipitable water values increasing
to 1.75 inches or greater continuing to be fed into the region by
south to southwesterly 850 mb flow with divergence aloft being
provided by the right entrance region of an upper level
jet...the 27/00Z HREF neighborhood probability values of 10 to 15
percent for 2+ inch amounts in an hour seem pretty reasonable.
Assuming that the GFS was too fast with the forward speed of the
front during the period...he risk of training cells and repeat
convection are additional concerns for excessive rainfall in
addition to the intense rainfall rates.

...Tennessee Valley and Surrounding Areas...

A stationary front turned warm front will track northeastward up
the Tennessee Valley through late Sunday night/early Sunday morning.
Increasing atmospheric moisture associated with the advance of an
airmass originating off the Gulf will allow for potential training
and backbuilding of strong storms capable of increasingly heavy
rainfall. The surface warm front will likely slow as it approaches
the southern Appalachians...supporting slower moving storms that
with interactions with the terrain will be capable of flash
flooding. With very efficient warm rain processes at
work...multiple inch per hour rainfall rates . Urban areas are at
a higher risk for localized flash flooding with any repeating
storms.

...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.

Bann


Day 3
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER
THE TENNESSEE VALLEY...

...Southeast US...
Focus for excessive rainfall shifts eastward and is mainly focused
in the central and eastern portion of the Tennessee Valley as mid
level height falls approach from the west and precipitable water
values at or above 1.75 inches will already be in place. While the
height falls are associated with a trough that was beginning to
fill...the diffluent flow located above an instability axis should
be enough to support downpours from slow moving storms in the
afternoon and evening. Elsewhere in the Southeast US...the forcing
remains weak and nearly impossible to time with any accuracy at
this range but localized downpours from any convection that can
develop and was covered by an expansive Marginal Risk area.

...Upper Midwest to far Western Great Lakes...
The same cold front helping to focus heavy to potentially excessive
rainfall on Sunday/Sunday night will continue to make its way
eastward on Monday. The pattern becomes somewhat less favorable for
heavy to excessive rainfall but the potential for some isolated
enhanced amounts linger...especially from far northern Minnesota
into the western Great Lakes where a Marginal Risk was placed.

...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert)...meaning
a slight westward expansion in the Marginal risk area over
portions of Arizona and New Mexico compared with the previous
couple of days.


Bann
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FOUS30 KWBC 280827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sun Jul 28 2024

Day 1
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND THE OHIO/TENNESSEE VALLEYS...

...Northern Plains...
Heavy to potentially excessive rainfall is possible as a mid- and
upper system ejects northeastward from the Mid-Mississippi Valley
on Sunday into the western Great Lakes by early Monday morning. The
inherited Slight Risk was once again expanded towards the south and
east as moisture streaming northwards from the Mid-Mississippi
system interacts with the strengthening of mid-level deformation.
With model guidance showing 1 to 3 inches of rainfall...much of
which could be delivered in an hour or two...there is the potential
for flash flooding.

...Central Ohio/Tennessee Valley...
Mid-level height falls approaching from the west should help focus
and support showers and thunderstorms capable of producing some
downpours from the afternoon into the evening...mainly across the
western portion of the Ohio and Tennessee Valley. Northerly flow to
the east and ridging to the east should slow the eastward progress
of storms...at least initially. Overall...the GFS looks to be the
most progressive while the ECMWF was on the slowest side of the guidance...leaving the NAM as a compromise in terms of position and
its 1 to 2 inch amount as being reasonable given the amount of the
instability and moisture.

...Southwest...
The Marginal Risk in this area was extended east to once again
include the Sacramento Mountains of south-central New Mexico, given
the sensitivities of the soils in this area. Expect otherwise
typical afternoon convection in the monsoonal moisture to develop
once again, with several very sensitive areas into southern
Arizona and New Mexico.

Bann


Day 2
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO/TENNESSEE VALLEY TO SOUTHERN APPALACHIANS...

The Slight Risk across this area was shifted eastward...again...largely
due to guidance shifts in the axis of heaviest deterministic rainfall,
which now appears to be focused along much of the central and
southern Appalachians, but extending back across much middle and
upper Ohio Valley. Given areas where flash flood guidance has
lowered values around 1.5 inches per hour or 1.9 inches per 3 hours
due to due to recent rainfall...the risk of excessive rainfall
persists given the amount of moisture still in place.

Upper Midwest...
Rainfall should be diminishing in areal coverage and intensity as
deeper moisture and the better dynamics get shunted eastward.
Maintained a Marginal risk area here given some on-going rainfall
early that could overlap with flash flood guidance lowered by rains
on Sunday.

...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert). This
warrants keeping the previously issued Marginal risk along/near the
Arizona/New Mexico border.

Bann


Day 3
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...There is a Slight Risk of Excessive Rainfall Lingering Over
Portions of the eastern Ohio Valley to the Appalachians...

...Eastern US...
With lingering potential for rainfall over a region of lowered
Flash Flood Guidance in the eastern Ohio/Tennessee Valleys and
adjacent areas of the Appalachians...opted to maintain a Slight
Risk area in a similar position to that of the Slight Risk on
Monday. Model QPF is actually pretty modest...but the environment
remains capable of producing downpours on scales that are difficult
for models to handle this far out.

Surrounding the Slight Risk...kept a Marginal risk area in place
for areas to the north where the ECMWF and GFS show energy in the
northern stream making its way eastward across the eastern Great
Lakes. The model spread on timing this energy is large enough to
warrant a Marginal although an upgrade is certainly in the realm
of possibilities if the large scale pattern supports more
organization over a broader area...especially upstream.

...Southwest...
Steering flow in the mid and upper levels and persistent monsoonal
moisture will still be in place along the Arizona/New Mexico border
that begins to ease northward towards southern Colorado during the
period.

Bann
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FOUS30 KWBC 290759
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
359 AM EDT Mon Jul 29 2024

Day 1
Valid 12Z Mon Jul 29 2024 - 12Z Tue Jul 30 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...

West northwesterly mid to upper level flow will persist during the
upcoming day 1 period from the Northern Plains, east southeastward
through much of the Mississippi Valley, Ohio Valley, Southern to
Central Appalachians, Mid-Atlantic and Southeast. Embedded
shortwaves in this flow will support another day of active
convection across much of these areas where PW values will be
mostly above seasonal averages. A slight risk area was drawn where
the latest HREF neighborhood probabilities were the highest for 1,
2 and 3"+ totals and overlapped the lower FFG values and observed
precip axis from recent rains. This results in a slight risk from
southern Illinois, east-southeast through the Ohio Valley, Upper
Tennessee Valley into the Southern Appalachians. Changes from the
previous issuance were to elongate the previous slight risk farther
westward through the Lower Ohio Valley and southward into the
Southern Appalachians where heavy rains fell over the past 24
hours. The eastern portion of the slight risk over west central NC
was trimmed westward given relatively high FFG values and no precip
overlap potential from past 24 hours and the upcoming day 1
period.

...Florida...
Concentrated the marginal risk area across the southern portion of
the peninsula where model consensus is for heavy rainfall in the
high PW axis along and south of the stationary frontal
boundary/trof currently analyzed off of the FL east coast. HREF
neighborhood probabilities are high for 1, 2 and 3"+ amounts across
the southern half of the Peninsula, supporting isolated runoff
issues, especially in the more urbanized regions.

...Southeast Arizona into southwest New Mexico...
No significant changes made to the marginal risk area across
southeast Arizona into southwest New Mexico. Scattered convection
likely again associated with shortwave energy rotating northward on
the western side of the Southern Plains upper ridge and across
portions of the Southwest. There is the usual low confidence in qpf
details, but potential for isolated heavy totals and localized
runoff issues. The marginal risk area fits well with the axis of
the 1"+ HREF neighborhood probabilities.

Oravec


Day 2
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC INTO THE CENTRAL APPALACHIANS...

There is not expected to be any significant changes to the overall
large scale flow across the northern tier of the country from the
Upper Mississippi Valley, east-southeastward across the Great
Lakes, Ohio Valley, Central to Southern Appalachians into much of
the east coast. West northwesterly mid to upper level flow will
again persist across these regions with additional embedded
shortwaves once again supporting potential for large regions of
convective activity. The lead area of height fall expected to
support potentially organized convection across portions of the
Central Appalachians into the northern Mid-Atlantic in an axis of
PW values 1.5-2.5 standard deviations above the mean. The previous
slight risk was expanded northeastward into the northern Mid-
Atlantic to cover this heavy rain potential. The southern portion
of the previous slight risk area was extended farther south into
eastern TN/far western NC to cover the area of qpf overlap day 1
and day 2.

...Ohio Valley into the Upper Mississippi Valley...
To the west and northwest of the slight risk area, there is a lot
of model qpf spread through the Ohio Valley into the Upper
Mississippi Valley in the region where additional mid to upper
level vort will be tracking east southeastward in a continued axis
of above average PW values. To cover the model qpf spread, the
previous marginal risk area was expanded significantly to the
northwest into the Upper Mississippi Valley. At the moment, not a
lot of confidence in any one heavy model qpf, so the risk level was
kept at marginal across these areas.

...Southwest Florida...
The previous marginal risk area was decreased in size and
concentrated over Southwest Florida. This corresponds to where the
HREF neighborhood probabilities for the 12 hour period from 12Z Tue
to 00Z Wed are highest for 2 and 3"+ amounts. Isolated runoff
issues possible, especially over urbanized regions.

...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.

Oravec


Day 3
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST...

The progressive northern stream flow will continue into day 3 with
the next strong area of height falls expected to push eastward
across the Northern Plains and into the Upper Mississippi Valley.
There is potential for additional organized convection/heavy
rainfall to push eastward with these height falls across eastern
portions of the Northern Plains and into the Upper Mississippi
Valley. There are some typical latitude differences with the max
qpf axis. The day 3 slight risk area was drawn to favor the more
southern solutions of the EC, NAM and CMC. The previous slight risk
area over the Upper Mississippi Valley was suppressed more to the
south and southwest to better match the more southern solutions
with the next round of potentially organized convection.

...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 2, will progress across eastern
NY State into New England day 3. There is a typical amount of qpf
spread for the day 3 period, but general consensus for potential of
moderate to locally heavy rainfall amounts in a region of much
above average PW values ahead of these height falls. A broad
marginal risk area was maintained from the previous issuance to
capture the heavy rainfall spread in the models across this area
day 3.

...Southwest...
Similar to the days 1 and 2 period, the mid to upper level ridge
will remain centered over the Southern Plains. Scattered convection
again likely over portions of the Southwest as shortwave energy
again rotates northward on the western side of this ridge and into
the Southwest. Localized heavy rains and isolated runoff issues
again possible for southeast Arizona into southwest to central New
Mexico.

Oravec
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FOUS30 KWBC 300802
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Tue Jul 30 2024

Day 1
Valid 12Z Tue Jul 30 2024 - 12Z Wed Jul 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE MID MISSISSIPPI VALLEY, LOWER OHIO VALLEY, UPPER TENNESSEE
VALLEY INTO THE SOUTHERN APPALACHIANS...

...Mid Mississippi Valley, Lower Ohio Valley, Upper Tennessee
Valley and Southern Appalachians...

Another day of active convection likely in the broad west
northwesterly mid to upper level flow from the Mid to Upper
Mississippi Valley, into the Ohio Valley, Tennessee Valley and
Central to Southern Appalachians. Shortwaves in this flow will be
pushing through an area of above average PW values, 1 to 1.5+
standard deviations above the mean. While there is a lot of model
qpf detail spread, there is consensus for a northwest to southeast
oriented axis of potentially heavy rains across these areas. The
two previous slight risk areas were consolidated into a more
elongated slight risk area that corresponds well with the latest
HREF mean and the higher HREF neighborhood probabilities for 1 and
2"+ amounts and the axis of HREF EAS probabilities for 1"+ amounts.
There has been some overlap in the slight risk area of precip over
the past 48 hours and potential for additional heavy rains in this
axis.

...Central Appalachians, Mid-Atlantic into NY State...
Much above average PW values, 1 to 2.5+ standard deviations above
the mean also expected to push northeast from the Central
Appalachians, across the Mid-Atlantic and into NY State. There is
a lot of qpf spread in the latest model suite, but consensus for
potential for widespread moderate to locally heavy totals. HREF
neighborhood probabilities are high for 1"+ amounts, but drop off
significantly for 2"+ amounts. The HREF EAS for 2"+ amounts is
showing probabilities of less than 5%, illustrating poor overlap in
HREF heavy rain areas. A marginal risk is depicted across these
areas, with the previous issuance slight risk along the northeast
PA/Southern Tier of NY removed.

..Northern Rockies...

Only some small changes made to the marginal risk area across
portions of the Northern Rockies from northern Idaho into southwest
Montana. Latest guidance continues to show potential for an
organized area of moderate to locally heavy precip ahead of a fast
moving vort pushing east from the Pac NW into the Northern Rockies
in the 1200 UTC Tue to 0000 UTC Wed period. The marginal risk
corresponds to where the HREF neighborhood probabilities show
spotty high values in the 1200 UTC Tue to 0000 UTC Wed period.
The overall quick movement of the vort should keep precip amounts
from being very heavy, but anomalous PW values 2 to 2.5 standard
deviations above the mean will support some short term rainfall
rates of 1 to 1.5" per hour, supporting potential for isolated
runoff issues in areas of steep terrain and over burn scars.

...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.

...Southwest Florida...
No significant changes to the marginal risk area over Southwest
Florida. Above average PW values expected to persist across the
southern portion of the FL Peninsula day 1, along and south of the
east to west trof axis stretching from the Atlantic into South
Florida. The marginal risk continues to fit in well with the high
HREF neighborhood probabilities for 1, 2 and 3"+ totals. This
heavy rain potential will support isolated runoff issues,
especially in more urbanize regions.

Oravec


Day 2
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST AND OVER PORTIONS OF
NORTHERN NEW ENGLAND...

...Upper Mississippi Valley into the Midwest...

The progressive northern stream flow will continue into day 2 with
the next strong area of height falls expected to push eastward
across the Northern Plains and into the Upper Mississippi Valley.
There is potential for additional organized convection/heavy
rainfall to push eastward with these height falls across eastern
portions of the Northern Plains and into the Upper Mississippi
Valley as low level southerly flow strengthens into the west to
east oriented frontal boundary expected to stretch from the NE/SD
border, east across northern IA into northern IL. 850-700 mb moisture
flux anomalies into this front expected to rise to 2 to 3 standard
deviations above the mean in the 0000 to 1200 UTC Thursday period.
There are still a lot of model qpf spread with some typical
latitude differences with the max qpf axis. The day 2 slight risk
area was drawn to favor the more southern solutions of the NAM,
NAMNEST and CMC.

...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 1, will progress across eastern
NY State into New England day 2. There is a typical amount of qpf
spread for the day 2 period, but general consensus for potential
of moderate to locally heavy rainfall amounts in a region of much
above average PW values ahead of these height falls. A small slight
risk area was added along the VT/NH border where very heavy
rainfall amounts have occurred early Tuesday morning and where
several models show potential for additional heavy rains during day
2. Otherwise,a broad marginal risk area was maintained from the
previous issuance to capture the heavy rainfall spread in the
models across this area day 2.

...Southwest...
Similar to the day 1 period, the mid to upper level ridge will
remain centered over the Southern Plains. Scattered convection
again likely over portions of the Southwest as shortwave energy
again rotates northward on the western side of this ridge and into
the Southwest. Localized heavy rains and isolated runoff issues
again possible for southeast Arizona into southwest to central New
Mexico.

Oravec


Day 3
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, ACROSS THE LOWER GREAT LAKES, OHIO
VALLEY INTO THE CENTRAL APPALACHIANS AND ACROSS PORTIONS OF THE
SOUTHWEST AND SOUTHERN CALIFORNIA...

...Upper Mississippi Valley, Lower Lakes, Ohio Valley...
The strong height falls pushing through the Northern Plains into
the Upper Mississippi Valley day 2 will sink more to the southeast
into the Mid Mississippi Valley/Mid West region day 3. Additional
heavy rains likely ahead of these height falls and the associated
surface low in a region of above average PW values and anomalous
850-700 mb moisture flux. There are some large model qpf
difference's with respect to the axis of the heaviest rains. The
marginal risk was drawn to encompass this range form the northern
solutions of the UKMET/GEM from southern WI into the southern L.P.
of MI, to the southern solution of the NAM and EC from southern IL,
southern IN, southern OH, into northern KY. Given the spread and
low confidence at the moment with which qpf axes is the best, the
risk level was kept at marginal.

...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest during day 3. This will help to push the axis of above
average PW values farther west into southern to central California
and southern Nevada. This will support an expanding area of
scattered diurnal convection into portions of central and southern
CA/southern NV from days 1 and 2. The marginal risk area was also
expanded westward from the previous issuance to include western AZ,
southern NV into southern to south central CA.


Oravec
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FOUS30 KWBC 310828
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed Jul 31 2024

Day 1
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN
PLAINS TO THE MIDWEST AS WELL AS PORTIONS THE SOUTHWEST US AND NEW ENGLAND...

...NORTHERN PLAINS THE MID-WEST...
Convection should be on-going across portions of Iowa at the start
of the Day 1 period at 12Z...with some risk of excessive rainfall
lingering early on. Renewed convection is expected later in the day
as stronger forcing moves across the Plains later today. WPC
favored the more southerly deterministic QPF solution offered by
the HREF...and that was reflected in expanding the previously-
issued Slight Risk area a bit southward in portions of Minnesota
and Iowa. Farther south...a large anomaly of 850 to 700 mb
moisture flux remains in place. With the HREF depicting an axis of
1 to nearly 1.5 inches of rain possible falling over lowered flash
flood guidance values in southeast Kentucky...felt confident enough
to agree extending the Slight Risk south and east into a region
where the deterministic QPF from GFS/ECMWF/UKMET were meager.
There is still a lot of model qpf spread with some typical latitude
differences with the max qpf axis.

...Northeast US...
Height falls with a wave approaching from the west will be
spreading over portions of New York into New England during the day...accompanied by a risk of moderate to locally heavy rainfall
given the abundant moisture already in place. Maintained the Slight
Risk without change given its placement over an area soaked by 4
to 8 inch amounts within the past 24 hours...making the area
especially vulnerable to any additional rainfall and locally
significant flash flooding remains a possibility.

...Southwest US...
Maintained the Slight Risk across portions of Arizona that was
introduced on Tuesday. Models continue to show an uptick in both
moisture and instability compared with the past couple of days,
with satellite supporting the model indications of a weak wave
moving in from the southeast. This combination of increased forcing
and better thermodynamic environment should support greater
convective coverage with locally heavy rainfall rates. HREF
probabilities are elevated as well...with 2"+ neighborhood
probabilities of 40-60% and 1" EAS probabilities of 5-15"
indicating at least some ensemble agreement on location/coverage
of convection.

Bann


Day 2
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY...

The suite of numerical guidance from 31/00Z continued to support a
Slight risk area over portions of the Ohio Valley from Thursday
into Thursday night as fairly impressive forcing approaches the
region. Low level moisture should be place as the closed mid-level
low drops in from the north...helping to focus moisture flux
convergence along a quasi-stationary front in the area at the same
time that divergence aloft increases in response to the presence of
an upper level speed maximum. Given uncertainty of where the low
will be based on the model spread and where the strongest forcing
gets directed introduces plenty of uncertainty with regard to
placement of heavier rainfall. As stated earlier...the ingredients
still suggest a Slight risk area is warranted and the WPC placement
remains south of global QPF consensus...in fairly close proximity
to the UFVS machine learning ERO.

...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest during day 2...helping push the axis of above average
precipitable water values farther west into southern to central
California and southern Nevada. This will foster an environment in
which scattered diurnal convection will be expanding into portions
of central and southern California/southern Nevada.

Bann


Day 3
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO/TENNESSEE VALLEYS AND IN THE SOUTHWEST US...

...Ohio and Tennessee Valleys...
A closed low initially near the Great Lakes at the start of the Day
3 period will continue to sage south and eastward into the Great
Lakes region. Given plentiful moisture in place and increasing
cyclonic circulation aloft...localized downpours are in the realm
of possibility. One camp of thought in the models is that higher
amounts will be tied to the upper dynamics early on in the period
while the NAM seems to champion the idea of higher amounts along
the leading edge of the height falls closer to the better
instability. With the spread shown...went with a broad and
unfocused Marginal risk area for the time being.

...Southwest US...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
international border...another round of late day/evening showers
and thunderstorms is expected. Some northward/eastward expansion
compared with the previous couple of days is expected.

Bann
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FOUS30 KWBC 010710
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Thu Aug 1 2024

Day 1
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST AND OHIO VALLEY...

Midwest/Great Lakes/Southern Appalachians...
The suite of numerical guidance continued to support a Slight Risk area
over portions of the Ohio Valley from Thursday into Thursday night as
fairly impressive forcing approaches the region. Low-level moisture
should be place (precipitable water values up to 2") as the closed
mid- level low drops in from the north...helping to focus moisture
flux convergence along a quasi- stationary front in the area at
the same time that divergence aloft increases in response to the
presence of an upper level speed maximum. The low-level inflow and
effective bulk shear should be high enough in magnitude (25-30 kts)
to lead to convective organization. Hourly rain totals to 2.5", and
local amounts to 5", are possible. The ingredients still suggest
a Slight risk area is warranted. Changes to continuity were
minimal.


...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest. An easterly wave under its base moves south of CA, helping
push the axis of above average precipitable water values (as high
as 1.75-2") farther west into southern to central California and
southern Nevada. This will foster an environment in which scattered
diurnal convection will be expanding into portions of central and
southern California/southern Nevada. Hourly rain totals to 2.5" are
possible, given the above. This would be problematic in area burn
scars and dry washes/arroyos.

