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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