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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