Roth/Bann


Day 2
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND
PORTIONS OF CENTRAL & EASTERN KENTUCKY...

....Ohio & Tennessee Valleys...
The upper pattern agrees on the approaching trough and focused
ascent over the Ohio Valley into the Mid Atlantic. Precipitable
water values up to 2" are advertised with this system. The low-
level inflow and effective bulk shear should be high enough in
magnitude (25-30 kts) to lead to convective organization. The
available ingredients suggest hourly rain totals to 2.5" and local
amounts to 5" would be possible. However, there is much spread in
scattered convective signals underneath the mean trough. The
environment is conducive for locally heavy rains anywhere from the
Central Midwest through the Central and Northern Mid Atlantic with
the primary likely falling under the center of the upper
circulation and mean trough. This pinpoints areas like Ohio and
Western PA down into WV/KY as the best chance for convective
coverage capable of more flash flooding capabilities. Much is
contingent on the pattern evolution and with time leading into the
setup, but the mass fields along with the GFS/NAM suggest that
portions of central and eastern KY should be most favored; added a
Slight Risk in this area to address that concern which was
coordinated with the JKL/Jackson KY and LMK/Louisville KY forecast
offices. The previous Marginal Risk east of the Mississippi was
expanded slightly, but remains fairly close to continuity.

...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to
1.75")...another round of late day/evening showers and
thunderstorms is expected. Continuity was generally maintained.
Hourly rain totals to 2" should be possible. Locally heavy
rainfall would be most problematic in any area burn scars and dry washes/arroyos.

Roth/Kleebauer/Bann


Day 3
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024

...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
SOUTHWEST, OREGON, MINNESOTA, FLORIDA, & IN AND NEAR THE MID-
ATLANTIC STATES...

...In and near the Mid-Atlantic States...
A generally wet and unsettled pattern is likely with ample
moisture (precipitable water values up to 2") and instability in
place as a slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. Inflow at 850 hPa of 25
kts implies some level of convective organization is expected.
Hourly rain totals up to 2.5" should be possible here, and local
amounts in the 2.5-5" range are advertised by the guidance, which
could easily fall within an hour or two. As there remains
uncertainty in placement of the higher amounts, and a decent
section of the Richmond/Washington D.C./Philadelphia megalopolis
has descended into drought, raising flash flood guidance values
fairly highly, believe the Marginal Risk continues best for the
time being. Urban areas are at the highest risk for flash flooding
in this region.

...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos. Expanded the area
westward to account for the signal seen in the 00z Canadian
Regional which implies a risk as far west of the Peninsular Ranges
of CA.

...Florida Peninsula...
Moisture and instability increase as a tropical disturbance moves
in the vicinity of the FL Peninsula. Precipitable water values rise
to 2.25"+, and plenty of instability resides offshore over the warm
waters. There has been a westward shift in the guidance over the
past 24 hours more into the Gulf of Mexico due to its slower than
anticipated (in model land) development, and the best indication as
to the center of any low- to mid-level vorticity has been near
Puerto Rico. With the potential for 3" an hour totals and a
possibly better organized system to enhance rainfall potential,
went ahead and broadened the existing Marginal Risk area to
encompass the FL Peninsula for the time being to account for the
uncertainty.

...Minnesota...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.

...Oregon...
Moisture advecting northward through the West gets drawn into the
leading edge of an upper level trough between southwest OR and
northeast WA. Both the GFS and NAM get the precipitable water
values up to 1-1.5", which would be most anomalous in terrain.
There is some reflection of the trough in the 850-700 hPa wind
pattern (the NAM appears more bullish on the low-level convergence
prospects). Considering the weakness of the upper level feature,
it shouldn't preclude daytime heating/diurnal insolation, so there
should be decent instability as well. The concern is highest in
the terrain, particularly near any burn scars. This led to a new
Marginal Risk area across portions of OR, which have the best QPF
signal, albeit not that high. Hourly rain totals to 2" are
considered possible.

Roth
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FOUS30 KWBC 020827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Fri Aug 2 2024

Day 1
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST, OHIO VALLEY, AND NORTHEAST, ALONG WITH THE EASTERN
TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...

...Eastern U.S....
Slowly advancing upper trough and surface front will focus broad-
scale ascent for scattered-numerous convective elements across
much of the eastern CONUS. Two areas of particular concern include
a W-E corridor from SE MI and the OH Valley eastward to NJ and
southern NY, where the deep-layer forcing (DPVA) ahead of the
opening mid-level low will be a bit more robust. The combination of
favorable deep-layer instability, especially during peak heating
hours (MLCAPEs 1000-2000+ J/Kg) along with TPW values of 1.75-2.00+
inches will make hourly rainfall rates of 1-2+ inches likely
underneath the strongest cells, with potentially higher hourly rates
(2-3") farther east toward eastern PA, NJ, and southern NY given
the closer proximity to the max TPW and low-level moisture
transport axes.

Farther south, the guidance is also favoring eastern portions of
the TN Valley into the southern Appalachians of northern GA-western
NC for more widespread convection and an enhanced risk of flash
flooding (i.e. Slight). This given the upslope low-level component
along with a more vigorous vort lobe traversing the area.
Elsewhere, any flash flooding is expected to be more
isolated/localized, thus the Marginal Risk areas.

...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to 1.75"),
another round of late day/evening showers and thunderstorms is
expected. Continuity was generally maintained, though we did expand
the Marginal Risk across the Sierra Nevada, Transverse Ranges, and
adjacent foothills. Hourly rain totals to 2" should be possible.
Locally heavy rainfall would be most problematic in any area burn
scars and dry washes/arroyos.

Hurley


Day 2
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTH FLORIDA ALONG WITH PARTS OF THE MID ATLANTIC AND SOUTHEAST
COASTAL PLAIN...

...Eastern U.S...
A generally wet and unsettled pattern is likely, with ample
moisture (precipitable water values up to 2" for most, 2.25-2.50
across FL along/ahead of the tropical disturbance) and instability
in place as a slow moving upper trough and surface/frontal systems
all support numerous concentrated areas of convection.

Across FL, per collaboration we have hoisted a Slight Risk across
parts of South FL given the uptick in QPF ahead of the tropical
disturbance. While most of the models and ensemble means keep the
max areal-average totals off the SW FL coast (3-5+ inches), TPWs
reaching 2.4-2.6 inches along with the possibility of more
destabilization east of the tropical distrubance's center will make
for the potential of very intense rainfall rates (2.5-3.0+ within
an hour) underneath the more isolated stronger cells.

Farther north into the Mid Atlantic, inflow at 850 hPa of 25 kts
implies some level of convective organization is expected. Hourly
rain totals up to 2.5" should be possible here, and local amounts
in the 2.5-5" range are advertised by the guidance, which could
easily fall within an hour or two. The guidance has come in better
alignment in advertising an axis of heavier rainfall across western
portions of the coastal plain and eastern Piedmont over the
Carolinas into parts of SE VA. This given the infusion of deep-layer
moisture surging north of the tropical disturbance toward the mid-
upper level trough and surface front. Per collaboration, have
included a Slight across this region as well.

Elsewhere to the north (north of Richmond VA), there remains
uncertainty in placement of the higher amounts, and a decent
section of the Washington D.C./Philadelphia megalopolis has
descended into drought, raising flash flood guidance values fairly
highly. Therefore still believe the Marginal Risk continues best
over these areas for the time being. Urban areas are at the
highest risk for flash flooding in this region.

...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos.

...Sierra Nevada into Oregon Cascades, Blue Mountains, and parts
of the Northern Rockies to western portions of the Great Basin...

Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations
above normal per the GEFS) will advect northward through the West
on the backside of the upper ridge and ahead of the shortwave
trough off the PAC NW coast. The guidance gets TPW values up to
1-1.25", which would be most anomalous in terrain. Given these
moisture profiles, along with the diurnal heating (MLCAPEs peaking
at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa
1" will make for localized runoff issues, especially over burn
scares.

...Upper Midwest...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.

Hurley


Day 3
Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN...

...Eastern U.S....
The Slight Risk across FL and the Southeast was expanded a bit,
based on the growing consensus in terms of the forecast track and
forward speed of the tropical disturbance. Areal-average QPF per
the smart blends/ensembles (including the NBM) is up to 3-5" over
parts of the FL peninsula. This given the highly favorable
environment (deep layer moisture, tall/skinny CAPE profiles) along
and ahead of the tropical disturbance. Per collaboration with the
WFOs, have expanded the Slight Risk area, but for now given
uncertainty by Day 3 in terms of the eventual track/speed/strength
of the potential tropical system, have held off in hoisting a
Moderate Risk.

...Montana into the Northern Plains and Upper Mississippi
Valley... Aforementioned axis of anomalous TPW out West during Day
2 will ultimately get absorbed in the upper level jet streak and
mid- level vort lobe that will cross MT and the Dakotas on Day 3.
Dynamical forcing along with the thermodynamical profile will favor
more widespread showers and thunderstorms, though the shear
profiles will certainly favor more progressive/forward-propagating
cell motions and thus make for more of an isolated flash flood
threat. Areas across northwest MT, including Glacier NP, would be
the most susceptible given the lower FFG (1" or less within 3 hours
for most areas). An upgrade to Slight may be needed over this
region in later outlooks, especially if we start seeing a consensus
of heavier QPF from the models.

Hurley
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FOUS30 KWBC 090758
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Fri Aug 9 2024

Day 1
Valid 12Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC AND NORTHEAST...

...Mid-Atlantic to Northeast...

Debby and it's remnant plume of tropical moisture are forecast to
lift northward today from the central Appalachians to the Interior
Northeast, while producing a swath of heavy rainfall and the
potential for numerous flash floods from near the West Virginia
Panhandle early this morning to Upstate New York this afternoon.
Precipitable water values of 2-2.5" stream along and just east of
the system will equate to above the 95th climatological percentile
and +3.0 standard deviations above the climatological mean.
Additionally, ample shear surrounding the low center and MUCAPE
values up to 1000 J/kg nearby along the Atlantic Seaboard being
advected northward will allow for efficient tropical downpours
within scattered to numerous showers and thunderstorms. A focus in
the heaviest rainfall is likely on the northern and northeastern
quadrant of the low pressure system as it transitions to an
extratropical phase, becomes frontal, and begins to increase
forward speed to the northeast by Friday night. A warm front
lifting northward will add increasing frontogenetic forcing to the
environment and lead to a more widespread precipitation shield
today across parts of PA and Upstate NY, with embedded rainfall
rates likely up to 2-3"/hr at times. The trend for this axis of
heavy rainfall has been for it to occur slightly westward than
previously forecast, now across far northwest NY. Additionally, the
trend is for faster northward progression with this system. 12z
NAEFS IVT valid 18z Fri is forecast to max all values in the
1979-2009 climo period, amplifying the moisture surge fueling
today's heavy rainfall event. 00z HREF neighborhood probs for at
least 3" in 6-hrs are high (60-80%) from central PA to the Finger
Lakes region and northwest portions of NY State, which would also
exceed local FFG for much of this region. As a result, the Moderate
Risk was maintained from the previous forecast with only minor
cosmetic changes. Numerous instances of flash flooding are expected
in this area, with locally significant flash flooding possible.
Elsewhere in the Northeast, convection developing along and ahead
of a cold front should have enough forward motion to lead to only
scattered to localized flash flooding concerns closer to the I-95
corridor, but with rainfall rates of 2"/hr possible. Areas that
received recent rainfall and poor drainage locations could have a
relatively greater flash flood risk, which supports the SLGT Risk
extending to parts of New England and the Mid-Atlantic.

...Eastern Carolinas...

A separate area along the East Coast at risk for flash flooding
today will be associated with a convergence axis along
coastal/eastern sections of North and South Carolina this afternoon
and evening. PWs elevated and above 2.25", along with ample
instability and mean winds parallel to the boundary/coastline will
support slow-moving and/or training thunderstorms across saturated
terrain. CAMs and the associated 00z HREF suite depict the
potential for 5"+ amounts throughout eastern North Carolina and
coastal South Carolina, but the expectation remains that these
totals would be pretty localized. The SLGT Risk was expanded from
the previous forecast across South Carolina given recent CAM output
and very saturated antecedent conditions in place. Will monitor
this region for upgrades in the future outlooks.

...Four Corners, Central Rockies, Central High Plains, and Southern
Plains...

Showers and thunderstorms are expected to fire up across the
Southwest and Rockies as shortwave energy passes through and
encounters the ongoing monsoonal moisture in place. There will be
increased focus for more organization and higher amounts
concentrating near the higher terrain of southern Utah, south/central
Colorado, and northern New Mexico, where a SLGT Risk area remains
in effect. Accumulations of 1-2" will be possible in the strongest
cells, a stronger correlation to flash flooding concerns compared
to historical precedence. The rest of the Southwest will still see
periods of storms capable of localized flash flood concerns with
the best threat located more into the Intermountain West and the
Mogollon Rim in Arizona. Convection (possibly organized) will also
spread eastward into western Kansas and the panhandles of Oklahoma
and Texas, as well as northeast NM. Here, 00z HREF neighborhood
probs are modest (20-40%) for at least 3" in 6-hrs. A broad
MRGL Risk covers much of the High Plains.

...Central Texas...

A MRGL Risk was introduced across parts of central Texas as
convection develops along the tail-end of a lingering cold front
and ahead of a stronger frontal boundary dropping south from the
central Plains. Shear will be weak, but steep low-level lapse rates
and sufficient moisture (PWs around 2") will allow for intense
rainfall rates over relatively wet ground conditions and a
historically flash flood prone region. 00z HREF neighborhood
probs have increased and now show elevated chances (30-50%) for
rainfall amounts exceeding 3" this evening.

Snell


Day 2
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS, AND THE COASTAL CAROLINAS...

...Southwest, Central Rockies, and Plains...

A rather broad MRGL Risk was maintained extending from the
Southwest to the central/southern Plains as diurnal heating and
monsoonal moisture will keep showers and thunderstorms developing
during the afternoon and evening hours for much of the Four Corners
region. Meanwhile, an area of more organized convection triggered
by shortwave energy from the D1 period will be making its way
eastward into the foothills and spreading out into the high
Central/Southern Plains. There is a fair amount of uncertainty with
how far east into the Plains the convection reaches, location of
heaviest amounts and the overall accumulations. Most of the
consensus is suggesting 1-3 inches however a couple models have
maxes near/over 4 inches. A Marginal Risk covers much of the Four
Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma,
northern Texas, southwest Missouri and northwest Arkansas. If
amounts trend up then a Slight Risk area may be need with further
updates.

...Coastal Carolinas...

A lingering frontal boundary and diurnal sea breeze convection will
continue chances for scattered thunderstorms across the Southeast
and more specifically the eastern Carolinas on Saturday. Soils
should remain saturated after prior heavy rainfall, increasing the
flash flood threat across an area that isn't usually as prone to
flash flooding. CAMs depict localized totals from pulse
thunderstorms could reach 1-3", which is supported by PWs over 2".

Snell


Day 3
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, FOUR CORNERS, CENTRAL ROCKIES INTO THE PLAINS, AND
THE EASTERN CAROLINAS...

...Four Corners, Southwest, and Central Rockies...

A deepening western U.S. trough by D3 will allow for another round
of diurnal monsoonal convection across the Southwest and Four
Corners region, with GEFS and ECENS depicting PWs above the 75th
climatological percentile. Opted to maintain the MRGL Risk for now
given the limited QPF signal in available guidance and the
possibility for fast storm motions.

...Plains and Mid-Mississippi Valley...

Northwest flow aloft and a gradually lifting warm front over the
Southern Plains will support MCS potential into the overnight
period on D3 along with increasing mid-level flow, which could
produce training and intense rainfall rates within convection.
Relatively high uncertainty remains with the exact placement of
heaviest rainfall (greatest potential currently near the western
Ozarks into northeast OK and southeast KS), which prompted the MRGL
Risk as opposed to a SLGT for now. Additional convection is
possible over SD and NE, with similar uncertainty in placement.

...Eastern Carolinas...

Another day of diurnal convection along a nearby frontal boundary
could be enhanced by an approaching shortwave diving over the Ohio
Valley around a deep upper-level low over SE Canada. Similar to D2,
rainfall amounts of 1-3" appear possible and are likely to occur
over saturated soils.

Snell
---
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FOUS30 KWBC 091546
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Fri Aug 9 2024

Day 1
Valid 16Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024

...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC AND NORTHEAST...

...16Z Update...

Debby's remnants continue to press rapidly to the north as the
remaining core analyzed at 15z is rushing northward at just over 30
kts (35 mph). Bands of heavier rainfall continue to push north out
of the Central Mid Atlantic with the heaviest rain focused near the
remnant center of now Post-Tropical Cyclone Debby. The current
forecast remains on track with little variance in the QPF and
associated probabilities as rainfall rates of 1-2"/hr will be seen
within the heaviest cores as the storm drives northward out of
Northern New England by late this evening. The previous MDT risk
was trimmed on the southern edge of the previous forecast to
reflect the radar trends of the heaviest precip shifting north with
more of a focus across Northern PA through Upstate NY with the max
for the period likely situated across the North Country of NY
state. A trailing cold front will allow for pockets of heavy rain
to develop this afternoon across portions of the Central Mid
Atlantic, but will quickly vacate eastward as low-level convergence
along the front will sweep east with a degrading threat of heavy
precip in wake of the boundary. Some of the SLGT risk across the
interior Central Mid Atlantic was also adjusted given the
environment improving with much of the heavier precip likely
falling along and east of the Blue Ridge.

The Eastern Carolinas exhibited very little in the way of variance
from the previous forecast issuance with the afternoon and evening
time frame remaining the period of interest with the convective
development anticipated. Hi-res was in strong agreement on the
placement of a line of convection extending from coastal SC up
through Eastern NC with much of the heavier cores likely to fall
east of the I-95 corridor. Embedded heavy thunderstorms within the
main convective axis will likely produce a few cells that can reach
upwards of 5" with a modest signature on the 12z HREF neighborhood
probability for the threshold settling between 30-50% over a large
area encompassing most of Eastern NC. HREF EAS signals for at least
2" is relatively high as well (20-30%) over the same areas, so the
threat is likely on the higher-end of the SLGT risk threshold,
especially given the antecedent saturated grounds after a multi-day
onslaught from Debby. This led to an easy decision to keep
continuity with locally significant wording reserved for any
urbanized areas within the above region.

Finally, out west the pattern remains on track with scattered
thunderstorm development during peak diurnal max with a complex
initiating over the High Plains of Southeast CO and Northeast NM,
shifting east with the mean flow focused along the northern
periphery of the mid-level ridge. Some pretty good signals for at
least 2" given a solid EAS probability between 30-50% over a small
zone between the NM/OK/TX borders. Neighborhood probability was
bullish for the >3" signal running between 60-70% over the
aforementioned area. There's little signal for >5", however so the
threat is likely capped between 3-5" at peak QPF. That's still
fairly prevalent to a localized flash flood concern within that
portion of the Southern High Plains, so the risk is likely within
the higher-end of the SLGT comparatively to what will occur over
NM. Much of the impact will be focused in those burn scar areas
where heightened sensitivity drives much of the risk. This was
sufficient to maintain what was forecast previously with only some
minor adjustments based on the latest HREF mean QPF footprint.

Kleebauer

...Previous Forecast...

...Mid-Atlantic to Northeast...

Debby and it's remnant plume of tropical moisture are forecast to
lift northward today from the central Appalachians to the Interior
Northeast, while producing a swath of heavy rainfall and the
potential for numerous flash floods from near the West Virginia
Panhandle early this morning to Upstate New York this afternoon.
Precipitable water values of 2-2.5" stream along and just east of
the system will equate to above the 95th climatological percentile
and +3.0 standard deviations above the climatological mean.
Additionally, ample shear surrounding the low center and MUCAPE
values up to 1000 J/kg nearby along the Atlantic Seaboard being
advected northward will allow for efficient tropical downpours
within scattered to numerous showers and thunderstorms. A focus in
the heaviest rainfall is likely on the northern and northeastern
quadrant of the low pressure system as it transitions to an
extratropical phase, becomes frontal, and begins to increase
forward speed to the northeast by Friday night. A warm front
lifting northward will add increasing frontogenetic forcing to the
environment and lead to a more widespread precipitation shield
today across parts of PA and Upstate NY, with embedded rainfall
rates likely up to 2-3"/hr at times. The trend for this axis of
heavy rainfall has been for it to occur slightly westward than
previously forecast, now across far northwest NY. Additionally, the
trend is for faster northward progression with this system. 12z
NAEFS IVT valid 18z Fri is forecast to max all values in the
1979-2009 climo period, amplifying the moisture surge fueling
today's heavy rainfall event. 00z HREF neighborhood probs for at
least 3" in 6-hrs are high (60-80%) from central PA to the Finger
Lakes region and northwest portions of NY State, which would also
exceed local FFG for much of this region. As a result, the Moderate
Risk was maintained from the previous forecast with only minor
cosmetic changes. Numerous instances of flash flooding are expected
in this area, with locally significant flash flooding possible.
Elsewhere in the Northeast, convection developing along and ahead
of a cold front should have enough forward motion to lead to only
scattered to localized flash flooding concerns closer to the I-95
corridor, but with rainfall rates of 2"/hr possible. Areas that
received recent rainfall and poor drainage locations could have a
relatively greater flash flood risk, which supports the SLGT Risk
extending to parts of New England and the Mid-Atlantic.

...Eastern Carolinas...

