878
FOUS30 KWBC 042008
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
308 PM EST Thu Dec 4 2025

Day 1
Valid 16Z Thu Dec 04 2025 - 12Z Fri Dec 05 2025

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

...16Z Outlook Update...
The ongoing forecast philosophy is generally on track. Latest
CAMs/12Z guidance and observations depict redevelopment of deep
convection over far northwestern open waters of the Gulf that will
stream northeastward into southern Louisiana through much of the
day today. Some of these showers/storms will occur over/near areas
that have experienced 1.5-4 inches of rain so far today, with
isolated instances of excessive runoff remaining possible. 1.75+ PW
values remain in place along the Louisiana coast today and upstream
mid-level shortwave troughs will migrate through the region from
Texas - both supporting locally heavy rainfall through the evening
hours. Given the latest observations and model guidance, the
Marginal Risk area has been maintained for this update with minor
spatial edits.

Cook

...Previous Discussion...

Upglide/warm-advective elevated convective activity is ongoing this
morning across portions of the central Gulf Coast with localized
hourly totals topping out near an inch or so (per MRMS estimates).
Right entrance ascent to 150-170 kt jet centered over the Ohio
Valley will keep isentropic moisture flux across the surface front
that will remain 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" PWs and the potential
for lingering marginal instability (MU CAPE up to 1000 J/kg now,
but forecast to decrease below 500 J/kg later this morning) may
maintain elevated convective development into midday. Internal
training of back-building convective elements will present the
greatest potential for higher rainfall totals across the Bayous of
south-central LA.

Hi-Res CAMs are in general good agreement with the heaviest
rainfall along and south of I-10 (where 3" exceedance probs from
both the 00z HREF and experimental REFS are contained). Little
change in the inherited MRGL risk, which will likely be pulled
around midday as convective activity wraps up.

Churchill/Gallina


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

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

Hurley


Day 3
Valid 12Z Sat Dec 06 2025 - 12Z Sun Dec 07 2025

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

The low-end Marginal Risk that was in effect across parts of the
Southeast (North FL and Panhandle, southern AL-GA, southern SC) has
been removed. Model QPFs from the 12Z have trended lower, while
also still showing latitudinal spread with the axis of max totals.
All of the rainfall is post-frontal, so any available instability
would be elevated, and even then there`s not much per the guidance
(<500 J/Kg). Probability Match Mean (PMM) of 12Z global guidance is
generally 0.75-1.00" in 24hrs, which is actually lower than the 17Z
NBM.

Given the latest areal-average deterministic QPF (again ~0.75-1.00
in the 24hr period from 12Z Sat to 12Z sun 4hrs), along with the
fact that much of the Southeast is between a D2-D4 drought with 3
hourly FFGs between 2.5-3", probabilities of even getting close to
those values are quite low -- certainly sufficiently low to remove
the Marginal.

Hurley

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