FOUS30 KWBC 290058
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Wed Aug 28 2024
Day 1
Valid 01Z Thu Aug 29 2024 - 12Z Thu Aug 29 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO THE UPPER MISSISSIPPI VALLEY,=20
AND PARTS OF THE MID MISSISSIPPI/OHIO VALLEYS AND MID-ATLANTIC...
...01Z Update...
Based on current radar/satellite trends and recent hi-res guidance
- the biggest adjustment was to reduce the extent of the Marginal=20
Risk area previously extending from the mid Mississippi Valley to=20
the Mid Atlantic. In the northern Plains and the Southwest, made=20
only minor adjustments.
Pereira
...16Z Update Summary...
Minor adjustments were made to the MRGL risk area across the Ohio Valley/Midwest/Mid-Atlantic with a general expansion on the
northern end into Southeast MI to account for a low-end urban flash
flood risk over the Detroit Metro. Little to no deviation in the
previous risk areas over the plains and Southwestern U.S were made
this update.
...Southwest...
16Z Update: Changes were very minor to the MRGL risk across the
Southwest with the main focus continuing to lie over the NM
terrain, mainly the burn scar locations within the Rockies and
Sacramento's. PWAT anomalies continue to meander around 1-1.5
deviations above normal with sufficient low-level buoyancy progged
through afternoon heating. Some remnant cloud cover this morning
will inhibit some of the convective potential early on, however the
focal points within the terrain will still manage to muster enough destabilization to initiate a period of thunderstorms capable of
localized flooding, especially within those sensitive burn scar
areas. The most notable areas within the MRGL are covered by a
Flash Flood Watch to account for the potential.
Will make a small mention of a non-zero flash flood threat within
the Lower Trans Pecos and Concho Valley in West TX. Antecedent soil
moisture is down <10% in the area leading to more beneficial
rainfall within cell development this afternoon. That said, the
rates within any stronger storms will be capable of exceeding 2"/hr
based off the latest HREF probability fields. This could allow for
an isolated flash flood risk within any urbanized areas and/or low
water crossings.
Kleebauer
..Previous Discussion..
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.
Hurley
...Eastern Dakotas-Western Minnesota...
16Z Update: Little to no change was needed for the MRGL risk across
the Dakotas into Northwest MN as guidance remains consistent within
their presentations of a quick-moving complex of thunderstorms
capable of locally heavy rainfall. Some discrepancy in exactly
where the heaviest rain will occur with signals mixed between the
SD/ND border up to the Canadian border. The environment is primed
for impact somewhere within the confines of the Central and
Eastern Dakotas, so didn't feel it was necessary to change the
previous forecast with some split in the specific locations of the
convection. The MRGL risk was maintained and falls within the low
to middle end of the risk spectrum.
Kleebauer
..Previous Discussion..
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.
Hurley
...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Mid Atlantic...
16Z Update: The biggest change this period involved a minor
expansion of the MRGL risk to the north to encompass the Detroit
Metro. Current visible satellite across Southern MI depicts a
small circulation migrating eastward off Lake Michigan with some
CAMs now picking up on this feature leading to convection
downstream later in the afternoon. Frontal boundary is just north
of the line from Grand Rapids over into Detroit with a southern
motion anticipated through the day time hours. Thin axis of
instability located across Southeastern MI coupled with the
approaching energy to the west will allow for a pocket of enhanced
low-level convergence capable of a few thunderstorms initiating
just to the west of Detroit, eventually moving overhead during the
peak instability. The threat is low-end within the flash flood
potential, but the proxy of Detroit with lower FFGs due to
urbanization factors corresponding to rates that could breach 1"/hr
allowed for the previous MRGL risk to be expanded northward to
account for the threat.
Further south, the front to the north will make progress into the
Ohio Valley with an axis of thunderstorms developing along and out
ahead of the front. A broad axis of instability is noted within the
current theta-E analysis extending from Southern IL across into
IN/OH with a secondary instability max located within the Central
Mid-Atlantic. Front over the Northern Mid-Atlantic will migrate
south with a convergent pattern developing in-of the front as it
approaches Southern PA into Northern MD. Guidance isn't as keen on
the potential, but there's enough of a signal remaining to maintain
the previous MRGL risk given the focal point of the front, as well
as the strong instability presence located over the above area.
Adjustments to the northern fringe of the risk area were made in
conjunction with the HREF blended mean QPF footprint and
neighborhood probability for >2".
Kleebauer
..Previous Discussion..
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 OVER PORTIONS OF
THE MIDWEST, CENTRAL MID ATLANTIC, AND ALONG THE SOUTHEAST TEXAS
COAST...
...20Z Update Summary...
The SLGT risk across the Midwest was generally maintained with only
minor adjustments necessary. Additional SLGT risks were added
across the Piedmont of VA into the WV Panhandle, as well as along
the Southeast TX coast. The MRGL risk was expanded further
southwest into NM.
...Lower MO Valley and Corn Belt into the Upper MS Valley...
20Z Update: The previous forecast remains steadfast with little
variance from run-to-run creating a general continuity in the MRGL
and SLGT risk proposals across the Midwest and Central Plains.