A separate area along the East Coast at risk for flash flooding
today will be associated with a convergence axis along
coastal/eastern sections of North and South Carolina this afternoon
and evening. PWs elevated and above 2.25", along with ample
instability and mean winds parallel to the boundary/coastline will
support slow-moving and/or training thunderstorms across saturated
terrain. CAMs and the associated 00z HREF suite depict the
potential for 5"+ amounts throughout eastern North Carolina and
coastal South Carolina, but the expectation remains that these
totals would be pretty localized. The SLGT Risk was expanded from
the previous forecast across South Carolina given recent CAM output
and very saturated antecedent conditions in place. Will monitor
this region for upgrades in the future outlooks.

...Four Corners, Central Rockies, Central High Plains, and Southern Plains...

Showers and thunderstorms are expected to fire up across the
Southwest and Rockies as shortwave energy passes through and
encounters the ongoing monsoonal moisture in place. There will be
increased focus for more organization and higher amounts
concentrating near the higher terrain of southern Utah, south/central
Colorado, and northern New Mexico, where a SLGT Risk area remains
in effect. Accumulations of 1-2" will be possible in the strongest
cells, a stronger correlation to flash flooding concerns compared
to historical precedence. The rest of the Southwest will still see
periods of storms capable of localized flash flood concerns with
the best threat located more into the Intermountain West and the
Mogollon Rim in Arizona. Convection (possibly organized) will also
spread eastward into western Kansas and the panhandles of Oklahoma
and Texas, as well as northeast NM. Here, 00z HREF neighborhood
probs are modest (20-40%) for at least 3" in 6-hrs. A broad
MRGL Risk covers much of the High Plains.

...Central Texas...

A MRGL Risk was introduced across parts of central Texas as
convection develops along the tail-end of a lingering cold front
and ahead of a stronger frontal boundary dropping south from the
central Plains. Shear will be weak, but steep low-level lapse rates
and sufficient moisture (PWs around 2") will allow for intense
rainfall rates over relatively wet ground conditions and a
historically flash flood prone region. 00z HREF neighborhood
probs have increased and now show elevated chances (30-50%) for
rainfall amounts exceeding 3" this evening.

Snell


Day 2
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS, AND THE COASTAL CAROLINAS...

...Southwest, Central Rockies, and Plains...

A rather broad MRGL Risk was maintained extending from the
Southwest to the central/southern Plains as diurnal heating and
monsoonal moisture will keep showers and thunderstorms developing
during the afternoon and evening hours for much of the Four Corners
region. Meanwhile, an area of more organized convection triggered
by shortwave energy from the D1 period will be making its way
eastward into the foothills and spreading out into the high
Central/Southern Plains. There is a fair amount of uncertainty with
how far east into the Plains the convection reaches, location of
heaviest amounts and the overall accumulations. Most of the
consensus is suggesting 1-3 inches however a couple models have
maxes near/over 4 inches. A Marginal Risk covers much of the Four
Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma,
northern Texas, southwest Missouri and northwest Arkansas. If
amounts trend up then a Slight Risk area may be need with further
updates.

...Coastal Carolinas...

A lingering frontal boundary and diurnal sea breeze convection will
continue chances for scattered thunderstorms across the Southeast
and more specifically the eastern Carolinas on Saturday. Soils
should remain saturated after prior heavy rainfall, increasing the
flash flood threat across an area that isn't usually as prone to
flash flooding. CAMs depict localized totals from pulse
thunderstorms could reach 1-3", which is supported by PWs over 2".

Snell
---
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FOUS30 KWBC 100746
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sat Aug 10 2024

Day 1
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CAROLINAS INTO SOUTHEASTERN VIRGINIA...

...Carolinas & Virginia...

A stalled out front over the western Carolinas and central Virginia
will act as a focus for storms to develop this afternoon on the
warm/moist southeastern side of the front. The atmosphere will be
characterized with plentiful deep moisture with PWATs over 2 inches
across much of the region. HRRR guidance shows several clusters of
storms forming across southeast Virginia and the central Carolinas
late this afternoon. The storms will then track southeastward
towards the coast into an area where instability could exceed 4,000
J/kg along the coast. This level of instability and moisture will
support potential for storms to produce rainfall rates to 3 inches
per hour. Fortunately the guidance suggests the storms will keep
moving into the evening, but those heavy rainfall rates would have
the potential to cause flash flooding even if antecedent conditions
weren't so favorable.

Debby's remnants tracking across this region over the last few days
has led to very saturated soils all across Virginia and the
Carolinas. With numerous streams, creeks, and rivers already well
into flood stage, additional rainfall from storms Saturday
afternoon will struggle to drain, resulting in additional flooding
and prolonging ongoing flooding. In coordination with all of the
impacted offices, a Slight Risk area was introduced with this
update.

00Z HREF guidance peaked at over 50% chance of exceeding 6 hour
FFGs in the Slight Risk area, with an over 70% chance of exceeding
3 inches of rain across much of eastern North Carolina. These
values added to the confidence to upgrade the ERO risk.

...Rockies into the Plains...

An approaching positively tilted shortwave trough moving into
California will increase lift to its east across the Four Corners
region. Storms are likely to be widely scattered across this area,
which would support an isolated flash flooding instance or 2,
currently covered with the large Marginal Risk. Additional moisture
in southern Arizona and lift into the Peninsula Ranges of southern
California may cause local flash flooding issues, but the guidance
is inconclusive at best as to where any potential Slight risk area
could be drawn. In coordination with the impacted offices, have
opted to hold at a Marginal for now, but will reevaluate each day's
chances for flooding based on the previous day's convective
behavior.

MCS development with potential repeating storms in the Plains will
be a concern across Kansas through Oklahoma late Saturday night.
Antecedent conditions are quite dry for soil moisture across this
area, and the CAMs are not in great agreement on the location and
behavior of the storms, despite ample moisture available. Here too
a Slight Risk is close, and may need to be considered with future
updates.

Wegman


Day 2
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS
AND FOR PORTIONS OF THE OZARKS AND CENTRAL PLAINS...

...Carolinas...

The stalled out front over the Carolinas will continue to draw deep
tropical moisture northward up the Southeast coast. The front will
have made enough progress to likely keep the rainfall down into
southeastern Virginia, but there will likely be more storms into
South Carolina. Thus, the Slight risk is just a bit south of the
Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from
Debby, additional rainfall, even if more widely scattered, will
have an outsized potential for impacts from additional flooding.
00Z HREF guidance through 00Z Monday suggests additional elevated
potential for exceeding FFGs in NC, and plentiful moisture and
instability with a stalled out front providing the forcing will
likely help organize the storms, increasing their flooding
potential.

...Central Plains/Ozarks...

MCS development on the leading edge of a low level jet (LLJ) is
expected from southeast Kansas through northwest Arkansas. Despite
recent dry weather, training convection with PWATs increasing to
around 2 inches will support storms capable of rates to 3 inches
per hour at times. This very heavy rainfall combined with terrain
interactions in the Ozarks will favor widely scattered instances of
flash flooding. In coordination with the impacted offices, a
Slight Risk upgrade was introduced with this update. Given CAMs
frequent trouble with correctly simulating storms with MCSs, there
are likely to be both temporal and intensity adjustments with the
expected rainfall. Despite this potential, there was good enough
agreement in the guidance to draw in the Slight risk, which for now
remains low-end given the recent dry weather and dry soils.

...Southwest...

More widespread convection is likely to impact the Four Corners
region Sunday afternoon. The shortwave that will start the day in
northern California will continue eastward across the Intermountain
West. Thus, it appears the best potential for storms capable of
flash flooding will be across central UT and western CO, generally
north of the UT National Parks. Nonetheless with better moisture
further south, AZ and northwestern NM will not be out of the woods,
and isolated flash flooding will remain possible there. Storms are
also likely along the Peninsular Ranges of southern California
again Sunday afternoon, where a small Marginal risk was introduced.

Wegman


Day 3
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS...

Ongoing rainfall from Sunday night may continue into Monday morning
across southern Missouri. With only a few hours of rain in the
morning, the Marginal across Missouri was maintained. However,
should rainfall from Day 2/Sunday overperform, then a future Slight
may be needed. Once again another round of storms Monday afternoon
across the Four Corners region is likely to result in isolated
flash flooding. However, there is greater uncertainty with
coverage by Monday in this region with lesser forcing than on
Sunday.

For the Carolinas, lingering moisture with the same front may cause
yet another round of storms Monday afternoon into the evening. With
greater potential that the bulk of the activity will be offshore by
Monday afternoon, the previously inherited Marginal risk remains
largely unchanged.

Wegman
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FOUS30 KWBC 110815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun Aug 11 2024

Day 1
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR CENTRAL UTAH
THROUGH NORTHWESTERN ARIZONA, PORTIONS OF THE OZARKS AND CENTRAL
PLAINS, AND FOR THE COASTAL CAROLINAS...

...Central Utah through Northwestern Arizona...

Monsoonal moisture will continue to draw northward across the Four
Corners region today. The jet stream and an upper level jet streak
will be set up across northern Utah today. PWATs increasing above
0.75 inches will boost portions of southern Utah to 2.5 sigma above
normal. With moisture and forcing in place, expect a renewed round
of afternoon convection across this area. With previous days'
storms having soaked the soils in some areas, the flash flooding
threat will be higher today as compared to previous days across
Utah and portions of northwestern Arizona. The Slight risk area was
expanded southward with this update to include much of the Grand
Canyon and points north, including Glen Canyon.

...Portions of the Ozarks and Central Plains...

Convection currently ongoing across central Oklahoma will continue southeastward into southeast Oklahoma and Missouri this morning.
This MCS will likely dissipate as often happens late this morning
with daytime heating, but the primary event is expected tonight,
and perhaps in the form of 2 separate areas. Gulf moisture racing
north with the LLJ will interact both with an upper level
shortwave, a stalled front over the area, and countering flow from
an area of high pressure over the Midwest helping hold the surface
front in place despite the deep moisture moving in from the
southwest on the other side of the front. Training convection is
likely to develop, especially over Oklahoma where the Slight risk
area remains in place with few changes. 00Z HREF neighborhood probabilities remain over 50% for 3 inches of rain in the 24 hrs ending at 12Z,
but much of that rain is likely during the overnight hours.

...Coastal Carolinas...

Guidance has been gradually shifting south and east/closer to the
coast with the axis of the heaviest rain with the afternoon
convection expected tonight. Thus, the Slight and surrounding
Marginal risk areas have been shrunk to the southeast accordingly.
Nonetheless antecedent conditions along the coast remain very
favorable in the recovery post-Debby. Expect similar coverage of
flash flood warnings as today's storms, but are likely to occur
further east. Given the uncertainty as to where the storms will
form, the Marginal risk still extends into central NC and western
SC for the potential for more widely scattered storms, as well as
across eastern Georgia.

Wegman


Day 2
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE FOUR
CORNERS REGION EAST TO THE MID-MISSISSIPPI VALLEY AS WELL AS FOR
PORTIONS OF THE COASTAL CAROLINAS...

...Four Corners Region to the Mid-Mississippi Valley...

Concern continues to increase with potential heavy rain and
subsequent flash flooding in various areas. For the Four Corners,
no major changes are expected, but with the greatest moisture
having shifted east, convective coverage should be lesser on Monday
as compared to today. Meanwhile, the signal for heavy rain remains
strong from eastern Colorado into western Kansas. Easterly flow
north of an ejecting low will draw plentiful Gulf moisture into the
High Plains of Colorado. Meanwhile, a stalled front to the north
will supply lower level forcing. The result will be an area of
heavy rain caused by training storms moving east along the front,
as moisture flowing northwestward on the warm side of the front
supports backbuilding. There remains some uncertainty both with the
placement of the heaviest rains, and for Kansas, the development of
drought conditions potentially resulting in less flooding. However,
it appears likely a Slight will be needed as plentiful moisture
will support storms capable of rates to 2 inches per hour, which in
areas of training storms should overcome otherwise high FFGs.

Further east into Missouri, lingering storms from tonight will
continue into Monday morning. Then, the stalled out front over the
area will begin to reorient, but pivoting over Missouri. This
should reduce the overall coverage of rainfall over Missouri for
the day, but will keep the area in the crosshairs for heavy
rainfall going forward.

...Coastal Carolinas...

Some lingering storms are possible along the coast Monday afternoon as
the front over the area dissipates. With much reduced storm
coverage, the Marginal remains over the area for the potential for
isolated instances of flash flooding. This would likely be the
result of sea breeze interactions.

Wegman


Day 3
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE FOUR
CORNERS REGION TO THE MID-MISSISSIPPI VALLEY, AS WELL AS FOR
EASTERN NORTH CAROLINA...

...Missouri...

A renewed push of Gulf moisture with the LLJ will impact a
persistent front across Missouri today. This will likely result in
an area of training thunderstorms capable of localized flash
flooding. The area is likely to be upgraded to a Slight with future
updates and higher confidence, but given the counter flow out of
the northeast on the dry side of the front over the Midwest, it's
likely that training storms are likely to impact a narrow corridor
over Missouri along the front. There is potential that the St.
Louis metro may be part of the heavy rain corridor.

...Eastern North Carolina...

A bit stronger moisture push off the Atlantic may result in more
widespread showers and storms a bit further inland into North
Carolina. With little to organize the convection however, the flash
flooding threat remains isolated.

Wegman
---
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FOUS30 KWBC 120833
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 12 2024

Day 1
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN COLORADO AND WESTERN KANSAS, EASTERN OKLAHOMA AND FAR
WESTERN ARKANSAS, AND THE COASTAL CAROLINAS...

...Northeast OK and Western AR...
Training convection is expected to be ongoing across this corridor
at 12z Monday. As of 07z the coverage and intensity of convection
over northeast OK is generally greater than most of the 00z high
res guidance. With the low level jet only intensifying, expect this
trend to continue, and thus would expect the convective training
threat into Monday morning to be more significant than the 00z high
res guidance would suggest. At least scattered flash flooding is
anticipated across portions of eastern OK, possibly pushing into
portions of far western AR as well. This convection should be on a
downward trend by late morning into the early afternoon hours.

...Eastern CO into KS...
Convection is expected to develop over portions of eastern CO by
later this afternoon. Models indicate that a weak shortwave will
eject eastward originating from the ongoing convective activity
over UT...with an upper jet to the north supporting at least weak
divergence aloft in its right entrance region. Meanwhile, easterly
flow in the low levels will advect in higher moisture, with PWs
forecast to increase towards 1.5", above the climatological 90th
percentile for mid August. Given the forcing and moisture in
place, the forecast ~2000 j/kg will allow for robust convective
development. With deep layer mean flow from the west, and low
level flow from the east, cells may initially end up slow moving,
with cell mergers a possibility. Given the environment in place,
tend to think some of more aggressive high res models may end up
closer to reality for this event. This would support localized
rainfall upwards of 3-5" over portions of eastern CO and far
western KS. HREF QPF, environmental ingredients in place, and the
CSU machine learning ERO are all supportive of an isolated to
scattered flash flood threat. Thus we will go ahead and upgrade
this region to a Slight risk.

This activity is likely to grow upscale overnight and then move
eastward across KS. Convection will likely be quicker moving by
this time, although can not rule out a west to east training axis
setting up. If this were to occur then Slight risk level impacts
may continue overnight across KS. However given some lingering
uncertainty on these convective details, and the fact that soil
saturation and streamflow conditions across KS are running below
average...will keep the risk level at Marginal for now.

...Carolinas...
Upgraded portions of the eastern Carolinas to a Slight risk for
today. We should see another round of convection near the stalled
front this afternoon, with HREF 3" neighborhood probabilities in
the 40-70% range. Totals of this magnitude will likely stay
isolated to scattered in nature...however given the increased
sensitivity over the area after Debby and post Debby
rainfall...expect another day of isolated to scattered flash
flooding. Today is a bit trickier of a forecast with a weak low
along the front. While the higher probabilities of excessive
rainfall are along the coast this afternoon, we could see at least
a localized flash flood risk a bit further inland this morning
into the early afternoon as well.

...Southwest...
A broad Marginal risk remains across much of the Southwest, with
isolated flash flooding possible over this large geographic region.
The coverage/organization of intense rainfall may be a bit less
than what we saw on Sunday, but nonetheless an isolated risk
likely continues. Do see some more elevated probabilities over
southern AZ, so may end up with a bit better concentration of
convection there, but still seems to fall shy of Slight risk
levels. Portions of central/northern UT also stand out as
potentially seeing greater convective coverage...but this signal is
generally north of the more sensitive basins in UT...and thus not
thinking a Slight risk is needed at this time.

Chenard


Day 2
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS INTO MISSOURI AND NORTHERN ARKANSAS...

...Portions of KS/MO/AR...
Likely to have some ongoing convection at 12z Tuesday across
portions of southeast KS and southwest MO. This will be what
remains of the Monday night organized convection that is expected
to move across KS. Most indications are that this activity should
be on a weakening trend after 12z, so while an isolated flash flood
threat may persist, not currently anticipating a widespread
threat. The greater threat is the likelihood of training
convection Tuesday night across this corridor.

This appears to be evolving into a potentially significant training
convective setup. Robust instability will be advecting in from the
southwest, likely resulting in a tight instability gradient
somewhere in the vicinity of eastern KS into western to central MO.
Meanwhile the low level jet will increase, resulting in strong and
persistent moisture transport into the region. Corfidi vectors end
up becoming quite weak, and what movement they do indicate is
generally parallel to the expected moisture convergence axis and
instability gradient. Thus we very well may end up with repetitive
backbuilding upscale convective growth over this area Tuesday
night. There remains some uncertainty on the exact details of
convective evolution and placement of highest totals...but
confidence is increasing in an excessive rainfall threat, with
potentially significant localized impacts. The coverage of excessive
rainfall will likely be quite a bit narrower than the Slight risk area...however it remains broader to account for uncertainty in the
maximum rainfall axis.

...Southwest...
Scattered convection is again likely from portions of AZ/NM
northward into ID/WY. There will be a bit more in the way of
westerly flow compared to earlier days, which may allow cells to
move along at a faster pace. This should keep the flash flood risk
isolated in nature...however heavy rainfall rates will again be
probable in the more intense cells...which could result in a
localized flash flood within the broad Marginal risk area.

...Southeast...
Convection is again likely Tuesday across portions of GA/SC/NC.
Overall it is an interesting setup, with model guidance suggesting
a more well defined low over SC by this time, with stronger
forcing and flow aloft moving in as well. This should seemingly
allow for some strong convective development...with the stronger
flow aloft supporting quicker cell motions off to the southeast.
These cell motions would typically suggest a lowered flash flood
risk...however we will need to keep an eye on how the convergence
axis east of the low plays out. This boundary could act as a focus
for some training convection, which would offset the quicker cell
motions and still produce a flash flood risk. Overall think enough
uncertainty on these details remains to keep the risk level at
Marginal for now. However will continue to monitor trends and
would not rule out an eventual Slight risk upgrade over portions of
the area.

Chenard


Day 3
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY...

The threat of excessive rainfall shifts into Plains and MS Valley
on Wednesday. Likely to have some ongoing risk to start the day
across portions of central MO...with the threat likely shifting to
the north by late in the day into the overnight hours. It again
looks like a favorable environment for training/backbuilding
convection with an increasing low level jet into a warm front.
Strong and persistent moisture transport into this boundary is
typically supportive of backbuilding convection and a flash flood
risk.

There is some model QPF spread, with convection and locally heavy
rainfall likely all the way from the Dakotas and MN into MO.
However it looks like the best ingredients for excessive rainfall
are over MO to start the day, and then across portions of IA and
western IL later in the day/overnight. This is where the better
instability and most persistent moisture transport is currently
forecast. And while the location of greatest QPF differs from model
to model and run to run...this axis is favored by the majority of
solutions. Thus will place the Slight risk here for now, with the
understanding that some adjustments are likely as the event nears.
So while confidence on the exact location is only average...do
think we are getting towards above average confidence in the
impacts from this event...with at least isolated to scattered flash
flooding expected, some of which could be locally significant.


Chenard
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FOUS30 KWBC 130825
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Tue Aug 13 2024

Day 1
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS INTO MISSOURI, NORTHEAST COLORADO, AND SOUTH CAROLINA...

...Portions of KS/MO/AR...
As of 07z organized convection is moving eastward across KS. The
00z HREF guidance has a decent handle on this activity and
indicates it should gradually take on more of a southeastward
motion and be impacting portions of southeast KS at 12z this
morning, moving into southwest MO through the morning hours. At
least some flash flood risk will probably continue into the
morning hours with some training potential persisting. Although
the overall expected progressive nature of convection by this time
should limit the magnitude of the threat, and convection should
see a weakening trend by afternoon as it moves into northern AR.

The greater flash flood risk is likely to evolve tonight into
Wednesday morning across portions of eastern KS into MO. The
ingredients remain favorable for a potentially significant
training convective axis. Robust instability will be advecting in
from the southwest, likely resulting in a tight instability
gradient somewhere in the vicinity of eastern KS into western to
central MO. Meanwhile the low level jet will increase, resulting in
strong and persistent moisture transport and convergence over the
region. Corfidi vectors end up becoming quite weak, and what
movement they do indicate is generally parallel to the expected
moisture convergence axis and instability gradient. Thus we very
well may end up with backbuilding upscale convective growth over
this area by Tuesday night into Wednesday morning.

Confidence in there being flash flood impacts continues to grow,
although the axis of heaviest rainfall will likely be quite
narrow...thus there remains uncertainty on where exactly this ends
up. HREF data supports a narrow axis of 3-5"+ rainfall, and given
the setup this seems reasonable. This is likely a higher end
Slight risk, with a narrow corridor of locally significant impacts
possible. If confidence in the location increases later today, can
not rule out the need for a focused MDT risk upgrade.

