253
FOUS30 KWBC 280050
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
850 PM EDT Thu Mar 27 2025

Day 1
Valid 01Z Fri Mar 28 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...
Maintained the Moderate Risk area of excessive rainfall in the face
of a complicated and messy forecast going through the rest of the
night across portions of South Texas through the Middle Texas Gulf
Coast but potential exists for numerous flash floods, some of
which. Adjustments were made to the outlook areas based on radar
and satellite trends through the late afternoon and early evening
and changes in Flash Flood Guidance over the past 24 hours. For
this update, the western boundary of the Moderate Risk was adjusted
eastward...but adjustments were tempered by guidance which showed
at least some potential for renewed convection associated with a
wave moving out of Mexico later tonight. Mesoscale Precipitation
Discussion 090 was issued just a bit ago to cover the ongoing and
near- term flash flood threat and a convective complex developing
and tracking east/northeast through South Texas. While instability
remained modest, a better shear environment and an increase in the
PWs should support greater storm organization and higher rain
rates. There are some suggestions for repeating/training clusters,
particularly in the southern extent of the updated Moderate Risk
area, where localized higher end additional QPF is possible (4-6").

This may result in numerous flash floods and locally significant
flooding into tonight. The previous discussion follows...

Bann


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...

The Slight Risk was maintained but confidence has lowered compared
to the previous cycle. The recent set of guidance and
probabilities for higher QPF have lowered, keeping the bulk of the
heaviest rainfall off the Gulf Coast as well as a subtle shift to
the west along the TX Gulf Coast. For now, opting to keep the
Slight in place but if trends continue in the next update,
probabilities for excessive rainfall and flash flooding may drop
below Slight Risk levels. Previous discussion follows...

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/Taylor


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 1-2" (mostly concentrated near the Gulf Coast where FFGs
are rather high). The inherited Marginal risk area was trimmed
further, as forcing and instability look rather minimal. A
localized flash flood threat should result for areas confined
closer to the Gulf Coast (and mostly limited to more sensitive
urban areas).


Taylor


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt