FOUS30 KWBC 280815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Mar 28 2025

Day 1
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 LOUISIANA...

An inherited Slight risk area was maintained (and adjusted) once 
again for this update, 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. However, considerably 
uncertainty remains (especially for an initial Day 1 outlook) as 
much is dependent on the evolution of convection this morning, as 
convection continues to consolidate farther south than virtually 
all of the guidance indicated at this time yesterday (resulting in 
the heaviest rainfall totals of 6"+ over the past 24-48 hours 
occurring over far South TX). Should a similar error occur today, 
a Slight risk may well not verify (as the consolidating MCS at the 
time of writing over far South TX will propagate offshore and may 
prevent critically necessary moisture flux transport northward into
the Gulf coast, as best depicted by the 00z HRRR). In addition, 
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). 

However, should more intense convection initiate and sustain 
itself farther north (as much of the rest of the 00z HREF
indicates), then localized totals of 3-6"+ totals will be possible
from Southeast TX and the Upper TX coast through much of 
southeastern LA (per 00z HREF PMM QPF and associated 40-km 
neighborhood probabilities for 3" and 5" exceedance of ~30-60% and 
~20-40%, respectively). Meteorological factors supporting this
potential include favorable synoptic-scale forcing (particularly
the continued impressive divergence aloft), precipitable water
values between 1.75-2.00", and CAPE building to 1000-1500 J/kg.
These same factors may still fuel intense convection offshore which
ultimately fail to materialize further inland, thus the Slight 
risk is considered to be conditional. On that note, the Marginal 
risk was also expanded southwestward to include South TX, given the
ongoing flooding and low-end potential for convective 
redevelopment once again with daytime heating (though this 
convection should be much less intense with the bulk of the forcing
shifting eastward). 

Churchill


Day 2
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 1 into Day 2, as ensemble guidance indicates the potential for
localized totals of 1-3" (mostly concentrated near the Gulf Coast 
where FFGs are rather high). The limited inherited Marginal risk 
area was maintained for this update, as forcing and instability
continue to look rather minimal (and most deterministic guidance at
this point indicates localized totals of less than 1" for the most
part, including the new WPC forecast, but decided to maintain the
Marginal for at least one more cycle to allow for new CAM data). 
Any localized flash flood threat should mostly be confined closer 
to the Gulf Coast (and limited to more sensitive urban areas).

Churchill


Day 3
Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS...

A much broader inherited Marginal risk area (relative to Day 2) has
been largely maintained (and expanded a bit), as multiple shortwave
troughs emerging from the Intermountain West and phase with the
polar jet and an associated longwave trough. This complex
interaction is expected to result in a broad area of convection
across much of the eastern CONUS, including portions of the MS, TN,
and OH Valleys. Despite the complexity of the synoptic and
mesoscale details, global models are in fairly good agreement in
indicating a convection breaking out across a broad warm sector
(characterized by PWATs of 1.5-2.0", SB CAPE of 1000-2000 J/kg, and
effective bulk shear of 40-60 kts). Most deterministic solutions
depict 1-3" localized totals (with 2-3" amounts most commonly
located farther south across the Lower MS Valley and Mid-South,
where better instability and moisture tend to be located) with both
the GEFS and ECENS indicating 2" exceedance potential (with the
GEFS favoring portions of the Mid-South from north MS into West and
Middle TN, while the ECENS favors locations farther southwest into
the MS Delta). These types of probabilities from the ensemble
guidance suggest that CAMs may well depict 2-4" localized totals,
and a targeted Slight risk introduction may be necessary in
subsequent outlooks (once better ensemble agreement and/or a
strengthening trend is more evident in the global guidance). 

Churchill

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