...Southeast...
Convection is again likely today across portions of GA/SC/NC. At
18z today the model consensus is for an area of low pressure to be
positioned near the GA/SC border. HREF guidance indicates a
relatively narrow axis of higher instability within the easterly
flow just north the stationary front extending from the low. Weak
mid level shortwave energy and stronger northwesterly flow aloft
moving over top this axis of convergence should aid in convective
development today. Quicker deep layer flow should mean faster cell
motions off to the southeast...however the stationary front could
act as a focus for some training. And easterly low level flow
countering the deeper layer westerly flow may also support some
cell merger activity. Absent wet antecedent conditions this is
probably more of a Marginal risk level threat. However with soil
saturation and streamflows continuing to run well above average,
and HREF 3" neighborhood probabilities of 40-70% and 5"
probabilities of 15-30%, think that scattered flash flooding could
evolve today. Thus will go ahead and upgrade to a Slight risk.

...Northeast CO..
Opted to introduce a small Slight risk area across portions of
northeast CO and adjacent areas of far southeast WY and southwest
NE. Frontal forecasts this afternoon/evening are for a stationary
front to be draped across this area potentially acting as a focus
for convective development. Upslope easterly flow north of this
front should aid in locally enhancing lift/convergence, and in
many ways this setup is similar to Monday. With several flash
flood warnings Monday, and some of these same areas expected to
see this round of storms, think isolated to scattered flash
flooding is again probable. Easterly flow may be a bit weaker
today, and guidance does suggest quicker forward propagation off to
the east than Monday...so it is possible that rainfall magnitudes
stay a bit lower. However the setup appears favorable enough for
some slower cell motions and mergers near the front before the
faster propagation takes hold...that think Slight risk level
impacts are possible.

..Western U.S...
Scattered convection is again likely from portions of AZ/NM
northward into ID/WY/MT. There will be a bit more in the way of
westerly flow compared to earlier days, which may allow cells to
move along at a faster pace. This should keep the flash flood risk
isolated in nature...however heavy rainfall rates will again be
probable in the more intense cells...which could result in a
localized flash flood within the broad Marginal risk area.


Day 2
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE AND UPPER MISSISSIPPI VALLEY...

Training convection will likely be ongoing at 12z Wednesday across
portions of MO. See the day 1 discussion for more details on this
expected evolution...but in summary the ingredients will be there
for a significant training threat to continue into Wednesday
morning. Just like on day 1, there remains some uncertainty on the
exact axis of maximum training, but this is likely a higher end
Slight risk at the moment, with some flash flooding likely
wherever the axis sets up. By late morning into the afternoon the
convection should become more progressive and/or weaken as it
shifts southeastward.

Thereafter the risk of excessive rainfall shifts northward into
portions of IA and southern MN. Convection here will likely
develop by afternoon and continue into the overnight hours, with
the focus expected to be near and just north of a warm front
slowly lifting north across the region. Stronger mid/upper forcing
will move into the region, which combined with strengthening low
level moisture transport, should allow for organized convective
development. With the front not moving much and the moisture
transport axis rather persistent, expect we will see some
backbuilding/training convection near the front. Where exactly this
sets up remains a bit of a question. The 00z GFS/NAM/UKMET have
trended northward into more of MN, however the GEM reg, AIFS and
GFS Graphcast all still support IA into far southern MN as the
preferred location. With models often too far north with convective
QPF, hesitant to expand the Slight risk too far north. Thus still
think keeping it centered over IA and only into far southern MN is
the way to go for now...generally favoring the the further south
model solutions mentioned above.

A secondary rainfall maximum may occur over portions of ND into far
northern SD along another slow moving convergence axis. The 00z
HREF data is supportive of Slight risk level impacts across this
region. However HREF skill at these longer lead times is sometimes questionable, and global model solutions show a bit more spread on
the convective evolution over this area. Given this uncertainty
and the fact that this area is typically not as flash flood prone,
decided to keep the risk level at Marginal for now and continue to
monitor.


Day 3
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ILLINOIS, INDIANA AND SOUTHERN WISCONSIN...

The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by
this time, but the overall ingredients remain in place for areas
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but with broad troughing
moving through, this will likely not be the end of the convective
risk. Instead we should be able to reload with instability and get
additional round(s) of convection into Thursday night. Exactly
where the best risk of multiple heavy convective rounds exists is
still uncertain, but the most likely axis appears to be across
portions of IL/IN. This is a bit south and east of the inherited
Slight risk, and so the risk was adjusted accordingly. There is
still some excessive rainfall threat further north over portions of
WI and MI, but at the moment think the greater training risk is
further south and east where the favorable ingredients should
linger longer.

Chenard
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FOUS30 KWBC 140830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Aug 14 2024

Day 1
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH DAKOTA AND MIDDLE MISSISSIPPI VALLEY...

A bit greater uncertainty than normal with how convection evolves
this morning across portions of IA/MO/AR, although things are
becoming a bit clearer as of 08z. The convection moving across NE
(as of 08z) will likely be over portions of western IA and
northwest MO by 12z this morning. It will likely align itself more
northwest to southeast by this time, allowing for a greater
training potential. However the longevity of this training is a bit
uncertain as the low level jet gradually weakens. Suspect that an
isolated to scattered flash flood risk will exist, but not
confident in anything more significant than that at this time.

Another axis of training convection is likely this morning across
portions of southern MO into northeast AR. As of 08z starting to
see the beginning of this scenario starting to evolve, and expect
to see an uptick in organization and training between now and 12z.
NAM Nest runs have indicated localized 5-10" rainfall with this
event, and while not impossible, it does seem probable that
amounts will stay below that level. However still thinking several
inches of rainfall is likely, locally higher, and thus do expect a
corridor of isolated to scattered flash flood risk to develop.
This activity will likely continue into the morning hours before
dissipating by later in the morning

By this afternoon into evening expect convection to develop across
far southeast NE into western IA and northwest MO, with the focus
expected to be near a warm front slowly lifting north across the
region. Stronger mid/upper forcing will move into the region, which
combined with strengthening low level moisture transport, should
allow for organized convective development. Suspect that the
convection this morning across NE/IA may help keep the warm
frontal position a bit further south today. Given the heavier
rainfall rates should generally be near and just south of the warm
front, we were able to trim some of the northern areas (MN and
northern IA) out of the Slight risk with this update...and this is
generally supported by 00z HREF QPF probabilities and FFG
exceedance probabilities. Area of heavy rainfall are still likely
over northern IA into MN, but rates should stay low enough to keep
any flash flood risk more localized in nature. Even over northern
MO into central IA this late day and overnight convection will
probably be rather quick moving, limiting rainfall total
potential. However we could see some brief training near the warm
front, and some of these areas will have also seen heavy rain from
the morning round of convection. Thus do think at least some flash
flood risk will exist with this later convection as well.

Overall the broad Slight risk stretches from central IA to
northeast AR, and covers the risk from both the convection this
morning and also later today/tonight. The southern part of this
Slight risk is primarily for this mornings activity, with the
northern portion of the risk potentially getting both rounds of
storms.

A secondary rainfall maximum should occur over portions of ND this
morning into early afternoon along another slow moving convergence
axis. Hard to ignore the impressive 00z HREF signal over this
area, with 3" neighborhood probabilities in the 50-80" range, and
6hr FFG exceedance probabilities over 40%. There was a localized
training cell that resulted in a flash flood warning last night
over ND, and the pattern does appear favorable for that potential
this morning as well, except probably a bit more convective
coverage today. Thus think adding a Slight risk is the way to go,
with isolated to scattered flash flooding a possibility.

Chenard


Day 2
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MISSOURI, ILLINOIS, INDIANA AND KENTUCKY...

The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by
this time, but the overall ingredients remain in place for areas
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but currently anticipating
this activity to be on a downward trend by this time and thus
should not be posing much of a flash flood risk. However with
deeper troughing still upstream at this point, would expect to see
additional convective development by the afternoon hours. This
convection is likely from portions of eastern MN and WI southward
into portions of MO/IL/IN/KY. Convection across the northern half
of this area will probably stay pretty quick moving off to the
east, likely limiting the extent of the flash flood risk, keeping
the threat more at Marginal risk levels.

The greater instability and moisture axis will likely end up
further south from southeast MO into portions of IL/IN and KY. Even
here cell motions should be rather quick, however with this area
further displaced from the forcing to the north and mean flow more
parallel to the front, do expect that we could at least see some
brief training of convection. Given the favorable thermodynamic
environment for heavy rainfall rates, any training would likely
result in at least some flash flood potential. This appears most
likely along this corridor, and thus will carry a Slight risk here.

Chenard


Day 3
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST INTO THE GREAT LAKES...

On Friday the likelihood of convection shifts eastward stretching
from portions of the Southeast up into the Great Lakes region. The
probability of flash flooding by this time appears to decrease,
but at least some risk likely persists. Stronger deep layer flow
supports quicker cell motions and not really seeing much of a low
level focus for training. However, the troughing moving overhead is
quite broad in nature, likely meaning the duration of lift will be
extended, potentially supporting a few rounds of convection. Also
deep layer flow, while quick, is fairly unidirectional, which can
support some brief backbuilding of convection at times. Thus while
the overall setup for flash flooding and model QPF output are not
all that impressive, there are enough favorable ingredients in
place to suggest at least an isolated flash flood risk will exist.
This warrants the continuation of the Marginal risk.

Chenard
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FOUS30 KWBC 150800
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Thu Aug 15 2024

Day 1
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY...

...Mid MS Valley to Lower Ohio Valley...
The active weather associated with the large troughing over the
Northern Plains and UPper Midwest this morning will move eastward
today. At the start of the period, convection should be ongoing
across portions of Missouri into Illinois and portions of Iowa,
tied with the overnight convective line segments. This activity
should be both relatively progressive and also weakening with the
loss of the nocturnal low level jet and less favorable influx of
moisture. However, a few pockets of heavier rainfall and rain rates
will be possible that could total a couple/few inches through mid
afternoon.

Further south/southwest, outflow boundaries and the approaching
cold front from the west/northwest will help spark an additional
round of showers and thunderstorms late this afternoon into the
late evening/overnight hours, most likely centered over portions of southern/central Missouri to southern Illinois, far southwest
Indiana, and portions of western Kentucky. Here, the setup is much
more favorable for 1) robust deep convection, 2)
training/backbuilding storms with the flow becoming more parallel
to the storm motions, and 3) an impressive evening/night low level
jet of 30-35 kts impinging on the boundary in place.

THe 00Z HREF neighborhood probabilities indicate potential for
some intense rain rates with the storms, where the 2" hourly total
probs reach 30 percent for several hours this evening/overnight
with a slight signal for 3" hourly totals at times. Overall,
isolated totals of 3-5" are possible with some localized higher
amounts not out of the question based on the 00Z hi-res guidance
and the HREF showing a few areas of 25 percent for 5" over the 24
hour period and a near 15 percent signal for 8" totals.

For the Day 1 Excessive Rainfall Outlook, still messaging a Slight
Risk for this area for the potential of isolated to scattered
instances of flash flooding, especially if the heaviest amounts
fall over portions of southern Missouri where soil moisture is
already elevated due to recent heavy rainfall events.

Taylor


Day 2
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GREAT LAKES TO LOWER OHIO VALLEY...

...Great Lakes to Ohio Valley...
By Friday, the cold front and associated upper level troughing will
shift eastward toward the Great Lakes and Ohio Valley. Expect
another day of scattered to numerous showers and thunderstorms as a
result of the higher moisture ahead of the front along with
favorable instability developing during the peak of the daytime
heating. The set up does favor faster/progressive storm motions but
a few repeating rounds or backbuilding storms will be possible so a
few instances of flash flooding can't be ruled out so the Marginal
Risk remains in place for the area.

...Mid MS Valley...
Late in the forecast period, the latest guidance is keying on a
subtle shortwave trough embedded in the northwest flow between the
upper ridge over Texas and the large troughing over the Great
Lakes. With the nocturnal low level jet reaching 25 to 30 kts
impinging on the surface boundary expected to be draped across
portions of Kansas and Missouri, there is increased likelihood of
convection developing early Saturday morning. While confidence in
exact placement is a bit higher than normal, the setup and
environmental ingredients suggest potential for some localized
heavy rainfall and this is showing up in the various deterministic
and ensemble probabilities this cycle. Isolated/localized flash
flooding will be possible.

Taylor


Day 3
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S. AND THE EASTERN U.S...

...Southwest U.S...
The return of deeper monsoonal moisture is expected across the
region, initially late Friday night across portions of southern
Arizona, but by Saturday a plume of higher moisture surges
north/northeast through the region. This is in response to the
anomalous upper ridge axis becoming centered over eastern New
Mexico and western Texas while a strong upper trough approaches the
coast of California. This will set up a favorable flow of this
higher moisture in between across much of Four Corners region
during the period. PW anomalies reach +2 to +2.5 standard
deviations above normal during the period while the latest model
guidance is suggesting a subtle shortwave trough rippling through
the flow, which will act to enhance the forcing for ascent across
the region. Combined this with the expected instability during
peak heating and the set up will become quite conducive for
scattered daytime to evening showers and thunderstorms. The latest
guidance is favoring northern Arizona and much of Utah for the
greatest QPF and potential for higher rain rates, but the threat
will exist across the larger region. Isolated to widely scattered
instances of flash flooding will be possible, particularly for the
more vulnerable locations and a Marginal Risk covers the region.

...Mid Atlantic to Northeast...
A pronounced upper trough is forecast to move from the Great Lakes
region at the start of the period to the Upper Ohio Valley and
Central Appalachians by 12Z on Sunday. At the surface, a cold front
is expected to pass through with a ribbon of higher moisture
surging ahead of it. The combination of the favorable forcing for
ascent and higher moisture, along with the daytime heating
instability, should be sufficient to bring scattered to numerous
showers and thunderstorms to the region. While the setup should
favor relatively progressive storm motions, some areas could see a
few repeating rounds or backbuilding storms on the
southern/southwest flank of any line segments that may bring
locally higher rainfall totals. Based on the latest deterministic
and ensemble guidance plus ML first guess fields, a broad Marginal
Risk is advertised for the region for localized/isolated flash
flooding.

...Southeast...
The advancing cold front will become more west to east oriented
across the Southeast during the period. The moisture profiles in
the region will remain elevated (around 1.5 std deviations above
normal) and with robust daytime heating and instability, strong
thunderstorms are forecast to develop along/ahead of the frontal
boundary then quickly move to the east/southeast. A
localized/isolated flash flood threat exists and the Marginal Risk
was extended southward to account for pockets of heavy rainfall
totaling a few inches in places over a relatively short period of
time.

Taylor
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FOUS30 KWBC 161235
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 AM EDT Fri Aug 16 2024

Day 1
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST U.S...

...Southeast U.S...
Convection across portions of the lower Ohio Valley early this
morning is forecast to reach portions of southern/eastern Kentucky
and Tennessee later today and eventually into northern
Alabama/Georgia. There is lower confidence on how the activity will
hold together or what additional line segments will develop but
with plentiful moisture and sufficient instability in place this
afternoon, several areas of heavier rainfall will be possible.
Based on the latest guidance and 00Z HREF probabilities, pockets
of 1-2" hourly totals will be possible, most likely across portions
of middle/eastern TN, far southeast KY, and into northern
AL/northwest GA where the Slight Risk in the Excessive Rainfall
Outlook exists. Some localized totals of 2-4" will be possible
within that area and could lead to scattered instances of flash
flooding.

...Ohio Valley to Great Lakes...
For today and tonight, a cold front is expected to move eastward
from the Midwest through the Ohio Valley as the main upper level
shortwave energy digs further into the Great Lakes region. This
combination should provide plenty of support for scattered to
numerous showers and thunderstorms to develop along the advancing
cold front across the Ohio Valley into the Central Appalachians.
The activity should be fairly progressive and quick moving,
limiting the overall duration of any pockets of heavy rainfall at
any one location, but a few spots of merging storm cells or
training could develop that pose a flash flood risk. Further north
into the Great Lakes (Wisconsin/Michigan), storm motions under the
core of the upper trough are likely to be slower and when combined
with the higher PWs and somewhat elevated soil moisture percentiles (Wisconsin), a few instances of flash flooding will be possible.

...Southwest U.S...
Increasing monsoonal moisture will begin to lift northward into
portions of the region today, the highest PW anomalies will likely
be tied to far southern Arizona but the 00Z guidance is keying on
enough moisture and sufficient instability developing today
combined with the subtle shortwave lifting through to spark at
least isolated thunderstorms into portions of southern NV and
southern/central New Mexico. Some of the convection will be
capable of producing intense rain rates and could lead to flash
flooding over the typical vulnerable locations.

...South Florida...
A trailing frontal boundary and trough over the region combined
with a very high moisture axis will bring another round of
scattered to numerous thunderstorms to South Florida later today
through the evening hours. The combination of high PWs and
sufficient instability will lead to very intense and efficient rain
producing thunderstorms, capable of producing 2-3" hourly totals at
times. The 00Z guidance is quite bullish for southeast FLorida
coastal areas with potential for some locations to see several
inches today (00Z HREF probs are moderate for 5" and show a slight
signal for 8"). The most concerning area is for the highly
urbanized corridor of southeast Florida but also the southwest
coast from near Naples/Fort Myers southward could also see some of
the excessive rainfall from thunderstorms. A Marginal Risk was
introduced this cycle.

Taylor


Day 2
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN U.S...SOUTHWEST U.S...SOUTH FLORIDA...AND PACIFIC NORTHWEST...

...Eastern U.S...
The anomalous upper trough and associated cold front will make its
slow progression eastward into the Eastern U.S. Saturday into
Saturday night. Another day of mostly loosely organized convective
segments are expected in the modest axis of higher moisture and
available instability. THe latest guidance points toward areas of
the OHio Valley, Central APpalachians, and Mid Atlantic being the
focus for higher rainfall totals which could total a couple inches.
Based on the environmental ingredients, pockets of 1-2" hourly
totals will be possible where any boundary collisions or storm
mergers allow for a slightly longer duration in rainfall at any one
location but the storm motions overall should be fairly
progressive. However given some of the terrain sensitivities and
urban areas in the risk areas, a few instances of flash flooding
will be possible.

Further south, the lingering boundary across portions of the
Southeast will again be the focus for another round of mainly
afternoon/evening thunderstorms. Storm motions may be slightly less
and with more robust moisture/instability, some of the rain rates
may be rather intense but short-lived. This may lead to isolated
instances of flash flooding.

...Southwest U.S...
The return of deeper monsoonal moisture is expected across the
region, initially late tonight across portions of southern
Arizona, but by Saturday a plume of higher moisture surges
north/northeast through the region. This is in response to the
anomalous upper ridge axis becoming centered over eastern New
Mexico and western Texas while a strong upper trough approaches the
coast of California. This will set up a favorable flow of this
higher moisture in between across much of Four Corners region
during the period. PW anomalies reach +2 to +2.5 standard
deviations above normal during the period while the latest model
guidance is suggesting a subtle shortwave trough rippling through
the flow, which will act to enhance the forcing for ascent across
the region. Combined this with the expected instability during peak
heating and the set up will become quite conducive for scattered
daytime to evening showers and thunderstorms. The latest guidance
is favoring northern Arizona and much of Utah for the greatest QPF
and potential for higher rain rates, but the threat will exist
across the larger region. Isolated to widely scattered instances of
flash flooding will be possible, particularly for the more
vulnerable locations and a Marginal Risk covers the region.

...Pacific Northwest...
There is a increasing signal for heavy downpours across portions
of the Washington and Oregon Cascades. During this period there
will be and approaching trough along with a steady influx of PW
values of 1+ inches. Some of the hires guidance project convection
to spread/track northward along the terrain with hourly rain rates
upwards of 0.75 inch/hour. In collaboration with the local forecast
office a Marginal Risk area was raised to highlight the elevated
threat for excessive rainfall and the potential impacts leading to
debris flows on the recent burn scar areas from active wildfires.

...South Florida...
Another round of deep convection capable of producing flash
flooding is expected on Saturday, thanks to the lingering trough
over the region. While the 00Z guidance isn't as robust for Day 2
as it is for Day 1 (and perhaps a bit to the south of the major
urban areas), the repeating rounds / cumulative effect may bring a
few round of flash flooding to the area again on Saturday/Saturday
evening.

Taylor


Day 3
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...Mid Atlantic and Northeast...
By Sunday, the timing of the cold front and upper trough is
expected to allow for another round of scattered to numerous
showers and thunderstorms across the Mid Atlantic and portions of
the Northeast. Based on the latest guidance, a focus area of
locally heavy rainfall due to the combination of favorable
dynamics, moisture (PWs near 2 std deviation above normal), and
potential for training/backbuilding is setting up across portions
of urban corridor from near Baltimore through southern New York.
This is where the potential exists for a few rounds of heavier
rainfall and given the anomalous moisture in place, pockets of
intense rain rates will be possible over the more urban areas. Some
areas may pick up a quick couple of inches. With this in mind, a
Slight Risk was introduced for portions of the area where
scattered instances of flash flooding will be possible.

...Southwest and Intermountain West...
A continuation of the deep monsoonal moisture and daytime to
evening showers and thunderstorms are expected across much of the
Southwest U.S. and into portions of the Intermountain West Sunday
into Sunday evening. Convection will be capable of producing hourly
totals up to 0.75" at times, particularly from northern Arizona
through Utah where the better instability is forecast to setup.
This will bring another day of localized to widely scattered
instances of flash flooding for the more vulnerable locations in
the region.

Taylor
---
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FOUS30 KWBC 170745
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sat Aug 17 2024

Day 1
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHWEST INCLUDING THE SLOT CANYON REGION OF UTAH AND PORTIONS OF
THE MID ATLANTIC...

