Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
743 PM EST Mon Dec 1 2025

Day 1
Valid 01Z Tue Dec 02 2025 - 12Z Tue Dec 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST AND DEEP SOUTH...

A negatively- tilted shortwave trough moving through the Southern
Plains at the present time is leading to a forming stationary from
across the Deep South to the north of a wave of low pressure in the
Gulf of Mexico. An area of low-level convergence which started as
a frictional convergence zone near the northwest coast of Cuba is
curling up towards the MS/AL coast at the present time. Moisture is
sufficient for heavy rainfall, but until recently, instability was
scant. However, ahead of the shortwave, MU CAPE has risen to 500
J/kg near the southeast tip of LA and is surging north towards the
AL/MS barrier islands, which should further increase from there
and spread into nearby areas of the Central Gulf Coast with time,
as far east as the FL Panhandle. MU CAPE could rise towards 1000
J/kg along portions of the Central Gulf Coast through early Tuesday
morning. Effective bulk shear of 25-45 kts lurks across the
region, which could lead to cell organization.

A band of heavy rain with embedded thunderstorms has recently led
to hourly rain amounts up to 1.75" in southeast LA and southern AL
thus far. The 18z HREF and 12z RRFS are not exactly in agreement
regarding the potential for 2"+ by 12z...their overlap is in a
narrow zone between Dauphin Island AL and Pensacola where
downstream instability and moisture overlap would be greatest. The
18z hi-res NAM is doing the best with the convective evolution
thus far. The three hour flash flood guidance values regionally are
quite high though they might be reachable. The risk for 1.75"
amounts in an hour with local totals up to 4" should continue to
exist overnight, which would be problematic in urban areas, so left
much of the Marginal Risk area from continuity intact. The
Marginal Risk area has been constrained on its west side per radar
reflectivity trends and trends noted in the 18z HREF/12z RRFS
guidance.

Roth


Day 2
Valid 12Z Tue Dec 02 2025 - 12Z Wed Dec 03 2025

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

Surface/low level convergence associated with a frontal boundary
forecast to translate eastward into the western Atlantic after 18Z
may allow for a band of SW to NE training heavy rain/thunderstorms
early in the period over eastern North Carolina. Weak instability
and precipitable water values increasing to over 1.5 inches along
the coast could support rain rates over 2 in/hr, however, recent
hires guidance maintains the potential for 3 to 6 inch totals
offshore/east of the Outer Banks. While the threat is non-zero, it
is low enough to not warrant a risk area at this time.

Otto


Day 3
Valid 12Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025

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

The very beginning of a multi-day rainfall event will begin to
unfold Wednesday night along portions of the Upper Texas and
Louisiana Gulf Coasts. Mainly for the 06-12Z time frame Thursday
morning, the 12Z guidance continues to suggest most of the
heaviest rainfall from heavy showers/thunderstorms will remain
offshore. However, at least some of that rain could make its way
inland. The threat remains sub- Marginal, but the area will
continue to be evaluated for a later introduction of a Marginal
Risk with future updates.

Otto/Wegman


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