FOUS30 KWBC 250822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Tue Mar 25 2025

Day 1
Valid 12Z Tue Mar 25 2025 - 12Z Wed Mar 26 2025

The probability of rainfall exceeding flash flooding guidance is
less than 5 percent.

Hurley


Day 2
Valid 12Z Wed Mar 26 2025 - 12Z Thu Mar 27 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL AND SOUTH TEXAS...

Diffuse mid-upper level trough moving slowly eastward across the
Southwest-Four Corners Region will lead to an uptick in upper
diffluence across northern Mexico and SC-South TX Wed and Wed
night, with a WNW-ESE lobe of mid level vorticity (shear axis)
lifting northeast across the outlook area along the elevated warm
between 18Z Wed and 12Z Thu. Strengthening low-level inflow (30-40
kts at 850 mb) will boost the low-level moisture transport, with
TPW values climbing between 1.75-2.00". 

Convection initiating across the Sierra Madre in northeast Mexico 
will cross into SC and South TX Wed afternoon and overnight, and 
while considerable shear in the low-mid levels should keep the 
convection moving along, the slow progression of the mid-upper 
trough will likely result in slow-moving and/or repetitive areas 
of more organized convection, as evidenced by the higher-end QPF 
totals per the latest (00Z Tue) guidance...as much as 5-7+ inches 
per the Regional GEM, ECMWF, and FV3. Dry antecedent soils and 
initially high FFG values (3-4" over a large portion of the outlook
area) will likely result in a more limited or localized flash 
flood threat, and as a result, have hoisted a Marginal Risk across 
these areas in the Day 2 ERO. 

Hurley


Day 3
Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTH-CENTRAL AND SOUTH TEXAS AND MUCH OF THE TEXAS GULF COAST...

...Southern Texas...
Aforementioned southern stream trough pivots slowly eastward across
the Southern Plains Thu-Thu night. Rather healthy, deep-layer exit
region forcing continues, with low-mid level moisture flux
anomalies increasing to near 3 standard deviations above normal
(TPW anomalies per the 00Z 3/25 GEFS) while 1.75-2.00" TPW values
overspread the outlook areas. Continued healthy low-level inflow 
off the Gulf, becoming more parallel to the mean 850-300 mb flow 
with time by late Thursday, will lead to an increase in upward 
propagation and thus potential for cell training Thursday evening 
and overnight. This as low-level inflow taps the more unstable 
airmass (1000-1500 J/Kg mixed-layer CAPEs) across the western Gulf.

The guidance is beginning to converge with the heavy rainfall
signal with the 00Z 3/25 models, as the GFS, ECMWF, GEM (esp. GEM
Regional), and UKMet now all show a corridor of 4-8+ inches of 
rain across portions of SC-South TX into the Gulf Coast during the
24hr period from 12Z Thu-12Z Fri. While there is still some areal
and timing differences, the growing model consistency (especially
among these global runs) has bolstered the confidence for a more
targeted Slight Risk area, including areas closer to the Coast 
that for now would likely end up being a higher-end Slight Risk 
despite the dry antecedent soils. 

These areas (closer to the Gulf coast) would also be an area to 
watch for a potential upgrade to Moderate Risk during subsequent 
outlooks, given the closer proximity to the more unstable airmass 
just off the coast and thus the greater potential for multiple 
hours of 2.0+ in/hr rainfall rates (higher probability of >6" of 
total rainfall), especially late Thu-Thu night with any cell training.

...Pacific Northwest...
Another modest Atmospheric River (narrow ribbon of 500+ kg/m/s
IVTs) will result in pockets of 3-5+ inch 24 hour rainfall totals 
along the West Coast from Northern CA northward through OR-WA.
Among the 00Z 3/25 models, the Regional GEM is by far the wettest,
with isolated totals of 6-8+ inches. Despite the lack of 
instability, the wet antecedent soils along with the potential of
0.3-0.5 in/hr rainfall rates could lead to localized short-term
runoff issues, particularly along small streams and over burn
scars. 

Hurley


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