...Southwest U.S...
An active convective day is in store for portions of the Southwest
U.S. as the deep monsoon moisture surges northward through the
region. PWs are forecast to reach nearly +2 to +3 standard
deviations above normal, with values as high as 1.25" into Idaho
and locally 1.5"+ across Arizona and southern Utah. Aloft,
shortwave energy will be moving through the flow, providing greater
forcing for ascent. Finally, with daytime heating and favorable
lapse rates, upwards of 1500-2000 J/kg of instability is expected
to develop. All of this points to robust thunderstorms capable of
producing intense rain rates (localized up to 1" in an hour) over
the more sensitive regions of the Southwest U.S. including the
Slot Canyon region of Utah. HREF neighborhood probabilities are
moderate (50%) for localized 2" totals over southern Utah and even
show a slight signal (15-20%) for 3" totals.

No significant changes were needed to the Slight Risk area and the
rest of the risk areas were adjusted minimally to the latest
guidance. The most vulnerable locations will be the slot canyons,
normally dry washes, and areas near recent burn scars for
potentially localized significant flash flooding.


...Eastern Great Lakes, Mid-Atlantic, and into the Mid-Mississippi
Valley...

The slow moving upper trough will continue to make a progression to
the east today, primarily affecting areas from the eastern Great
Lakes through the central Appalachians and Mid Atlantic region.
Fairly decent forcing for ascent provided by the approaching height
falls and upper divergence, which will provide the best support for
convection across the Mid Atlantic later today. Further west,
convection along the Appalachians/Ohio Valley will be tied to the
advancing cold front. Moisture profiles are sufficient but not
overly impressive and instability is expected to be enough to
produce scattered to numerous showers and thunderstorms, though
storm organization is generally expected to be weak and overall
progressive as well. This should limit the scope of any flash
flooding concerns.

With that said, there are signals for some pockets of higher rain
totals, one of which is across the Mid Atlantic where some of the
00Z hi-res guidance paints some hefty QPF totals, although spotty
in places. This drives the HREF probs for totals for 3" above 50
percent across northern VA through eastern/east-central PA.
Confidence in this is lower with the various scenarios is
conditional based on how earlier/morning activity plays out.
However, given the run to run continuity of the HREF signals and
potential for at least a couple rounds of heavier rains, a Slight
Risk for mainly urban flash flooding was introduced from portions
of MD (near Washington, DC) through eastern PA.

Farther south along the stalled boundary, some training of
convection is possible as the mean flow becomes more parallel to
the expected storm motions, particularly across portions of the
Tennessee Valley southward. Despite the decent HREF signal for localized/pockets of higher rain totals, generally high FFG and
drier soils should keep the flash flood threat at the Marginal
level.


...Pacific Northwest...
An impressive upper level trough will amplify into a closed low off
the northern CA coast, dropping 500mb heights to less than -3 sigma
this afternoon. Downstream of this low, pinched flow will
transition into broad divergence over the Pacific and Interior
Northwest, surging moisture and ascent into the region. PWs are
progged to reach above 1 inch, and possibly above 1.25" in parts of
the Cascades, reaching nearly +3 sigma from climatology. As the
upper low pivots slowly northeast, this moisture fetch will be
persistent into the region, and IVT values peak around 350 kg/m/s,
reflective of a rare but weak August AR into WA/OR/northern CA.

During this time, ascent increases within the RRQ of a
strengthening upper jet streak, and mesoscale upslope ascent
occurs into the Cascades. This will result in widespread showers
and even scattered thunderstorms in response to MUCAPE reaching
500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach
as high as 30-40%, so despite generally rapid storm motions, brief
torrential rainfall is likely, with total rainfall being enhanced
by repeating rounds of cells. August ARs usually do not result in
anything more than isolated flood/flash flood instances, but the
MRGL risk was extended along the Cascades into northern CA, and
rapid runoff/instances of flash flooding are possible, with
impactful flash flooding, including debris flows, possible across
recent burn scars.

...South Florida...
Another day of enhanced coverage of convection is expected across
South Florida and the Keys in the vicinity of the lingering
surface trough draped across the area. This feature combined with
fairly anomalous moisture should lead to another day of scattered
to numerous showers and thunderstorms, some of which will be
capable of producing intense rain rates (2-3" hourly totals) and
potentially produce a quick 2-4"+ over some areas. The latest
guidance points to the greatest focus area to be south of the most
urbanized corridor but nonetheless, some isolated/localized
flooding concerns will be possible and the Marginal Risk remains in
place.

Taylor


Day 2
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...Mid Atlantic and Northeast...
A cold front will advance eastward into the region as the main
upper trough slowly advances through the Great Lakes. This will
bring another day of scattered to numerous showers and
thunderstorms to much of the region. In the low level southwesterly
flow ahead of the front, with enhanced training likely on S-N
oriented mean flow parallel to the from from near Washington, D.C.
through Upstate NY. Here, PWs will be nearly +2 sigma according to
NAEFS, which will be acted upon by the robust synoptic lift to
deepen convection. Rainfall rates of 1-2"/hr are likely, which
through training could produce locally more than 3" of rainfall
based on the latest ensemble probabilities.

Where this training occurs across urban areas or sensitive soils,
scattered instances of flash flooding are possible. The recent CSU
FG fields depict an enhanced risk for flash flooding along I-95 and
into the Tri-State area, leading to higher confidence in the Slight
Risk.

Further south, the Marginal Risk extends into the Piedmont and
coastal Carolinas where the combination of heavy rainfall over more
sensitive soils due to recent heavy rains may result in isolated
instances of flash flooding.


...Southwest and Intermountain West...
Impressive and deep monsoonal moisture will continue to stream
around the western periphery of the monsoon ridge and combine with
daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will
be capable of producing hourly totals up to 0.75" at times,
particularly across portions of Utah where the better instability
and highest PW anomalies are forecast. It's possible a Slight Risk
may be needed in future updates across portions of northern AZ into
southern UT but there is some uncertainty on cloud cover
potentially limiting the available instability and thus the rain
rates may end up lower.

...South Florida...
Another day of scattered, slow-moving thunderstorms are expected
across the region where rain rates may approach 2-3"/hr at times.
This is due to the lingering surface trough in the vicinity
focusing the higher PWs. Given the potential for this to occur over
the more urban areas, the inherited Marginal Risk was minimally
changed to highlight the continued isolated flash flood risk.

Taylor
---
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FOUS30 KWBC 180812
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Sun Aug 18 2024

Day 1
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...

...Mid Atlantic and Northeast...
An approaching cold front will bring another day of scattered to
numerous showers and thunderstorms to portions of the Mid Atlantic
and Northeast today into tonight. Aloft, the broad upper trough
will slowly advance eastward into the eastern Great Lakes and
central Appalachians. Ahead of it, southwesterly flow will allow
for warm and moist air to continue lifting into the region and will
be characterized by dewpoints well into the 70s and PWs above 1.5"
with locally higher values exceeding 1.75", which is around 2-2.5
standard deviations above normal. With peak heating, instability of
between 1500-2000 J/kg is forecast to develop.

The 00Z guidance continues to key on the urban corridor from near
Washington, DC to near NYC metro for the greatest QPF and
probabilities for intense rain rates today into tonight. Several
clusters of thunderstorms are likely to develop (some this morning) and
with the mean flow out of the southwest near parallel to the storm
motions, some enhanced training or backbuilding will be possible,
particularly over eastern PA, northern New Jersey, and into
southern New York. Here, the HREF neighborhood probabilities are
high for 3" totals (above 60 percent) and show a slight signal for
5" (15-20 percent). Rain rates of 2-3"/hr will be possible given
the PW environment. Given the proximity and likelihood for this to
occur over the more urbanized corridor, scattered instances of
flash flooding will be possible, some of which could be locally
significant.

Elsewhere in the region outside of the Slight Risk, thunderstorm
activity is expected to be more isolated in nature, but given the
environmental ingredients could still result in localized flash
flooding. This includes much of PA and NY states where there is
still a decent signal in the HREF probabilities for 2-3" isolated
totals. Another area of concern is further south from eastern VA into
the Piedmont and coastal regions of the Carolinas where the
combination of locally heavy rainfall over more sensitive/saturated
ground conditions could lead to additional flash flooding
concerns.

...Southwest and Intermountain West...
Impressive and deep monsoonal moisture will continue to stream
around the western periphery of the monsoon ridge and combine with
daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will
be capable of producing hourly totals up to 0.75" at times,
particularly across portions of northern AZ and southern Utah where
guidance continues to show potential for locally higher rainfall
amounts.

...South Florida...
Another day of scattered, slow-moving thunderstorms are expected
across the region where rain rates may approach 2-3"/hr at times.
This is due to the lingering surface trough in the vicinity
focusing the higher PWs. The consensus among the 00Z guidance is
that this should largely stay south of the major metro areas, but
will be close enough to continue the Marginal Risk for portions of
South Florida and the upper Keys.

...Ozarks Region...
Shortwave energy rounding the top of the upper ridge will quickly
drop through the Ozarks region late in the period
(this evening/overnight). With a modest low level jet forming in
response and a decent moisture influx, a narrow strip of locally
heavy rainfall is likely to develop in the region. Given the
northwest flow aloft and storm motions, some backbuilding is
possible. While confidence in these scenarios isn't very high,
some of the outlook area has seen recent heavy rainfall and the
potential for a narrow/localized area of 1-3" is possible based on
the 00Z hi-res guidance and HREF neighborhood probabilities. This
could lead to a few instances of flash flooding, mainly in the
06Z-12Z period early Monday morning.

Taylor


Day 2
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHEAST U.S. AND SOUTHWEST INTO THE INTERMOUNTAIN WEST...

...Northeast...
The upper trough will make a final pass across the Mid Atlantic to
Northeast on Monday with the cold front expected to move across
the coastal areas during the late morning into the evening hours.
With peak heating and sufficient destabilization, vigorous
convective elements are expected to develop again, mainly from the
coastal Mid Atlantic through much of eastern New York and portions
of New England. Storm motions should be fairly progressive, but a
few storms could repeat/train due to merging/colliding cells and
storm totals could approach 1-2" in spots leading to flash
flooding.

...Southwest/Intermountain West...
By Monday, the bulk of the anomalous moisture is expected to have
lifted northward through the Four Corners region and into the
Intermountain West/Rockies. With another shortwave trough embedded
in the flow moving through, this should be sufficient to produce
another day of isolated to scattered thunderstorms, most likely
over portions of northern Utah, Colorado, and into western WY, far
eastern Idaho. Flash flooding will be possible, particularly for
the most sensitive locations like areas near recent burn scars.

Taylor


Day 3
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

The upper ridge over the Southern U.S. is expected to build/grow
and expand westward into more of the Southwest U.S. on Tuesday.
This will likely limit the extent and coverage of monsoon
thunderstorm activity across the region, but with PWs still
elevated across Arizona and the expectation for at least a modest
amount of instability, isolated thunderstorms are possible through
the late morning into evening hours. If they do develop, these
storms would pose an isolated flash flood risk for the typical
vulnerable locations like areas near burn scars, dry washes, and
slot canyons.

Taylor
---
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FOUS30 KWBC 190738
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Mon Aug 19 2024

Day 1
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHEAST U.S...

...Northeast...

Broad meridional flow across the Eastern CONUS will continue with a
dual surface low structure progged to move northeast through the
period with a cold front swinging eastward out of the adjacent Ohio
Valley up through Quebec, clearing the eastern seaboard later in
the evening. The deep cyclonic flow ahead of the trough will
continue advecting warm, moist unstable air poleward with
increasing theta-E's and accompanying instability along and ahead
of the cold front situated across Southern New England up through
Interior Northern New England by the late-morning and afternoon
time frame. The combination of increased mid-level forcing from the
trough and surface convergence along the cold front will aid in
developing a line of convection in-of the above area with a primary
focus north of I-80 with the best chance for heavy rainfall likely
north of the LI-NYC-Hudson Valley corridor. This puts areas that
have seen significant rainfall within the crosshairs of another
locally enhanced rainfall prospect before the cold front moves past
and drier air sweeps through in wake of the front.

In terms of potential, the upper threshold will be less than what
occurred today due to a shorter time frame of impact due to the
frontal progression, as well as a more narrow sector of favorable
instability that will play a significant role in the heavy rain
footprint, and overall flash flood prospects. 00z HREF neighborhood probabilities paint some of that story with a focal point of 20-40% probabilities for >3" located within the confines of Southern CT up
through Interior New England, including parts of the complex
terrain within MA/VT/NH. The alignment of these higher
probabilities coincide with the better MUCAPE field forecast during
the morning and afternoon hours before the cold front passes the
area. A favorable footprint of at least 1"/hr rates off the
ensemble (HREF) also give credence to the potential with an area
of 50-90% encompassing the above region with low-end probabilities
for at least 2"/hr showing up across Northern VT and NH. These
areas have been impacted a few times in recent memory with the area
across CT very fresh from today's barrage that brought locally 8+"
to parts of the state. Area FFG's across New England are relatively
low with soil moisture averages falling just above the 50th
percentile, or near normal. CT is not so lucky with the top soil
layer completely inundated from this past event leading to very
sensitive grounds for flooding in this area of interest.

Considering the above factors, a moderately favorable environment
for heavy rainfall, and with a strong agreement from the UFVS First
Guess Fields denoting a recommended high-end SLGT risk, a Slight
Risk was added across much of CT, West-Central MA, up through much
of VT and NH. A MRGL risk was maintained further south for isolated
flash flood prospects from scattered convection migrating eastward
during the frontal passage that could lend a few chances to those
within the urban corridor from Hampton Roads up through the NYC
metro.

...Southwest U.S...

The region will continue to lie within the western flanks of mid-
level ridging situated over the Southern Plains into the
Continental Divide. Relatively moist airmass with differential
heating during the afternoon will yield scattered thunderstorms
across the Desert Southwest with locally enhanced rainfall
plausible within the terrain areas of AZ up through the Inter
Mountain West. A few shortwaves embedded in the flow will aid in
some better organized convective clusters capable of providing 1+"
rainfall amounts in a short time, enough to cause some issues if
they fall within complex terrain, remnant burn scars, or slot
canyons located across the region. The threat remains on the lower
end of MRGL, but still within the threshold leading to a general
maintenance of the risk from previous forecast.

...High Plains...

Multiple stronger shortwaves will eject eastward along the northern
periphery of the ridge, encountering a formidably unstable airmass
as they enter into the High Plains of the Northern and West-Central
U.S this afternoon and evening. Consensus has grown on a few areas
within the High Plains of MT into SD, as well as the CO/WY Front
Range for a cluster of thunderstorms capable of locally heavy
rainfall developing later this afternoon, carrying through the
evening as they ridge along the theta-E gradient in place across
the center of the CONUS. HREF neighborhood probabilities were
favorable for at least 1-2" of rainfall (60-90%) in either of these
areas with some lower-end probabilities for upwards of 3" in any
cells that propagate through the High Plains. The setup should be
on the progressive side overall, however some backbuilding
potential behind any MCS development over the Northern Plains could
produce a more locally significant rainfall footprint where if/when
it develops. The zones of potential include the Southeast WY and
Northeast CO corridor along with Southeast MT into Western SD as
the secondary focal points in the setup. ML output was targeting
this area as the latest runs with an axis of higher QPF positioned
within the same zones as above. First Guess Fields are also
pin-pointing a broader MRGL risk extended into the above zones with
even a some signals for a potentially higher risk (SLGT) embedded.
With the top soil layers trending drier than normal, the prospects
for a more widespread significant rainfall are not anticipated,
thus an additional MRGL expansion into the High Plains was enough
to cover for the potential.

Kleebauer


Day 2
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

Mid-level ridge across the Southern U.S is still on track to
maintain a stronghold on the overall pattern for much of the
Central and Western U.S by Tuesday. Ridge axis will expand and
strengthen according to the model consensus leading to unfavorable
conditions for convective development over under the ridge itself.
Arizona will continue to lie right on that western fringe of the
ridge placement with enough lingering instability and moisture
promote a threat of isolated thunderstorms capable of producing
locally heavy rainfall. The threat is certainly on the low-end of
the potential, but non-zero, especially along the International
border between Santa Cruz and Pima counties in AZ. The previous
MRGL was maintained with no change in the alignment.

A small note to add that lingering convection across the Northern
and Central Plains stemming from activity on D1 will have the
ability to maintain enough intensity to promote an isolated threat
for flash flooding the very beginning of the period before
diminishing. The global model consensus is all over the place on
exactly where the heaviest rain could occur, so there was not
enough agreement to institute a MRGL risk. There is a chance for a
small addition in later updates if there is more agreement within
the model suite, but there's still time to hash out those details
in the next succession of updates. CAMs guidance will hopefully
shed more light on the necessity, if any for an addition.

Kleebauer


Day 3
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

A more robust moisture advection pattern will ensue across the
Southwest U.S with some Pacific moisture being pulled northward
thanks to the combination of the Southern ridge shifting eastward
and becoming more elongated north-south, along with a shortwave
trough quickly moving inland through CA leading to a pathway for
elevated moisture and embedded shortwaves pivoting out of Sonora
and nearby Baja to move northward. Ensemble QPF is much greater
compared to the recent D2 time frame thanks to the poleward
advancement of anomalous moisture (+2 to +3 deviation PWATs based
off the latest NAEFS) and subsequent instability increase. The
ascent pattern across the Great Basin is the driver for a better
signal further north of AZ thanks to diffluent flow out ahead of
the shortwave. A widespread signal of 0.5-1" of rainfall at this
lead is indicative to the potential for more widespread convective
impacts across the Southwest and parts of the Inter Mountain West.
As of now, the MRGL in place will suffice, but a targeted upgrade
is plausible considering the expected evolution of the pattern
heading into mid-week.

Kleebauer
---
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FOUS30 KWBC 200752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Tue Aug 20 2024

Day 1
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

Current upper air analysis across the CONUS depicts a sprawling
mid-level ridge positioned over much of the Southern Plains into
the Southwest. The ridge itself will maintain its orientation with
some modest strengthening heading through the afternoon with the
center of the ridge positioned across the Southern Rockies. Enough
diurnal destabilization and moisture trapped under the ridge will
offer a threat of widely scattered convection across the terrain in
AZ, extending into Southern CO and Northern NM. The overall QPF
footprint is fairly lackluster in the areal average, but a few
pockets of heavier precip can be found within the CAMs indicating a
low-end potential for heavy rain and potential flash flooding. The
primary concerns will remain tied to the complex terrain and
adjacent towns that could be impacted by rapid onset flooding. Slot
canyons and remnant burn scars located across the Southwest will be
under the threat as well, one of the main reasons for continuity
from the previous forecast, as well as an expansion eastward into
Southern CO and Northern NM where multiple burn scars are located.
This remains the primary area of interest for the period.

A small area across east-central SD will also experience a threat
of locally heavy rainfall as remnants of a complex of thunderstorms
migrates eastward along the leading edge of favorable mid-level
ascent as a shortwave ejects out of the High Plains. Favorable
instability axis across the Northern Plains favors points further
west with the storm complex likely to enter a less favorable
convective regime to maintain a strong enough intensity to induce
flooding prospects. LLJ will also be on the decline leaving behind
a relatively mundane setup for prolonged convective enhancement
that would be necessary for flash flood prospects. Considering the
above variables, and the addition of drier soil moisture located
within the forecasted zone of impact, a nil risk area was
maintained, however the threat is non-zero (<5%).

Kleebauer


Day 2
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN ARIZONA...

A more robust moisture advection pattern will ensue across the
Southwest U.S with some Pacific moisture being pulled northward
thanks to the combination of the Southern ridge shifting eastward
and weakening, along with a shortwave trough quickly moving inland
through CA leading to a pathway for elevated moisture and embedded
shortwaves pivoting out of Sonora and nearby Baja to move
northward. Ensemble QPF is much greater compared to the D1 time
frame thanks to the poleward advancement of anomalous moisture (+2
to +3 deviation PWATs based off the latest NAEFS) and subsequent
instability increase. The ascent pattern across the Great Basin is
the driver for a better signal further north in-of AZ and UT
thanks to diffluent flow out ahead of the shortwave. Ensemble QPF
has increased since the last series of runs with a more pronounced
convective depiction signaling some totals between 1-1.5" now
introduced in the means. The back end of the 00z HREF is already
showing the increased convective pattern at the beginning of the
diurnal period with the remainder of the evening still to go.
Ensemble bias corrected QPF is running between 0.75-1.25" for an
areal average over a good portion of AZ with the focal points
situated over the terrain in Southern AZ and across the Mogollon
Rim and Grand Canyon areas of Central and Northern AZ. The
prospects are certainly favorable for more localized flash flood
concerns just based off the anomalous PWAT advection anticipated.

After collaboration with offices out west, a targeted SLGT risk was
added across Northern AZ encompassing places like Flagstaff to the
Grand Canyon, representative of the strongest heavy precip signals
expected during the D2 period. Additional upgrades to a SLGT risk
are also possible within the next series of updates, especially
within Southern AZ and Southern UT pending the run-to-run
consistency within guidance.

Kleebauer


Day 3
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...

A continuation of the elevated moisture pattern across the
Southwest will transpire in D3 with an axis of scattered to
widespread convection likely across much of AZ through the Inter
Mountain West. Small mid-level perturbations are still forecast to
rotate northward around the western flank of the ridge positioned
across the Southern Plains. A digging upper low off the PAC
Northwest coast will allow the flow to become even more meridional
across the West allowing for the more anomalous PWAT advection to
occur further in the interior leading to the best heavy rain axis
to likely situate over Northern AZ through much of UT and Western
CO. The growing consensus on the specifics of where the heaviest
precip will focus will likely correlate to an upgrade at some point
in the future, but timing of the mid and upper pattern still has
some points to hash out before going into more detail. As of now,
the MRGL from the previous forecast was maintained but expanded on
all sides as the 00z ensemble QPF footprint expands with agreement
from the recent ML output on the precip placement.

