Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
638 AM EST Wed Dec 3 2025

Day 1
Valid 12Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...

CIRA LPW shows the slug of enhanced moisture along the central
Mexican Gulf coast starting to lift northward through the western
Gulf jet under response from the digging mid-level shortwave across
the Four Corners of the Southwest U.S. Twenty to 30kts of southerly
low level flow will advect the moisture north, bring the surface
front toward the central Texas coast through late afternoon into
the evening; CAPE values will rise into the 2000-3000 J/kg range.
As night-falls, stronger isentropic ascent will increase through
depth as the upper-level southwesterly jet stream flow expands and
strengthens providing the solid ascent plane for sufficient
moisture convergence with total PWats over 2" for expanding region
of elevated thunderstorm activity across the central and upper
Texas Coast.

Northward expansion remains the greatest uncertainty but overall
00z CAMs continue denoting solid rainfall rate potential mainly
along and south of the I-10 corridor with cell motions being slow
but also slightly veering to more easterly with solid back-building
potential to allow for some repeating training. HREF probability
driven by the 00z CAMs, still suggest best signals will remain
off-shore but there was also a slight northeastward trend in some
of the convective elements into southwest to south-central LA with
streaks of 2-3" totals. This will retain the solid Marginal Risk
(5-15% coverage) of FFG exceedance with isolated rapid inundation
flooding possible.

Gallina


Day 2
Valid 12Z Fri Dec 05 2025 - 12Z Sat Dec 06 2025

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

Upglide/warm-advective elevated convective activity should be
ongoing start of the forecast period (04.12z) across the central
Gulf coast. Steering flow will continue to flatten within fairly
unidirectional west-southwesterly deep layer flow. Right entrance
ascent to 150-170kt jet centered over the Ohio Valley will keep
isentropic moisture flux across the surface front that will
oriented along and south of the coast. A weak mid-level shortwave
will slide from west to east along the southern periphery of the
jet streak maintaining 20-25kt of west-southwesterly 850mb flow
which given 1.75-2" total PWats and modestly unstable ribbon (up to
1000 J/kg) and should maintain the elevated convective
development. Internal training of back-building convective elements
should be the greatest potential for higher rainfall totals across
the Bayous of south-central LA. A slow eastward propagation of the
SSW- NNE oriented line will occur with the translation of the flat
shortwave energy and potentially expand into coastal MS/AL and
perhaps as far as western FL Panhandle by the end of the forecast
period.

There remains some north/south spread in placement of heaviest
rainfall cores/training corridor. The GFS continues a traditional
northward bias along with the NAM relative to the ECMWF and other
global guidance; with some suggestion of overall totals in excess
of 5" in spots. Hi-Res CAMs generally trend toward the southern
solutions with heaviest rainfall along and south of I-10, which
appears more plausible given the overall flow orientation (unstable
air upstream over the waters). Still, there is some solid
suggestion of rainfall rates/totals and environmental conditions
that may warrant an upgrade to a Slight Risk, however, the split in
placement and wider overall total rainfall total spread continues
to be too great to delineate one at this time, but will continue to
evaluate with future updates. As such, the broad Marginal from
Galveston Bay to the Pensacola area of FL remains, though have
broadened it slightly northward to account for the latitudinal
variance.

Gallina


Day 3
Valid 12Z Fri Dec 05 2025 - 12Z Sat Dec 06 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL
GULF COAST THROUGH CENTRAL GEORGIA...

The deep layer unidirectional west-southwesterly flow regime will
remain in place for the day 3 period as well. The subtle, flat
mid-level shortwave feature will be translating quickly across the
South, through the Carolinas and offshore. The connection to the
western Gulf moisture pool/theta-E source will continue to further
narrow and stretch through an elongated isentropic ascent plane in
the wake of the wave draped across SC, central GA toward the
central Gulf coast. By this time period, the ascent across the
front will be fairly oblique and lapse rates will have moderated
toward limited elevated MUCAPE below 500 J/kg...becoming more of
an Atmospheric River with prolonged moderate rates with 24hr totals
of 1-2" across the length of the stream with scattered localized
maxima of 2-3". To be expected, much like the day 2 period, there
remains a latitudinal spread to the placement of the AR stream and
the width of the inherited Marginal Risk reflects this. However, at this
point, FFG exceedance will be more likely in the longer duration
of 6, 12 and 24hr exceedance, with any flash flooding potential
most likely upstream across LA/S AL, where multiple days of
rainfall should saturate the upper soil profiles and FFG values may
be much lower than currently analyzed.

Gallina


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