FOUS30 KWBC 270827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Thu Mar 27 2025

Day 1
Valid 12Z Thu Mar 27 2025 - 12Z Fri Mar 28 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF SOUTH-CENTRAL TEXAS AND THE MIDDLE/LOWER TEXAS GULF COAST...

...South Texas...
Periods of heavy rainfall over South TX will continue into today 
and tonight, as ample moisture, shear, and instability (which will
reload today via both daytime heating and decreasing lapse rates 
aloft) remain in place. That said, spatially uncertainty remains
relatively high for a Day 1 outlook, as mesoscale factors this 
morning may have a large impact on the ultimate evolution of 
convection (best expressed by the lack of consistency from hourly
runs of the HRRR, as well as the 00z HRRR being an outlier from
most of the other CAMs with regard to QPF maxima). On the synoptic
scale, ingredients are more favorable for heavy rainfall than
yesterday, and large scale forcing should reload with the mid- 
level trough axis and associated shortwave energy moving across 
the area (enhancing the upper divergence/DPVA).

However, the extent of the excessive rainfall risk will be at 
least partially dependent on mesoscale factors, and observational
trends have already diverged somewhat significantly from the 00z
HREF consensus (with convection propagating farther south than
indicated, to near Brownsville). Despite these trends, the
inherited Moderate risk remains largely unchanged, as localized 
3-5" totals over the past 12-24 hours have occurred within the
inherited area where additional training is expected. In fact, the
one area of expansion of the Moderate risk was to bring the contour
farther inland where FFGs have been depressed. While there is still 
some chance that the bulk of convection is offshore by 12z,
observational and model trends suggests it may struggle to clear 
the coast, leaving coastal areas susceptible to additional heavy 
rainfall and flash flooding. 

While a break in convective activity is somewhat likely by late
morning to midday, there is high confidence that scattered to
numerous instances of flash flooding will become likely again by
this afternoon and evening. In a worst case scenario, the outflow 
stalls near the coast allowing for repeat convection and a 
significant flash flood event could evolve (though an increase in
CAMs suggesting that the convection propagates fully offshore has
reduced this risk a bit). When combining rainfall that has already
occurred with what is forecast, it seems probable that some areas 
receive 5-8" of storm totals, and localized amounts of 8-12" are 
certainly possible. 

...MS and OH Valley...
A Marginal risk was maintained (and shifted a bit west with
observational and model trends) across portions of northern MO, 
southern IA, IL and central/southern IN. Warm-air advection and 
overrunning near a stationary front draped across this region will
gradually lift north as a warm front through the period. In the
meantime, a short period of training this morning may result in
short-term 1-3" totals (per HREF PMM). A localized, and generally 
minor, flash flood risk could result.

...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.
Despite the relative 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.

Churchill/Chenard/Hurley


Day 2
Valid 12Z Fri Mar 28 2025 - 12Z Sat Mar 29 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER TEXAS COAST THROUGH SOUTHWEST AND SOUTH-CENTRAL LOUISIANA...

An inherited Slight risk area was maintained (and shifted a bit
west based on model trends) as a slow-moving mid-upper trough will
push the area of most favorable deep-layer forcing (upper divergence
and DPVA) farther east along the Gulf Coast. Coincident with the 
favorable synoptic-scale forcing, TPWs between 1.75-2.00", along 
with mixed layer CAPE around 1000-1500 J/Kg, will result in a more
enhanced excessive rainfall threat (i.e. Slight) across the Upper 
TX Coast to Southwest and South-Central LA. Attributing to a lower
risk (Slight) compared to the previous days (Moderate) will be the
weakening low-level flow, which given the more limited deep-layer 
shear, would more likely result in more effective convective 
outflows and thus shorter duration of the more intense updrafts and
heavier rainfall cores (as well as generally drier antecedent
conditions). Considerable uncertainty still exists, as convective
details remain unclear at this point even on Day 1, but confidence
in 2-4" localized totals is increasing per the latest guidance. 

Churchill/Hurley


Day 3
Valid 12Z Sat Mar 29 2025 - 12Z Sun Mar 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS 
OF THE CENTRAL GULF COAST...

A continuation of the decreasing flash flood risk is expected from
Day 2 into Day 3, as guidance largely indicates localized totals 
of only 1-3" (mostly concentrated near the Central Gulf Coast 
where FFGs are rather high). The inherited Marginal risk area was
trimmed rather significantly to the north (into the Mid-South), as
forcing and instability look rather minimal (with the ECMWF being 
the wettest global model only indicating localized totals of 1-2".
A localized flash flood threat should result for areas confined
closer to the Gulf Coast (and mostly limited to more sensitive
urban areas). 


Churchill

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