Across Florida, a stalled frontal boundary will become the focal
for scattered convection on Thursday afternoon with the areal QPF
average relatively elevated, but still not depicting a substantial
enough signal to warrant a risk addition to the area. This will be
a period to monitor for perhaps a targeted risk along either coast,
pending sea breeze pattern evolution where heavy rain could affect
the the urban corridors either side of the Peninsula, or an area
inland like Orlando proper.

Kleebauer
---
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FOUS30 KWBC 210752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Aug 21 2024

Day 1
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARIZONA AND SOUTHWESTERN UTAH...

...Southwest...

A relatively strong mid-level ridge will be broken down on the
western flank as a series of mid-level shortwaves begin ejecting
off the Pacific with a more amplified, closed-low pattern evolving
towards the back end of the forecast period off the Pacific
Northwest. The weakening force of the pattern will allow for a more
north-south alignment of streaming mid-level energy to ride out of
Sonora and traverse the Desert Southwest to the Great Basin. In
tandem, a pretty formidable moisture flux will begin advecting into
the region, highlighted by a well-forecast IVT pulse (500-700
kg/ms) pivoting through much of AZ with roots from the tropical
Eastern Pacific. This will allow for PWATs to surge closer to +2 to
+3 standard deviations, a factor that typically enhances the
convective pattern across the West and has a history of more
widespread thunderstorms with heavy rainfall. Latest 00z HREF is
privy to the enhancement with the blended mean QPF signal depicting
a more robust convective threat with local maxima bordering upwards
of 2" in spots confined to the terrain. The greatest threat(s)
will be located across Southern to Southeast AZ where the initial
surge will be taking place, intersecting the terrain situated near
the border. The second areas will be within the Mogollon Rim and
Grand Canyon areas of AZ into the Southwestern portion of UT. Those
areas will see their best potential the second half of the period
as the surge of elevated moisture reaches the region and continues
the convective threat into the evening. HREF probability fields for

1" of precip are high (60-90%) across much of the above areas with
2" markers a respectable 25-40% within the neighborhood

probability field. These are textbook signatures for elevated
convective flash flood threats leading to a general maintenance of
the SLGT risk from previous forecast with only minor modifications
to the risk area.

Interior areas across UT and CO will also see an uptick in
convective coverage, but the deeper moisture plume will still be
making progress around the northern periphery of the ridge, so the
threat isn't expected to be as robust as further southwest. The
environment is favorable for some isolated impacts, especially
within any stronger mid-level perturbations that rotate overhead
allowing for better organized cell structures in their presence.
The MRGL risk was modified only to outline the trends within the
ensemble QPF where locally heavy rainfall could produce some flash
flood prospects.

...New York and Northern New England...

Upper low across Quebec will slowly migrate south into Upstate NY,
eventually drifting eastward through VT/NH during the second half
of the period. Large scale ascent pattern under the low will
generate periods of showers with embedded thunderstorms as the
disturbance pivots through the region. Some of the signals across
the Green/White Mountains of New England are sufficient for
potential locally heavy rain cores that become tied to the terrain
thanks to a slower mean storm motion. The lack of surface
instability will limit the threat to a more elevated convective
regime which typically doesn't allow for pronounced rainfall rates.
The antecedent moist soils still lingering after what transpired
the past week and the slow mean storm motions are still enough to
sway to the side of a MRGL risk which was added the previous
update. There was little deviation in the setup from that point, so
decided to maintain continuity and keep the MRGL risk over portions
of NY state and Northern New England.

Kleebauer


Day 2
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S...

An increasingly amplified pattern out west will take shape as the
digging upper trough and closed-low positioned off the Pacific
coast will begin interacting with the anomalous moisture feed
advecting northward around the western flank of the ridge to the
southeast. PWATs will remain settled between +2 to +3 deviations
above normal across much of the Southwestern U.S with advancement
deeper into the Inter Mountain West leading to an enhanced
convective regime extending from Northern AZ through UT and Western
CO. Areal average MUCAPE across the Southwest will range from
500-1000 J/kg with a maximum of 1500-2000 J/kg situated within the
core of the IVT pulse bisecting portions of Southern UT down
through Northern AZ. This area in question will be the primary
region of focus for heavy rainfall and relevant flash flooding,
especially given the ongoing nature from the D1 time frame, as well
as the complexity of the terrain with numerous slot canyons and
remnant burn scars still situated over the area.

Ensemble QPF footprint between 1-1.5" on average is a testament to
the increasingly bullish outlook for heavy rainfall as these
signals are usually reserved for the better flash flood setups in
recent past. The back end of the 00z HREF for reference has a
sharp increase in the 1"/hr rainfall rate probabilities between
18-00z in the period with a blossoming convective pattern likely
extending through the following evening when correlating to global deterministic. The pattern will evolve with convective lingering
late into the period with even some nocturnal storm prospects
further inland into UT and Western CO. The previous SLGT risk
forecast was maintained with the higher-end SLGT potential located
within that zone between Flagstaff up through Zion National Park,
including the Grand Canyon. The SLGT extends further inland through
the interior of UT into Western CO with a secondary maxima being
depicted within the western slopes of the Rockies, east of Grand
Junction.

Elsewhere:

The setup across Maine and Florida will lend to non- zero chances
for flash flooding, but the overall signal is still just below the
5% risk threshold to necessitate a MRGL risk. The previous MRGL
across Maine was removed due to timing of the heaviest convection
occurring mainly within the D1 period with an overall decay in the
heavy rainfall prospects during the front half of the period.
Across Florida, the setup is conducive for locally heavy rainfall
thanks to a stalled front and pooling moisture near the boundary.
Right now, the threat is borderline with the area of interest
mainly over the Northeast FL coastal plain between Jacksonville to
Daytona. Area FFGs are very high in this area leading to the
primary threat being almost pure urban flash flooding with less
concern for flooding outside those local zones. With some time
remaining leading up to the period, will continue to assess the
signal to see if a MRGL addition is prudent.

Kleebauer
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FOUS30 KWBC 220749
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Thu Aug 22 2024

Day 1
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S...

...Western U.S...

A continued amplified pattern across the west will evolve further
with a retaining deep moisture fetch arcing through the Desert
Southwest and further into the Inter Mountain territories within UT
and CO. A general +2 to +3 deviation PWAT anomaly will bisect the
region of Northern AZ through Central UT and Western CO by the
afternoon hours today leading to an enhanced convective output
during peak diurnal destabilization. A growing consensus amongst
CAMs members for widespread thunderstorm development with stronger
cores able to produce 1"/hr rates or better aligns well with the
pattern and will induce a greater potential for flash flooding
along the aforementioned areas.

A strong HREF EAS signal for 1" of rainfall exists within the
Southwest portion of UT, including some of the busier National and
State Parks where slot canyon positioning and stronger flash flood
prospects overall create a heightened prospect for impacts during
the expected convective time frame. HREF neighborhood probabilities
for >2" of rainfall are also very high (50-80%) within the local
area of Southwest UT down into the Northern tier of AZ, a signal
generally befitting for flash flood scenarios in recent history.
The threat is not as robust further south into the Mogollon Rim as
the deeper moisture advection regime will be located further north,
but the environment will still be primed for scattered heavy rain
signatures within that zone, so continuity on the southern end of
the SLGT risk was maintained from previous forecast. Further north,
the SLGT risk remains, but the threat is on the higher-end of
potential for the areas near Flagstaff up through that area of
Southwest UT thanks to the alignment of the best instability, deep
layer moisture, and correlated IVT pulse traversing the area.
Pending the evolution of hi-res guidance and radar trends leading
into the afternoon, a targeted Moderate Risk is plausible,
especially within the slot canyon corridor that has the highest
flash flood susceptibility. The areas further north across UT and
CO will see their fair share of convective impacts, especially late
in the afternoon and evening when instability reaches peak and any
mid-level perturbations stemming from convection to the Southwest
advect northeastward allowing enhanced ascent locally as they
maneuver overhead.

Another area of interest will occur across the CO Front Range as
combination of elevated moisture, instability, and a stronger
shortwave ejecting out of the Central Rockies will ignite another
round of heavy thunderstorms to the east of Denver, an area that
has seen impacts as of late with regards to flash flooding. A
secondary QPF max located across the area allowed for a pretty
elevated signal within the 3hr FFG exceedance probabilities
(40-60%), mainly after 00z when LLJ influence peaks the intensity
of the forecasted convection. A MRGL risk is currently located over
the area, but that corridor across Eastern CO, away from the divide
is the focal point for that time frame.

...Florida...

Two separate zones for flash flood potential will exist across FL
this period. The first of which will be situated across the western
coast of the Peninsula with the Clearwater/Tampa/Sarasota corridor
as the primary focus. A quasi-stationary front will meander over
the northern half of FL with smaller mid-level impulses wandering
eastward off the Gulf, creating a multi-wave convective scenario
during the entirety of the forecast. 00z sounding out of KTBW
indicated a sufficiently deep warm cloud layer (>15.5k ft) with
areal PWATs running over 2" with expected continuity during the
period. Mean flow running parallel to the front will create an,
"Express lane" for multiple mid-level shortwaves off the adjacent
Gulf to bisect the area, each carrying their own convective
conglomeration as they move ashore across the region. Rainfall
rates of 2-3"/hr will be plausible in the scenario, impacting an
already water logged area of FL that is still dealing with the
aftermath of Debby. The lower FFG intervals within the urban
corridors, and now surrounding areas with the slow receding waters
create a higher likelihood of flash flooding to occur, especially
with the multitude of convective waves anticipated during the
forecast period. A MRGL risk was added to account for the threat.

The second area of interest lies further northeast, closer to the
Jacksonville metro and surrounding coastal portions of Northeast
FL south of Jacksonville, as well as far Southeast GA where the
recent CAMs output is the most aggressive with regards to heavy
rain prospects. A strong HREF neighborhood probability for >5"
(50-80%) exists within that zone between GA down to Jacksonville
proper with a low-end probability for >8" also depicted within the
prob fields. This all stems from the front location and expected
instability axis to develop during diurnal heating leading to
development along the boundary with slow storm motions anticipated.
In coordination with the Jacksonville WFO, a MRGL risk was added
for that area encompassing Southeastern GA down through Northeast
FL with emphasis on the urban areas along the coastal plain.

Kleebauer

Day 2
Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION AND INTERMOUNTAIN WEST...

The anomalous moisture feed that allowed for the active periods
prior will vacate to the northeast with a skinnier axis of elevated
PWATs generally focused from the AZ/NM state line up through the
Inter Mountain West of UT/CO. This positions the the Four Corners
area up through the Wasatch as the the main targets for heavy
convection during the afternoon and evening Friday before the setup
dwindles the back end of the period. A secondary focus across the
terrain in Southeast AZ over into the NM Bootheel will also be an
area that could see some scattered heavy rain signals with some
support from global deterministic and ensemble members. Current QPF
signal is modest with areal averages closer to 0.25-0.5" with a
maxima focused near the Four Corners over Southwestern CO (>1").
The secondary max is across the Wasatch through the terrain in
Western CO where isolated totals >1" are plausible, but more
sporadic in coverage compared to the Four Corners region. Overall,
the setup is more benign in the grand scheme, especially
considering the previous active periods.

Kleebauer
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FOUS30 KWBC 230829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Fri Aug 23 2024

Day 1
Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION/INTERMOUNTAIN WEST AND PARTS OF THE SOUTHEAST US...

...Southeast US...
Maintained the Marginal risk area that was introduced on Thursday
over portions of northeast Florida...and expanded the area
northeastward along the Georgia coast line. 00Z runs of the HREF
increased the neighborhood probabilities of 1 to 2 inch per hour
rates in response to a frontal boundary..and associated
convection...being a bit closer to the coast than shown by earlier
runs.

...Southwest US...
Moisture feeding northward into the Southwest US will begin to be
channeled into a more narrow corridor disrupted today...leading to
a somewhat smaller coverage of late day and evening convection
capable of producing heavy rainfall. A secondary focus across the
terrain in Southeast AZ over into the NM Bootheel will also be an
area that could see some scattered heavy rain signals with some
support from global deterministic and ensemble members. Current QPF
signal is modest with areal averages and the setup is more benign
in the grand scheme, especially considering the previous active
periods.

Bann


Day 2
Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST/FOUR CORNERS AND SOUTH FLORIDA...

...Southwest and Four Corners...

Lingering moisture and relative instability coupled with
progressive shortwaves cutting through the Interior Mountain West
will set up another round of isolated convection during the
afternoon and early evening hours on Saturday before a gradual
dissipation with the loss of diurnal heating. Pockets of heavy
rainfall are still forecast across a similar corridor as on Day 1,
so there is some concern that the antecedent conditions could be
favorable for greater impacts pending what occurs leading up to the
start of the forecast period on Saturday morning. Overall ensemble
QPF footprint still indicates a max of 1-2", at best, within the
strongest cores. This is still enough for flash flooding within
complex terrain and especially around any burn scars. The previous
MRGL risk was relatively unchanged.

...South Florida..

Incoming moisture flux with roots from the Caribbean will make
headway into South Florida on Saturday with area precipitable water
anomalies approaching 2 deviations above normal. The increasing
moisture will be in proximity to a quasi-stationary front to the
north...with a sharp delineation point of where the moisture
pooling will reside. A Marginal risk area was maintained across
the Southern FL Peninsula.

Bann


Day 3
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST/FOUR CORNERS AND SOUTH FLORIDA...

As an upper high over the Southern Plains begins to move eastward
and away from the Southwest...another push of moisture and
shortwave energy will move northward. This will sett up another
round of isolated to widely scattered thunderstorms capable of
producing flash flooding or run off problems within complex
terrain and especially around any burn scars. Given the fetch of
moisture from the southeast...the areal coverage was expanded
somewhat compared with the size of the Marginal risk area on Dy 2.

Bann
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FOUS30 KWBC 240846
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Sat Aug 24 2024

Day 1
Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST...MIDWEST...AND SOUTH FLORIDA...

...Southwest...

Another round of daytime convection is expected in the Southwest
again on Saturday, although the primary threat area should be
relatively narrow, focused in a plume of deeper moisture along the
periphery of an advancing deep trough. The Marginal Risk was
generally maintained in the same area as the previous outlook
issuance with only minor adjustments around the southern periphery.
Localized heavy downpours will remain possible given the plume of
moisture still being drawn northward out of Mexico and northward
around the western side of a sprawling upper level ridge. However,
the mid-level flow will be much stronger than usual for this time
of year helping to mitigate the flash flooding concern by providing
a strong and persistent steering flow. Nevertheless, around an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.

...Midwest.

A complicated forecast exists with several conditional threat
areas that still need to be sorted out. Models are generally in
good agreement in showing several rounds of elevated convection
along the periphery of a pronounced elevated mixed layer, with
700mb temperatures approaching record highs in the sounding archive
and ERA reanalysis dataset. One of the rounds is focused early in
the Day 1 period in or near Missouri. Satellite and radar imagery
from the overnight hours was showing thunderstorm development along
the Kansas/Nebraska border moving westward around the 1000-700 mb
thickness gradient and being fed by south to southwesterly winds of
35 to 50 kts per area VAD Wind profilers upstream over Kansas and
Oklahoma. The 06Z run of the HRRR continued to generate a narrow
stripe of moderate to heavy rainfall in western Kansas...its areal
coverage apparently being in check from the warm mid level
temperatures noted above but providing support to maintain the
Marginal risk area without much need to adjust the placement. The
other round looks to be focused after 00Z farther to the north from
southeast Iowa into northwestern Illinois. Ingredients favoring
heavy to excessive rainfall extend to the north and northwest in
so those solutions extending as far north and northwest into
Minnesota or southeast South Dakota cannot be ruled out yet.
Multiple hi- res models also show much of the northern sections of
the region remaining almost entirely dry. However, the risk should
an inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.be assumed to
be non- zero in the areas described above, and above 5 percent in
the Marginal Risk areas. Further updates and expansion of the
Marginal Risk is possible.

...South Florida..

Incoming deep moisture with roots from the Caribbean will make
headway into South Florida today with area precipitable water
around the 90th percentile, which is well above 2 inches. The
increasing moisture will be in proximity to a quasi-stationary
front to the north...with a sharp delineation point of where the
moisture pooling will reside. A Marginal risk area was maintained
across the Southern FL Peninsula.

Bann


Day 2
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NEW MEXICO AND COLORADO...AND THE MIDWEST...

...New Mexico and Colorado...

As the western periphery of the Southern Plains upper level high
begins to erode, the heavy rain threat area should expand eastward
on Sunday into more of New Mexico and Colorado. Scattered daytime
convection should develop in a plume of deeper moisture and
moderate instability, and the thunderstorms will be capable of producing
flash flooding within complex terrain and especially around any
burn scars. Hourly rain rates may locally exceed 1 inch. The
Marginal Risk was mostly maintained in the same areas.

...Midwest...

The latest guidance shows potential for for organized convection
and an associated heavy rainfall threat to persist into the Day 2
period over portions of southeast Iowa into western Illinois and
northeast Missouri. The latest CSU machine learning Excessive an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.
Rainfall Outlook still appeared to be displaced north and west of
where the ensembles favored heaviest rainfall...and the 00Z runs
did little to resolve the considerable degree of mesoscale
uncertainty both from the overall setup across the region. The
model consensus was best on the idea that heavy rainfall was
possible but they varied widely on placement. Therefore, the
Marginal Risk should still be viewed as the most likely placement
for heavy rain on Sunday morning, but future updates are possible.

Lamers/Bann


Day 3
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST ON MONDAY...

...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but strengthening steering flow should
help limit residence time to mitigate some of the flash flooding
concern. As mentioned earlier...though...even brief heavy rainfall
rates in a short period of time may still lead to flash flooding,
and a Marginal Risk was maintained in the region.

...Northern Plains...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains helping to sharpen a front
across parts of the Upper Midwest/Northern Plains. Precipitable
water values are forecast to approach 1.75 inches over portions of
the eastern Dakotas prior to the arrival of the surface front and
increasing divergence/difluence associated with the upper level
system. The expectation is that moisture flux convergence along the
front will focus convection moving into an unstable environment
driven by diurnal heating that leads to locally heavy downpours.
Farther west where instability is more driven by cold mid level temperatures...rainfall should be less focused and less intense.

Bann
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FOUS30 KWBC 250838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Aug 25 2024

Day 1
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST ...

...Arizona/New Mexico and Colorado...

The axis of highest precipitable water values start the period
extending from eastern Arizona and western New Mexico into
southwest Colorado...with maximum values at or slightly above an
inch near the international border. This should help fuel
additional late day and evening convection...with at least a low-
end chance for rainfall rates that approach an inch per hour.
Flow increases across the region as the western portion of the
upper high erodes and a closed low over Nevada this morning makes
its way eastward...which should help convection be progressive
enough to mitigate at least some of the excessive rainfall threat.
Even so...even brief heavy rainfall can produce flash flooding
within complex terrain and especially around any burn scars.

...Midwest...

With the upper high/warm temperatures firmly in place and
apparently sufficiently strong to suppress buoyancy...and high
resolution models now struggling to generate more than light
showers...have removed the Marginal Risk area over portions of Iowa
and Illinois.

Bann


Day 2
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST...

...Southwest US...

The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow
should help limit residence time to mitigate some of the flash
flooding concern. As mentioned earlier...though...even brief heavy
rainfall rates in a short period of time may still lead to flash
flooding, and a Marginal Risk was maintained in the region.

...Northern Plains...

An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains on Monday...helping to
sharpen a front across parts of the Upper Midwest/Northern Plains.
Precipitable water values are forecast to approach 1.75 inches
over portions of the eastern Dakotas prior to the arrival of the
surface front and increasing divergence/difluence associated with
the upper level system. The expectation is that moisture flux
convergence along the front will focus convection moving into an
unstable environment driven by diurnal heating that leads to
locally heavy downpours. Spread lingers in the guidance with
respect to how progressive the system will be and how much of
excessive rainfall threat will be associated with the upper system
compared to the threat associated with the convection pushing into
the better instability. Kept the westward extension introduced on Saturday...but there was a consensus that some territory could be
trimmed out near the international border.

Bann


Day 3
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE UPPER GREAT LAKES...

...Southwest US...

The risk of excessive rainfall will linger on Tuesday afternoon and
evening across portions of the Southwest US as a moisture plume
lingers across the area and winds aloft/steering flow weakens in
response to the a mid level wave moves away over the Northern
Plains as it begins to fill. Model based ingredients suggest the
potential for locally heavy downpours may extend a bit west of the
area shown by the deterministic runs...so made a minor expansion
that way.

...Upper Great Lakes...
There is a Marginal risk of excessive rainfall over portions of the
western Great Lakes as the upper trough initially over the Upper
Midwest continues to move northeastward during the day.
Precipitable water values of 1.75 to 2 inches should already be in place
before the flow aloft becomes increasingly difluent along and
north of a quasi-stationary boundary helping enhance rainfall rates
locally. The operational guidance tended to the potential for a
couple of inches of rain...although there was considerable north-
south spread in the guidance limiting the confidence that excessive
rainfall will occur at any spot.

Bann
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FOUS30 KWBC 260833
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 26 2024

Day 1
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...

...Summary...
Per collaboration with WFOs ABR, FSD, MPX, and ARX, have hoisted a
Slight Risk across eastern SD into southern MN, western WI, and a
small portion of northeast IA. Have also added Marginal Risk areas
across the Mid-Upper TX Coast to eastern portions of the Hill
Country, much of the west coast of FL, and across parts of New
England.