Increasing ascent from a vigorous upper low over the Northern
Plains will generate a period of scattered to widespread convection
with embedded heavy rainfall within the strongest cell cores. HREF
neighborhood probabilities for >2" and >3" are both fairly
aggressive (40-70%) within the outlined SLGT risk area with the
core of the potential lying across the Corn Belt of IA into
Southern MN. 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.
Kleebauer
..Previous Discussion..
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...
20Z Update: A SLGT risk has been introduced across the VA Piedmont
down into portions of Southeast VA and as far north as the WV/MD/PA
line, including the WV Panhandle and Allegheny Front. Backdoor
front will be co-located within the confines of the above area with
a fairly well-depicted axis of low-level instability and
anticipated surface convergence. 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. Widespread convective activity will initiate
across the Blue Ridge and adjacent Appalachian Front, drifting
eastward under the guide of a very slow forecast storm motion as
indicated from the latest forecast soundings off 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. Local amounts of 2-4" will be plausible
based on the recent HREF blended QPF output and enhanced
probabilistic signals based off the 12z CAMs. In coordination with
the local WFOs in the region, a SLGT risk was added with emphasis
on areas west of the Potomac in WV/VA.
Kleebauer
...Louisiana and Middle-Upper Texas Coast...
20Z Update: The premise noted in the previous forecast still
maintains merit with little change in the overall pattern. The most
notable difference is the uptick in the QPF based within the
ensemble mean(s), as well as a strong consensus within the CAMs
that elevated the probabilities for totals exceeding 5" with some
lower end potential for upwards of 8" along the TX coast. HREF EAS
signal for 2" is also relatively robust with a solid 40-60% marker
along the coastal plain extending from Eastern Matagorda up through
Galveston into the Upper TX coast into the Lower Sabine. EAS for 3"
is also within the 15-20% range, which considering the conservative
nature of the parameter is a pretty notable output over the areas
mentioned above. The current evolution maintains the heaviest
rainfall and accompanying rates away from the urban center of
Houston, but sits just south of the city. Considering the recent
trends and the anomalous moisture source off the Gulf helping aid
the pattern, a SLGT risk was added along the TX coast between
Matagorda up through Port Arthur and the far Southwest corner of
LA.
Kleebauer
..Previous Discussion..
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
Day 3
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS THE
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA COASTAL PLAIN...
...Southeast Texas and Southwest Louisiana Coasts...
A continuation from D2 will spill over into Friday as a weak
disturbance from the Western Gulf will pivot northward into Eastern
TX with a prevailing south to southeast flow funneling tropical
moisture into the coastal plain of Southeast TX and Southwest LA.
Still some discrepancy within the recent global deterministic on
the depth of the protrusion of the elevated moisture field and
accompanying heavy rain footprint across the region. Despite the
differences, the environment is very much favorable with the
recently ML and ensemble mean QPF output indicating another round
of heavy precip impacting areas that will have seen a considerable
amount of rain the prior 24 hrs. The trajectory of the mid-level
disturbance off the Gulf will make the difference in exactly where
the best heavy rain axis will occur as ascent provided under the
guide of the complex will determine where the more appreciable
totals would occur. As of now, a general 2-4" with locally higher
is expected, leading to a chance for 2-day totals to exceed 8" in
areas that see repeated action in both D2 and D3 time frames. A
SLGT risk was added to very similar locations to D2 with a small
extension eastward into more of Southwest LA as the southerly fetch
off the Gulf is forecast to bisect that region when analyzing
ensemble outputs.
Kleebauer
...Southern Rockies-Plains into the Mid MS Valley, Northern OH
Valley, and Lower Great Lakes...
20Z Update: There were very little changes necessary to the
expansive MRGL extending from the Southwest through the Great
Lakes. Convective pattern will be driven by relevant moisture
anomalies aligned along the cold front as it slowly migrates to the
south and east with high pressure pressing down from the north.
Currently, there is no one favored area of convective
organization, however the zone within the Mid- Mississippi Valley
and down across West TX is the most likely candidate for any
potential upgrade. Both areas have been dry recently, so the
antecedent conditions work against the potential, so maintained
continuity given the pattern remaining the same from the previous
issuance.
Kleebauer
..Previous Discussion..
Elongated Marginal Risk inherited from yesterday's Day 4 ERO
continues to look good; made a minor adjustment by extending the
southern periphery farther south across TX-AR-MO-KY given the
latest guidance trends. QG forcing, kinematic, and forcing along
the expansive frontal boundary will doesn't appear to be robust
given the upper level pattern; however, pooling of above normal PWs
ahead of the front along with the diurnal instability will support
the potential for locally excessive rainfall and thus an isolated
flash flood threat.
Hurley
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgbh_dC06c$ Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgb0A_wYck$ Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt= __;!!DZ3fjg!-ZVoYxoFS8XPd5u26iRL782pAKlgHOhM4Fib5knQC3YCcyJQzD1wfFii82O7LG2= YmmcKVajMrq7unY3gJGgblHgtigE$=20
=3D =3D =3D
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