Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
727 PM EST Sat Dec 7 2024

Day 1
Valid 01Z Sun Dec 08 2024 - 12Z Sun Dec 08 2024

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

Hurley


Day 2
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

...2030Z Update...

In coordination with LCH/Lake Charles, LA and LIX/Slidell, LA
forecast offices, the inherited Slight Risk was downgraded to a
Marginal with this update.

The latest run of both the global and hi-res guidance have brought
the rainfall forecast down from previous forecasts. The heaviest
rains for the period are now expected in the middle of the Marginal
Risk area over northern Mississippi, where interaction with a
front in that area will lead to greater forcing than areas further
south along the Gulf Coast. Instability will be a major limiting
factor in flooding potential area-wide by greatly limiting rainfall
rates. MUCAPE values throughout the duration of the rainfall will
struggle to get much above 300 J/kg, suggesting that nearly all of
the rainfall will be stratiform. Where the front adds forcing,
there could make local convective bursts embedded within the
broader rainfall shield, but do not expect there to be many, if any
instances of rates above an inch per hour.

Hydrologically, most if not all of the soils in the are are at or
below average for moisture. Low/dry creekbeds should easily handle
most of the light to briefly moderate rainfall rates. Any isolated
flooding will be in any urban areas, as well as other low-lying or
flood prone areas.

Wegman


Day 3
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN LOUISIANA AND COASTAL MISSISSIPPI...

...2030Z Update...

In coordination with LIX/Slidell, LA forecast office, the inherited
Slight was shrunk largely to the I-10 & I-12 corridors from Baton
Rouge east to Biloxi. The flooding potential in this area appears
greatest in urban and flood-prone areas, so most of the more rural
areas of Louisiana to the Florida Panhandle were removed from the
Slight. Further, there appears to be better agreement in the
guidance that the heaviest rains and potential for training will be
focused in the new Slight Risk area, with areas further east
including Mobile and Pensacola largely missing out on heavy rains
through 12Z Tuesday.

As on Day 2/Sunday, instability remains a major limiting factor
supporting most of the rainfall on Monday staying stratiform and
light in intensity. Latest guidance keeps most of the low-level
instability over the Gulf, with relatively little of it making it
inland as we are in the climatological minimum for diurnal heating.
The rain won't persist nearly long enough with mostly stratiform to
raise big concerns for flooding. However, given the flooding
history in the New Orleans area, think there still remains some
potential for flooding in those urban and flood prone areas.

Wegman

...Previous Discussion...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally under 1") to much of the
Northeast. Farther south along the central Gulf Coast, there are
hints of another (weaker) shortwave trough within the southern
stream that may enhance heavy rainfall along a trailing cold front.
Given the slowing of the front, the opportunity for training areas
of heavy rainfall will be greater on Day 3, best depicted by the
00z ECMWF and ECENS (depicting localized banded totals of 2-5" and
probabilities for 2" exceedance as high as 30%). This risk appears
greatest from southeast LA to southern AL and the tip of the FL
Panhandle, though the EC model system is the most extreme and
farthest east of all the models. Maintained the inherited Slight
Risk area with some adjustments, limiting the eastern extent a bit
given the lack of agreement from the other models. Uncertainty is
still relatively high, but subsequent outlooks should get a better
handle on things as these details begin to enter the CAM period.

Churchill


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