...Northern Plains into the Upper Mississippi Valley...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains on Monday, becoming less
amplified with time as it encounters the central U.S. ridge.
Precipitable water ahead of the upper trough and associated surface
cold front area expected to peak between 1.75-2.00 inches over the
outlook areas. Pre-frontal surface low tracking slowly from northeast NE-southwest SD early Mon to the eastern MN-western WI border by
12Z Tue will coincide with a slower eastward frontal progression
later Monday and especially Monday night. As a result expect
multiple rounds of organized convection across the outlook areas,
with multicellular upscale growth potential along with some
supercells. MLCAPEs of 3000-4000+ J/Kg this afternoon-evening within
the very moist environment will support intense instantaneous
rainfall rates; however, given the relatively weak llvl flow
compared to the mid-upper levels, expect the convection to be
forward propagating at a fairly decent clip through this evening.
This will cut down on the rainfall potential during the daylight
hours. However, given the slow moving surface low and front, again
expect another round of convection Mon night as PWs remain between
1.75-2.00 inches with MUCAPEs still between 1000-2000 J/Kg. The
aggregate of both events will likely enhance the flash flood risk
within the Slight Risk area, given the growing support from the
high-res CAMs.

Elsewhere, the Marginal Risk area remains, though did pull the
southern edge south a bit into much of southern WI (including the
MKE area) towards the max instability gradient.

...Mid-Upper TX Coast to Eastern Portions of the Hill Country...
Well defined low-mid level circulation near the TX Gulf Coast early
this morning will traverse slowly into South-Central TX this
evening and overnight. Instability will remain greatest offshore,
though SBCAPEs near 1000 J/Kg distributed in a tall/skinny profile,
with WBZ levels over 13 kft and PWs of 2.00-2.25 inches, will
produces areas of heavy rain. Given the weak deep-layer flow (0-6km
bulk shear 20kts or less), expect the more intense rainfall rates
to be short lived before the more intense updrafts collapse as the
cells become outflow dominated. Therefore have opted for a Marginal
Risk over this region, with the anticipation that any flash
flooding/short term runoff concerns will be isolated.

...Western FL Peninsula...
The guidance shows a more enhanced ECSB (East Coast Sea Breeze)
given the added synoptic low-level easterly component. This will
focus peak diurnal convective coverage along the west coast of the
FL Peninsula, which is supported by the model QPFs (especially
CAMs). PWs between 2.1-2.3 inches, along with SB CAPEs peaking
between 1500-2500 during the afternoon and early evening will
support intense short term rainfall rates. In fact, the 00Z HREF
40km neighborhood probability of 2"/hr rainfall rates climbs to
40-60% over/near the Tampa Bay area between 20-00Z (while 3"/hr
rain rates climb to around 25%). Per the CAMs, localized totals of
3-5" can be expected, much of which would be within a 3 hour
period. Cannot rule out localized flash flooding as a result,
particularly in urban areas in and around the Tampa Bay area.

...Parts of New England...
Vigorous shortwave energy and compact LFQ forcing pivoting south-
southeast on the western periphery of the upper low will set the
stage for fairly robust deep-layer forcing over a relatively narrow
corridor across parts of New England Today. The mid/upper ridge
impinging from the west is likely causing the uptick in upper level
flow (localized 90-100kt 250mb jet streak) across the Hudson
Valley/eastern NY. Steepening mid level lapse rates along with
MLCAPEs ~1000 J/Kg will help fuel numerous showers/tstms during the
peak heating hours this afternoon, with storm motions south-
southeast. The 00Z HREF depicts high probabilities of >1"/hr rates
between 18-00Z, peaking between 60-70% across eastern
CT/RI/southeast MA. Localized storm totals of 3+ inches may lead to
isolated instances of flash flooding.

...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow
should help limit residence time to mitigate some of the flash
flooding concern. As mentioned earlier, although even brief heavy
rainfall rates in a short period of time may still lead to flash
flooding, and a Marginal Risk was maintained in the region.

Hurley


Day 2
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...

...Summary...
Have expanded the Marginal Risk southward across the Upper Midwest,
to account for the potential of locally heavy rainfall before the
frontal passage.

...Upper Midwest-Upper Great Lakes...
Upper shortwave will continue to dampen somewhat by Day 2, which
will weaken the deep-layer Qs forcing. Prior to the surface frontal
passage, PWs will remain between 1.75-2.00 inches (around 2
standard deviations above normal). Instability will be much greater
over the southern portion of the outlook area (down towards the
I-80 corridor, which was a big reason to nudge the ERO Marginal
Risk farther south (matching up with SPC's Day 2 Severe Slight
Risk). Expect the activity to move rather swiftly before dropping
south of the outlook area Tue night. Still appears to be a
localized flash flood risk though, given the weakening deep-layer
forcing and degree of downwind propagation. Therefore, have
continued with the Marginal Risk.

...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest US as a moisture plume
lingers across the area and winds aloft/steering flow weakens in
response to the a mid level wave moves away over the Northern
Plains as it begins to fill. Model based ingredients suggest the
potential for locally heavy downpours may extend a bit west of the
area shown by the deterministic runs...so made a minor expansion
that way.

Hurley
---
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FOUS30 KWBC 270841
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Tue Aug 27 2024

Day 1
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...

...Summary...
Compared to yesterday's Day 2 ERO, we have expanded the Marginal
Risk area to include much of the Upper to Mid MS Valley, along with
parts of the Lower MO Valley. Only minor tweaks were made to the
Marginal Risk area across the Southwest.

...Lower Missouri Valley-Mid to Upper Mississippi Valley-Upper
Great Lakes...
Upper shortwave over the Northern Plains early this morning will
continue to dampen somewhat during the day, which in term will
weaken the deep-layer Qs forcing. By the same token, as the
shortwave becomes sheared, the confluence aloft will sharpen a
compact upper level jet streak (90-100 kts) in the lee of the
flattening shortwave, thereby increasing the Qn forcing within the
right entrance region (strengthening the low-level frontogenesis).
The guidance, including the CAMs, appears to be having a tough time
resolving this transition of favorable dynamical forcing. The
ensemble of model QPFs show a bimodal distribution with the max
rainfall axes -- one to the north (central MN-northern WI-Upper
MI), behind Monday night MCS, where the DPVA/Qs forcing is most
favorable, and another farther south (northeast KS/southeast NE,
southern IA, central-northern MO, much of IL-IN), where the uptick
in right entrance region forcing (upper divergence and low-level
frontogenesis) coincides with a more moist and unstable
environment. TPWs climbing between 1.8-2.1 inches over this
southern portion, along with ML CAPEs of 1500-2500+ J/Kg, will
support locally intense shorter-term rainfall rates. Moreover, as
the front flattens (becomes more W-E oriented), more parallel to
the mean 850-300 mb flow, suspect cell training will become a
greater risk. What will likely keep this event from needing a
Slight Risk is the relatively weak low-level inflow. The mean flow
is fairly weak as well (850-300 mb wind 15-20 kts), however there
is sufficient shear to keep any organized segments forward
propagating.

Farther north, the instability won't be as robust, however the
soils will have already been primed from the MCS Monday night.
Therefore with this secondary max QPF axis that the 00Z CAMs are
depicting, another round of heavy rainfall could lead to isolated
flash flooding given the antecedent wet soils and reduced FFG.

...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest U.S., as the moisture
plume lingers across the area and winds aloft/steering flow
weakens in the wake of the departing Northern Plains shortwave. PWs
peaking around 1.3 to 1.4 inches (1.5 to 2 standard deviations
above normal for late August), along with MLCAPEs of 500-1000 J/Kg,
will support isolated hourly rain rates of 0.75-1.00" underneath
the strongest convective cores. The 00Z HREF probabilistic
guidance depicts this, with the risk spreading a bit farther west
(into eastern AZ) compared to yesterday. Have maintained the
Marginal Risk in the Day 1 ERO given the continued localized flash
flood threat during the peak heating hours (afternoon/evening).
evening.

Hurley


Day 2
Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND
PARTS OF THE MIDWEST INTO THE OHIO VALLEY AND NORTHERN MID-ATLANTIC...

...Summary...
Compared to yesterday's Day 3 ERO, the Southwest and northern-tier
Marginal Risk areas were largely unchanged. However, based on the
00Z guidance suite, the Marginal Risk across the northern Mid
Atlantic Region was expanded westward to include much of the OH
Valley and parts of the Mid MS Valley.

...Southwest...
Guidance mid-level vorticity progs continue to support a more
north-south outlook area compared to Day 1, though nonetheless
still quite similar. Instability and moisture parameters (afternoon
and evening CAPE and PW profiles) remain similar, and thus the
anticipation of more isolated/localized instances of flash flooding
than otherwise, particularly after 18Z coinciding with peak
diurnal heating.

...Upper Midwest...
Strong, deep-layer warm and moisture advection ahead of the next
vigorous mid-upper level trough (closed 500-300 mb low) and
associated surface fronts will favor more organized/widespread
elevated convection by Wed night ahead of the advancing low-level
warm front. Models are still indicating spread with the QPFs,
enough to keep the ERO risk level at Marginal for now. Later
shifts will need to assess the need to upgrade at least a portion
of the outlook area to Slight however, as the heavy rainfall signal
exists given the degree of dynamical forcing and thermodynamical
profiles. 00Z ECMWF and regional GEM in fact indicates localized
totals of 3+ inches between 00-12Z Thu.

...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Northern Mid Atlantic...
W-E frontal boundary will sink southward on Wednesday, then remain quasi-stationary Wed night as the mid- upper trough traverses the central-northern Rockies and High Plains. Deep-layer moisture and
instability will be sufficient for areas of heavy rainfall along
this boundary, while the easterly storm motions (parallel to the
low-mid level fronts) would support the risk of cell training.
Outside of the 00Z Regional GEM, the guidance at this point isn't
overly robust with the QPF within the outlook area, at least not
yet, despite some uptick in right- entrance region forcing from the
upper level jet streak over northern NY-New England. Thus for now,
have kept the ERO risk at Marginal.

Hurley


Day 3
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL PLAINS INTO THE UPPER MIDWEST, ALONG WITH THE MID ATLANTIC REGION...

...Summary...
Few changes were made to yesterday's Day 4 ERO Marginal Risk areas
across the Central Plains to Upper Midwest and over the Mid
Atlantic.

...Eastern Portions of the Central Plains into the Upper Midwest... Aforementioned upper trough and associated surface cold front will
continue eastward on Thu-Thu night, though with the mid-upper low
lifting north into western Ontario by 12Z Fri. This will keep the
bulk of the deep-layer forcing north of the U.S., however
sufficient low-level frontogenesis will persist ahead of the mid-
upper shortwave well south of the main mid-upper low. Much
cooler/drier post-frontal airmass will lead to a fairly tight
moisture gradient along the front, with strengthening negative PW
advection behind it. Given the anticipated swift movement of the
front (strong deep-layer shear), for now it would appear any
organized, more widespread linear convective segments ahead of the
front will too be rather progressive. Therefore, have maintained
the Marginal Risk area.

...Mid Atlantic Region...
Lingering quasi-stationary boundary across the Mid Atlantic will
couple with increasing right entrance region forcing via an upper
level jet streak traversing NY-New England. Over time, the CAD
signature becomes evident from the guidance -- i.e. cooler/more
stable low-level air and a south-southwest dip in the surface
boundary occur along and near the Mid Atlantic Coast. However
plenty of deep-layer instability should linger west of the
DelMarVa, which along with the forcing, should allow for more widespread/organized convection and a localized flash flood threat.

Hurley
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FOUS30 KWBC 280829
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Wed Aug 28 2024

Day 1
Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND
PARTS OF THE MIDWEST INTO THE OHIO VALLEY AND NORTHERN MID-ATLANTIC...

...Summary...
Few changes were made to yesterday's Day 2 ERO Marginal Risk
areas.

...Southwest...
Guidance mid-level vorticity progs continue to support a more
north-south outlook area compared to Day 1, though nonetheless
still quite similar. Instability and moisture parameters (afternoon
and evening CAPE and PW profiles) remain similar, and thus the
anticipation of more isolated/localized instances of flash flooding
than otherwise, particularly after 18Z coinciding with peak
diurnal heating.

...Eastern Dakotas-Western Minnesota...
Strong, deep-layer warm and moisture advection ahead of the next
vigorous mid-upper level trough (closed 500-300 mb low) and
associated surface fronts will favor more organized/widespread
elevated convection by Wed night ahead of the advancing low-level
warm front. After 00Z, 850 mb southerly flow increases to 40-50 kts
from west to east across the outlook area, which is around 4
standard deviations above normal per the SREF and GEFS. Not
surprisingly, 850-700 mb moisture flux anomalies also climb to +3
to +4. Given the strength of the low-mid layer moisture transport
and uptick in IVT, isolated QPF totals of 3-5" per some of the 00Z
CAM runs should not come as a surprise. However, at least for now,
reasons for keeping the ERO below the Slight threshold over this
region Wed night include (a) the continued high spread in guidance
QPFs (including CAMs), along with (b) progressive southeast motion
of the convection growing upscale (MCS) per the forecast Corfidi
Vectors, given the shear between the low-level flow and mean
850-300 mb wind.

...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Northern Mid Atlantic...
W-E frontal boundary will sink southward on Wednesday, then remain quasi-stationary Wed night as the mid-upper trough traverses the central-northern Rockies and High Plains. Deep-layer moisture and
instability will be sufficient for areas of heavy rainfall along
this boundary, while the easterly storm motions (parallel to the
low-mid level fronts) would support the risk of cell training,
especially when the Corfidi Vectors drop to 5 kt or less). WPC QPF
and smart blends (NBM and HREF) have come up from the previous
forecast, with the D1 period now fully within the high-res CAM
window. Uptick in right-entrance region forcing (upper divergence...
low-level FGEN) from the upper level jet streak over northern NY-
New England will favor more widespread convection, especially when
coinciding with peak diurnal heating. However, lack of robust low-
level moist inflow/transport will likely not sustain/prolong strong
updrafts and intense hourly/sub-hourly rainfall rates. As a
result, have maintained the Marginal Risk from yesterday's Day 2
ERO, albeit with a few tweaks based on the latest guidance trends.

Hurley


Day 2
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
LOWER MISSOURI VALLEY INTO THE UPPER MISSISSIPPI VALLEY...

...Summary...
The biggest change from yesterday's Day 3 ERO was to upgrade a
portion of the Marginal Risk to Slight across parts of the Lower MO
Valley into the Upper MS Valley. Have also added a Marginal Risk
across the central-western LA and Middle-Upper TX Gulf Coast.

...Lower MO Valley and Corn Belt into the Upper MS Valley...
Aforementioned upper trough and associated surface cold front will
continue eastward Thu-Thu night as the mid-upper low lifting north
into western Ontario by 12Z Fri. Given the progression of synoptic
features, the bulk of the deep-layer forcing and more anomalous
low-level inflow/moisture flux will pivot across the Upper MS
Valley and southern Canada. Compared to the southern portions of
the Slight Risk area, generally speaking instability will not be
as high over the northern portion (i.e. along the path of the
triple point). However, owing largely to the SSW LLJ, especially as
it becomes more aligned with the mean 850-300 mb flow, the
eastward progression of the quasi-linear convective segments will
slow during the period as per the weakening Corfidi Vectors
(decreasing forward propagation). Therefore the risk for cell
training will increase along and ahead of the front.

Farther south (IA and eastern NE), the pre-frontal activity should
be more forward-propagating, however much higher deep-layer
instability south of the triple point, especially by late Thu-Thu
night, will maintain the risk of more intense hourly rainfall
rates (1.75-2.00+ inches) a little while longer (beyond 00Z Fri)
per the 00Z HREF exceedance probabilities.

...Mid Atlantic Region...
Lingering quasi-stationary boundary across the Mid Atlantic will
couple with increasing right entrance region forcing via an upper
level jet streak traversing NY-New England. Over time, the CAD
signature becomes evident from the guidance -- i.e. cooler/more
stable low-level air and a south-southwest dip in the surface
across MD-VA. However plenty of deep-layer instability should
linger west of the DelMarVa, especially west of I-95, which along
with the forcing, should allow for more widespread/organized
convection and a localized flash flood threat.

...Louisiana and Middle-Upper Texas Coast...
Underneath the broad upper ridge, models show a subtle mid-upper
level shortwave lifting gradually northward toward the central-
western LA Coast on Thu. Subtle upper-level forcing (DPVA/upper
divergence) along with persistent (albeit not overly robust) low-
level moisture transport from the GOMEX will make for a localized
flash food risk, especially coinciding with peak diurnal heating
Thu afternoon. PW values increasing to 2.3-2.4 inches along with
MLCAPEs between 1000-2000 J/Kg (tall/skinny distribution) and WBZ
heights climbing to aoa 15,000 feet will establish an optimal
thermodynamic profile for heavy rainfall. Sub-hourly rainfall rates
of 2-3" will be possible underneath the strongest cores. By the
same token, 0-6km bulk shear generally 20kts or less will lead to
more pulse-type convection and thus more short-lived intense
rainfall rates. Still enough of a flash flood risk to support a
Marginal Outlook area in the ERO, especially given the uptick in
southeasterly low-level flow from the Gulf on Thursday, which
would be of equal or greater magnitude compared to the 850-300 mb
wind. Corfidi Vectors aligning in the opposite direction than the
inflow would support increased upwind propagation and cell
training.

Hurley
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FOUS30 KWBC 290823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Thu Aug 29 2024

Day 1
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST AND THE MID ATLANTIC AS WELL AS ALONG THE GULF COAST...

Lower Missouri Valley to the Upper Midwest...

Height falls associated with an upper trough and surface cold
front will continue eastward today and tonight as a deep mid-
upper low lifts north into western Ontario by 12Z Fri. Given the
progression of synoptic features, the bulk of the deep- layer
forcing and more anomalous low-level inflow/moisture flux will
pivot across the Upper MS Valley and southern Canada. HREF
neighborhood probabilities for greater than 2 inch and greater than
3 inch amounts remain fairly aggressive (40-70%) within the
previously issued Slight risk area with the core of the potential
being located over parts of Iowa and southern Minnesota. As
mentioned before...this area has been dry as of late, so some of
the precip will be beneficial, but anomalous moisture and strong
mid- level ascent could generate rainfall rates capable of
flooding, especially within any urban footprint.

Mid Atlantic Region...

Maintained the Slight risk area across the VA Piedmont down into
portions of Southeast VA and as far north as the WV/MD/PA line with
only minor modifications. There will be a fairly well-depicted
axis of low- level instability and anticipated surface convergence
in proximity to a backdoor front settling in to the area. A core
of elevated theta-E will align with the front from northwest to
southeast during the afternoon hours while a jet streak to the
northeast catches the Central Mid Atlantic just within the right-
entrance region of the jet leading to appreciable upper forcing
during peak destabilization. Convective activity is expected to initiate
across the Blue Ridge and adjacent Appalachian Front then drift
eastward. various CAMs and global guidance. MLCAPE between
2500-3500 J/kg will be common within the vicinity of the front, so
ample surface based instability will be present to maximize
potential of any thunderstorm development.

Louisiana and Middle-Upper Texas Coast...

Models show a subtle mid-upper level shortwave lifting gradually
northward toward the central- western LA Coast today and tonight.
Subtle upper- level forcing along with persistent (albeit not
overly robust) low- level moisture transport from the Gulf of
Mexico will result in a localized flash food risk, especially
coinciding with peak diurnal heating . Precipitable water values
increase to 2.3-2.4 inches along with MLCAPEs between 1000-2000
J/Kg and WBZ heights climbing to aoa 15,000 feet will establish an
optimal thermodynamic profile for heavy rainfall as shown in the
NAM and ARW guidance. This should result in rainfall rates of 2-3
inch on a sub-hourly time scale will be possible underneath the
strongest cores. Given that the overall forecast reasoning has not
changed and the latest and the latest guidance continues to show 3
to 5+ inch amounts hugging the coastline...saw little reason to
make more than minor nudges to the Slight risk and Marginal risk
areas.

Bann


Day 2
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE
GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA ...

Gulf Coast...

There is a Slight Risk of excessive rainfall along the immediate
Gulf coast region of southeast Texas into a small portion of
adjacent Louisiana...with a Marginal risk area extending as far
north as the upper Great Lakes in response to gulf moisture
streaming northward ahead of a well-defined cold front advancing
eastward. In addition...a well defined circulation that has
persisted a couple of days over the Gulf of Mexico and Texas will
meander eastward into Arkansas. While its impact will be waning as
the system fills...it still could allow for additional convective
flare-ups along and near the Gulf of Mexico. Precipitable water
values of 2 to 2.25 inches are widespread from eastern Texas into
Louisiana and Arkansas with an axis then extending north and east
ahead of the front being some 2 to 2.5 standard anomalies above
climatology being an indication of the moisture-laden atmosphere
still in place. Discrepancies linger in the guidance on placement
and amounts in where the deepest moisture gets funneled...with the
above- mentioned upper level feature playing a key role. Between
the NBM still showing 2 to 4+ inch amounts along the southeast
Texas coast on Day 2 alone and a more than 6 inches for a 2-day
total in proximity to the 28/12Z guidance and already covered by
yesterday's Day 3 outlook...saw little reason to make any changes
to the Slight Risk area.

Great Lakes to Southern Rockies/Southern High Plains...

Convection will continue to be driven by deep gulf moisture that is
initially drawn northward and then northeastward towards the Great
Lakes ahead of a well defined cold front pushing southward and
eastward. By 00Z Saturday...moisture anomalies of 2 standardized
anomalies will be in place. The overall upper forcing still does
not appear to be terribly robust but the diurnal instability
supports at least the potential for locally excessive rainfall.

Bann


Day 3
Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG A SMALL
PORTION OF THE UPPER TEXAS COAST AND ADJACENT COASTAL LOUISIANA...

There is a Slight risk of excessive rainfall once again over
portions of the Upper Texas coast and adjacent portions of
Louisiana with a broad Marginal risk are extending northward and
eastward as gulf moisture streams northward ahead of...and
eventually intersects with...a cold front. The 29/00Z suite of
guidance focuses highest rainfall amounts from Saturday into early
Sunday over portions of the Upper Texas coast and nearby Louisiana
where a moisture plume with precipitable water values in excess of
2.25 inches should be positioned at the start of the period (a
nearly 2.5 standardized anomaly greater than climatology for this
time of year and above the 90th climatological percentile along the
gulf coastline shown by both the ECENS and GEFS). Two inch
precipitable water values are forecast to extend from eastern Texas northeastward in a corridor to far southern Indiana. During the
ensuing 24 hours...the northern portion of the front reaches
portions of New England and the Mid-Atlantic region but the
southern portion of the front makes little progress into Texas and
Arkansas. Expectations are that downpours will be possible along
and ahead of the front. Given the amount of moisture and weaker
steering flow, though, the southern portion of the Marginal area
has a better chance for excessive rainfall while storms to the
north should be more progressive leading to a nominally low/more
isolated excessive rainfall risk. The question remains whether or
ot the stronger dynamics farther north will enhance amounts/rates.
Will leave as a Marginal risk area for the time being in deference
to the NBM amounts and guidance from the CSU ML EROs.

Bann
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FOUS30 KWBC 300826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Fri Aug 30 2024

Day 1
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE
GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...

...Gulf Coast...

Moderate to heavy rainfall expected along the Gulf coast as deep
moisture advects northwards as a tropical low drifts in the
northwest portions of the Gulf of Mexico. Repeated rounds of slow-
moving heavy rain cells from the Gulf into the Texas/Louisiana
border area east of Houston. Guidance depict 1 to 4 inches for
areal average along the coast with isolated higher amounts
possible. Some of the hi-res guidance have very isolated maximums
of 6 to 11 inches. Areal averages of The focus for any potential
flooding in this small area will be mainly in the urban areas
around Beaumont/Port Arthur and points east. A Slight Risk is in
effect from the Houston area east to Vermillion Bay. The
surrounding Marginal risk was expanded to include areas near/west
of Mobile.

...Great Lakes to Southern Rockies/Southern High Plains...

Convection will continue to be driven by deep gulf moisture that
is streaming into the north/northeast towards the Great Lakes
ahead of a well defined cold front pushing southward and eastward.
Precipitable water values of 2 to 2.25 inches are widespread from
eastern Texas into Louisiana and Arkansas with an axis then
extending north and east ahead of the front being some 2 to 2.5
standard anomalies above climatology being an indication of the
moisture-laden atmosphere still in place. There continues to be
some variance on where the deepest moisture gets funneled... with
the above- mentioned upper level feature playing a key role.
Although the overall upper forcing still does not appear to be
terribly robust, the diurnal instability supports at least the
potential for locally excessive rainfall. A broad Marginal Risk was
maintained for this period from New Mexico/Texas to Wisconsin/New
York.

...Mid-Atlantic...

Portions of the region will have PW values greater than 2 inches
along with slow moving storms near the front. This environment
looks to be quite favorable for localized heavy rain with some of
the guidance showing probabilities of 3 inches/3 hour 30-50%
across Northeast North Carolina. A Marginal Risk area was raised
for portions of southeast Virginia and northeast North Carolina.

Campbell


Day 2
Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG A SMALL
PORTION OF THE UPPER TEXAS COAST AND ADJACENT COASTAL LOUISIANA...

...New Mexico and Texas...

Showers and thunderstorms are expected to fire across portions of
New Mexico and Texas as the trailing cold front slows its
progression over the region. Some of these storms will be capable
of producing hourly rain rates greater than 0.50/1 inch per hour.
A Marginal Risk area was maintained for this part of the country.

...Gulf Coast...

Showers and thunderstorms will persist along the Gulf coast along
the nearly stationary tropical low and frontal boundary. The Slight
Risk area that was in effect has bee maintained as well as expanded
eastward to just east of Vermillion Bay. A broad Marginal risk area
extends northward and eastward as gulf moisture streams northward
ahead of...and eventually intersects with...a cold front. Guidance
shows the highest amounts occurring from Saturday into early
Sunday over portions of the Upper Texas coast and nearby Louisiana
where a moisture plume with precipitable water values in excess of
2.25 inches should be positioned at the start of the period (a
nearly 2.5 standardized anomaly greater than climatology for this
time of year and above the 90th climatological percentile along the
gulf coastline shown by both the ECENS and GEFS).

...Lower Mississippi Valley to New England...

Two inch precipitable water values are forecast to extend from
eastern Texas northeastward in a corridor to far southern Indiana.
During the ensuing 24 hours...the northern portion of the front
reaches portions of New England and the Mid-Atlantic region but the
southern portion of the front makes little progress into Texas and
Arkansas. Moderate to heavy rain will be possible for areas along
and ahead of the advecting boundary.

Given the amount of moisture and weaker steering flow, though, the
southern portion of the Marginal area has a better chance for
excessive rainfall while storms to the north should be more
progressive leading to a nominally low/more isolated excessive
rainfall risk. The question remains whether or ot the stronger
dynamics farther north will enhance amounts/rates. Some of the
guidance do signal a concentration of QPF and increasing threat for
excessive rainfall over portions of central pennsylvania.
Antecedent conditions will have lowered FFGs across this part of
the region thus becoming more favorable for growing flooding
concerns. After coordination with the local forecast office decided
to hold off for another cycle to reassess the need for a possible
upgrade to a Slight Risk for central portions of Pennsylvania.

Campbell/Bann


Day 3
Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024

...New Mexico and Texas...

Showers and thunderstorm activity is expected to linger with the
capability of producing excessive rainfall. The threat for
localized flood concerns will remain elevated thus a Marginal Risk
was maintained for this period.

...Gulf Coast...

The highest rainfall in this multi-day heavy rain event will have
likely already occurred however showers and thunderstorms are
expected to linger over the region near the coastline. A Marginal
Risk remains in affect from the southeast Texas coastline to
eastern Louisiana coastline.

...Eastern Tennessee Valley and Mid-Atlantic region...

Moderate to locally heavy rainfall will persist across portions of
the Mid-Atlantic as a cold front approaches the region. Some of
these areas will have lowered FFGs by this period and may be more
susceptible for additional rainfall to increase the threat for
local flooding concerns. A Marginal Risk area was maintained from
eastern Tennessee Valley to the Mid-Atlantic region.

Campbell
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FOUS30 KWBC 310823
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sat Aug 31 2024

Day 1 Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE GULF COAST
FROM GALVESTON TEXAS THROUGH THE MISSISSIPPI COAST AS WELL AS FOR
THE CENTRAL APPALACHIANS...

...Texas to the Mid-Atlantic and New England...

A slow-moving cold front over the Midwest will move into the
central Appalachians this afternoon. Surface dew points in the 60s
and rising precipitable water values to around 1.75 inches (+2
sigma or 98th percentile) will support both a severe weather threat
(see SPC outlook) and a heavy rain threat. Though convection
should be progressive with the front (or pre-frontal trough), FFG
values are around 1-2"/hr and 00Z HREF neighborhood probs show
values rising to 40-80% for >1"/hr and 10-40% for >2"/hr. Focus of
highest threat will lie from eastern WV northward through central
PA and into the Southern Tier of NYS where the complex terrain can
add to the flash flooding sensitivity coincident with slightly
higher than average soil moisture. Maintained the Slight Risk from
the previous forecast with little change in shape. Along the front
in general, a Marginal risk was maintained from New England back
through the mid-MS Valley/Mid-South into TX where convection will
be a bit less coherent and the threat for any flash flooding will
be lower, but non-zero.

...Central Gulf Coast...
Decaying mid-level vort max will drift eastward along the coast,
detached from any surface trough just offshore. However, in-situ
moisture remains highly anomalous -- precipitable water values
around 2.25 inches which is about +2 to +3 sigma. Most of the hi-
res guidance keeps the heaviest rainfall today along and just off
the coast but hint at drawing some heavier cores inland a bit
toward the I-10 corridor in LA. Maintained the Slight Risk outline
for southern Louisiana given the continued threat of heavier rain
today.

...Florida...
Above normal moisture (PW > 2") will linger over southeast FL
where afternoon convection could support some local downpours
(2-3"/hr rates) that may induce some flooding over the urban areas
from ~PBI to MIA/HST and perhaps into the Keys.

...Arizona...
Added a Marginal Risk for the Mogollon Rim as a weak mid-level
vort center pushes through the region. Though precipitable water
values are around normal, sufficient instability this afternoon
could support some isolated heavier rain cores with 1"/hr rates.

Fracasso


Day 2 Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC...

...Southeastern New Mexico and West Texas...
250mb upper trough moving out of northern Mexico and southern
AZ/NM and mid-level vort max (perhaps weakly closed low) will creep
closer to far western Texas day 2. Sufficient moisture (though
near normal) will be in place over the region to sustain some
isolated excessive rainfall via afternoon convection.

...Gulf Coast...
Meandering weakness in the mid/lower-levels will again help drive
some heavier rainfall along the Upper TX Coast into southern
Louisiana. Maintained a Marginal Risk for now given the uncertainty
in the day 1 period with respect to heavier rain placement (on or
offshore) and amounts. Again, the highest rainfall may linger just
off the coast but potential is certainly there to nudge inland.

...Eastern Tennessee Valley and Mid-Atlantic region...
Cold front will continue to push through the area to the east and
south, squeezing the best forcing into southern Virginia and into
North Carolina where afternoon convection may yield 1-2" amounts
and a localized threat of flash flooding. There and to the west
over western VA into WV, FFG guidance is lower and some lingering
rain may eclipse those values as the front works through.

Fracasso


Day 3 Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS AND
THE CAROLINAS...

...Texas...
Mid-level vort and upper trough will move into West Texas on
Monday atop increased moisture from the southeast. Guidance
continues to show modest QPF of 1-5" in various locations but the
spread is quite large -- nearly covering the whole state in their
QPF max region among the deterministic, ensemble, and AI guidance.
The CSU machine learning first guess fields suggest a broad Slight
Risk over the Hill Country which is, perhaps, a best consensus
placement, but prefer to hold off an upgrade to a Slight Risk for
now until at least the ensembles come into better agreement.

...Carolinas...
Cold front will sink into the Carolinas which will nudge the
excessive rainfall threat south from day 2. Best dynamics move
offshore with the trough axis but the Marginal Risk should cover
any isolated flooding threat due to afternoon convection.


Fracasso
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FOUS30 KWBC 010838
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Sep 1 2024

Day 1
Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC...

...Southeastern New Mexico and West Texas...

An approaching 250 mb upper trough will move into far West Texas
today while a surface boundary lingers across the Hill Country.
Precipitable water values are forecast to be around 1.5-1.75" which
is about +1 to +2 sigma with the typical NW to SE gradient.
Afternoon convection should percolate today and could fall over
areas that have seen a few inches of rain on Saturday (with lower
FFG values as a result). Coverage should be somewhat isolated so
the Marginal Risk will suffice.

...Gulf Coast...

Meandering weakness in the mid-lower levels will again help drive
some heavier rainfall along the Upper TX Coast into southern
Louisiana. Maintained the Marginal Risk as the bulk of the guidance
keeps the heaviest rain offshore, though it will be close (again)
just like on Saturday.

...Eastern Tennessee Valley and Mid-Atlantic Region...

A cold front will approach the Appalchians this afternoon as the
lead boundary dissipates, leaving a trough over the coastal plain.
The best forcing for afternoon convection will lie there, generally
over far southeastern Virginia into central/eastern North Carolina.
00Z HREF probs of exceeding FFG values are around 10-20% (via 1-2
inch per hour rates), though there are hints at some higher rates
still. Activity should be mostly progressive, suggesting a
localized threat of flash flooding. Kept the Marginal Risk for this
area.

Fracasso


Day 2
Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
HILL COUNTRY OF TEXAS...

...Texas...

Mid-level vort and upper trough will move into West Texas on
Monday atop increased moisture from the southeast. Guidance
continues to show modest QPF of 1-5 inches near and around the Hill
Country, with various foci in the deterministic models. ECMWF-AIFS
was still decidedly north compared to the other models. The CSU
machine learning first guess fields still suggest a broad Slight
Risk over the Hill Country, so at least there is some consistency
over the past 24-36 hours in this guidance vs the models (which is
the point). FFG values are lowest near/south of San Angelo to Del
Rio (due to recent rain) where the HREF probs of exceeding these
values reach ~50%.

...Carolinas...

Cold front will sink farther into the Carolinas from day 1, which
will nudge the excessive rainfall threat south as well. Best
dynamics move offshore with the trough axis so the Marginal Risk
should cover any remaining isolated flooding threat due to
afternoon convection.

Fracasso


Day 3
Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS...

...Texas...

Mid-level vort will be exiting the region though precipitable
water values will remain high (+1 to +2 sigma initially, then
decreasing late Tuesday). Depending on how much (and where) rain
falls on day 2, there is room to upgrade some of this region to a
Slight Risk pending better model agreement in future runs. For now,
ensemble consensus is just east of the Hill Country but individual
models are quite varied north/south. First guess fields from the
CSU machine learning algorithm do show a Slight Risk outline,
lending credence to a future upgrade.

Fracasso
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FOUS30 KWBC 020830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Mon Sep 2 2024

Day 1
Valid 12Z Mon Sep 02 2024 - 12Z Tue Sep 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE HILL COUNTRY/CENTRAL TEXAS AND ALONG THE TEXAS GULF COAST...

...Texas...

An upper level trough with a mid-level vort will be advancing into
western Texas during this period. Meanwhile a surface low hugging
the South Texas Coast will maintain easterly flow from the Gulf
allowing PW values of 2+ inches to pool over much of the state. The
latest guidance continues to depict areal averages of 1 to 5 inches
of QPF across the Hill Country into east-central Texas and along
much of the Texas coastline. Recent rainfall has increased soil
saturation across the state thus lowering FFGs. The hi-res guidance
is showing training of slow moving convection capable of 1 to 2.5
inches/hour rain rates which could easily lead to scattered areas
of excessive rainfall and localized flash flooding. A Slight Risk
areas remains in effect for west-central and central Texas.
Additionally, a Slight Risk was added for the coastline today where
storms will be producing rates of 1 to 3 inches/hour.

...Carolinas...

A cold front sinking through the Mid-Atlantic region will continue
to move showers and thunderstorms southward over the Carolinas
today. Guidance keeps the bulk of the QPF offshore during this
period where there are better dynamics in place however there will
still be an isolated threat for localized excessive rainfall and
flooding. A Marginal Risk area was maintained for eastern portions
of the Carolinas.

Campbell


Day 2
Valid 12Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL TEXAS...

...Texas...

The mid-level vort is expected to exit the region although
precipitable water values will remain high (+1 to +2 sigma
initially, then decreasing late Tuesday). A large portion of the
flooding potential will be contingent on a verified forecast from
Day 1/Monday, as Monday will have the heavier rain/stronger storms
of the two days. However, even the lesser amounts of rain expected
Tuesday should still cause flooding issues in the area with a
widespread 1 to 1.5 inches forecast with locally higher amounts. A
Slight Risk remains in effect for central portions of the state.

The heaviest rains in the state are expected to be along the Gulf
Coast from Galveston south and west through Corpus Christi. Locally
heavy rains to 2 inches per hour will be possible. For now the
greatest flooding threat remains confined to any urban areas, but
should Monday's rains overperform here then a Slight may be needed
for the hardest hit areas with future updates.

...Idaho/Montana...

A potent shortwave trough will interact with up to 2 sigma above
normal atmospheric moisture on Tuesday. PW values rising to near 1
inch will support storms capable of heavy rain that may produce
isolated flash flooding should the storms impact particularly flood
sensitive or more urban areas.

Campbell/Wegman


Day 3
Valid 12Z Wed Sep 04 2024 - 12Z Thu Sep 05 2024

...Gulf Coast and Lower Mississippi Valley...

The influx of anomalous Gulf Moisture will persist all across the
southern tier states while the upper trough and vort exits the
plains and into the Mississippi Valley. The surface front will
continue to be draped west to east across the South continuing to
provide focus for showers and thunderstorms across the region.
During the late afternoon and evening hours there will be an
intensification of instability and forcing to boost rainfall rates
across the Lower Mississippi Valley, particularly over Louisiana,
western Mississippi and southeast Arkansas. Most of the guidance
are suggesting that 1 to 3 inches will concentrate over this part
of the region thus increasing the risk for excessive rainfall and
local flooding potential. There is the potential for a few
locations for local maximums to exceed 6 inches near northern Louisiana/Arkansas. Onshore flow across southeast Louisiana and
southern Mississippi could yield 1 to 4 inches as well. A Slight
Risk area was raised for parts of Louisiana, Mississippi and
Arkansas.

Campbell

d
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FOUS30 KWBC 031328
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
928 AM EDT Tue Sep 3 2024

Day 1
Valid 16Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024

....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTH TEXAS...

...13Z Outlook Update...

...Texas...
Slight risk areas were combined and expanded to include more of
south Texas (near the Del Rio area). This morning, slow-moving
convection has materialized and produced areas of 1-2.5 inch/hr
rain rates just east of the Rio Grande. These storms are in a
sufficiently moist/unstable enviornment with weak flow aloft for
continued heavy rainfall. Low FFG thresholds (near zero in spots)
were also noted. The flash flood threat should persist for most of
the D1 forecast period under this regime. Reference MPD #0968 for
additional mesoscale details.

...Southeastern Mississippi eastward to northern Florida...
Latest CAMs indicate a robust axis of convection materializing amid
weak flow aloft from the Pine Belt of Mississippi eastward to the
Jacksonville Metro area. HREF probabilities of 24-hr, 3 inch+
precipitation totals maximize near Jacksonville, where models
depict a persistent fetch of deep convection originating along a
frontal boundary over the Gulf Stream. Multiple rounds of
precipitation are expected today through the early overnight
hours especially across northeastern Florida. Localized flash
flood potential is expected where rainfall can occur over
sensitive/urbanized locales. The Marginal risk has been expanded
northwestward and eastward to include areas of isolated flash flood
potential.

The remainder of the forecast is unchanged. Refer to the previous
discussion below for more details.

Cook

...Previous Discussion...

...Texas...

The mid-level vort is expected to exit the region although
precipitable water values will remain high. Flooding issues will be
possible in the area with a widespread 1 to 1.5 inches forecast
with locally higher amounts. A Slight Risk remains in effect for
central portions of the state although it was expanded y 1 tier of
counties to the south to reflect the latest trends in the guidance.
A slow moving tropical low will continue to bring periods of heavy
rain in the form of slow- moving showers and thunderstorms from
the Gulf into the adjacent Texas Coast. Most areas will not see
steady rain the entire time, but very localized areas of training
storms resulting in flash flooding. The heaviest rains in the
state are expected to be along the Gulf Coast from Galveston south
and west through Corpus Christi. Locally heavy rains to 2 inches
per hour will be possible.

...Mississippi and Alabama Gulf Coast...

The Marginal Risk was maintained for the potential of slow moving
and training storms to the east of the low along a stalling front
into coastal Mississippi and Alabama. The abundance of atmospheric
moisture and instability, as well as, the slow-moving nature of
the synoptic systems increases confidence that an isolated flash
flood or two could develop along this stretch of coast.

...Idaho/Montana...

There is potential for isolated to scattered heavy rainfall as an
advancing shortwave encounters an anomalously moist airmass. PW
values rising to near 1 inch will support storms capable of heavy
rain that may produce isolated flash flooding should the storms
impact particularly flood sensitive or more urban areas.

Campbell


Day 2
Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI
DELTA REGION AND ADJACENT GULF COAST...

...Gulf Coast and Lower Mississippi Valley...

The surface front will continue to be draped west to east across
the South, albeit further south than previous forecast cycles. This
frontal boundary will provide focus for showers and thunderstorms
to spread across the entire Gulf Coast from south Texas all the
way to the Florida Peninsula on Wednesday. Most of the guidance
shows 1 to 3 inches will concentrate over this part of the region
thus increasing the risk for excessive rainfall and local flooding
potential.

There is the potential for a few locations for local
maximums to exceed 6 inches near northern Louisiana/Arkansas.
Onshore flow across southeast Louisiana and southern Mississippi
could yield 1 to 4 inches as well. The focus for the heaviest rains
will be in the central Gulf Coast from Louisiana east through the
Florida Panhandle. Locally heavy rains are likely, especially from
New Orleans east through Pensacola, where a higher-end Slight is in
place. With increasing amounts of rain forecast in and around New
Orleans, continued increases in rainfall there may require an
eventual targeted Moderate Risk should trends continue.

Given the trends, the back edge of the Marginal Risk was trimmed
eastward toward the Texas coast in addition to reducing the
northern boundary by 1 tier of counties across Alabama and
Mississippi.


Campbell/Wegman


Day 3
Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL
GULF COAST...

...Gulf Coast and Lower Mississippi Valley...

The area of precipitation mentioned for the Day 2 period will
shift eastward across the central Gulf Coast with the heaviest QPF
still focusing across portions of southeast Louisiana, southern
Alabama, southern Mississippi and clipping western portions of the
Florida panhandle. Again, there was a notable southern shift in
placement within a majority of the guidance from previous
cycles.coastlineThe inherited Marginal Risk areas was removed
from southern Tennessee and northern portions of Alabama,
Mississippi and northwest Georgia while trimmed eastward across
central Louisiana. The Slight Risk spans along the Gulf Coast from
west-central Louisiana to eastern Choctawhatchee Bay Florida.

Campbell
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