FOUS30 KWBC 081538
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1038 AM EST Sun Dec 8 2024

Day 1
Valid 16Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI AND TENNESSEE VALLEYS...

...16Z Update...

No changes were made to the previous Marginal Risk area, while the
threat is certainly on the lower end of the Marginal Risk threat,
previous days' rain in Louisiana and concern about training led to
keeping the threat as is. Meanwhile the guidance is in good
agreement that the heaviest rains will be focused over much of
northern Mississippi, where conditions are much drier, but more
rainfall could lead to isolated flash flooding. The previous=20
discussion remains valid.

Wegman=20

...Previous Discussion...

The inherited Marginal Risk area was maintained for this update,
suggesting that any flash flooding should remain localized today,
as rainfall rates will generally remain capped below 1"/hr (with
lacking instability being the primary limiting factor). The
heaviest rains for the period are expected in the middle of the
Marginal Risk area over northern Mississippi, where an interaction
with a front in that area will lead to greater forcing than areas
farther south along the Gulf Coast. Where the front adds additional
forcing, localized convective bursts may be embedded within the
broader rainfall shield.

Hydrologically, most if not all of the soils in the are are at or
below average for moisture. Low/dry creek beds 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 more flood-prone areas.

Churchill/Wegman


Day 2
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...

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 well under 1") to much
of the Northeast. Farther south along the central Gulf Coast, a
weaker shortwave within the southern stream may locally enhance
heavy rainfall along a trailing cold front. The inherited Slight
Risk in this area was maintained, mainly from the I-10 & I-12
corridors from Baton Rouge east to Biloxi, where the flooding
potential appears greatest in urban and more flood-prone areas
(given relatively dry antecedent conditions and streamflows normal
to below normal). There is still some question as to how far east
the heavy rainfall threat will extend (as the ECMWF and ECENS have
been persistent outliers suggesting an eastward extension into the
FL Panhandle and surrounding southeast AL and southwest GA), but
the signal for localized totals of 2"+ are maximized near the
Slight Risk (and confined to where instability and forcing are
greatest).

Churchill


Day 3
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEAST AND CENTRAL GULF COAST...

Large scale upper trough will be digging into the central US by
Tuesday, setting the stage for more organized heavy rainfall into
portions of the Southeast. Initially positively tilted, the trough
looks to quickly take on a more neutral tilt over the course of
the course of the day, as an associated 250 mb jet streak
strengthens from ~110 kts to ~170 kts over 24 hours (with our area
of interest for the Slight Risk located near the right-entrance
region). As with prior days, instability once again looks to be the
primary limiting factor, and this is particularly the case farther
north into the Carolinas. While the GFS/GEFS depict 2-4" localized
totals into northern GA and western SC, the ECMWF/ECENS largely
limit this threat (for 2" and 3" exceedance) a bit farther south
into central GA, southeast AL, and the FL Panhandle (where the
inherited Slight Risk was maintained and adjusted a bit based on
the new model data). Given relatively dry antecedent conditions and
associated 3-6 hr FFGs generally ranging from 3.0-4.0", the Slight
is on the lower-end of the 15-40% probability distribution,
suggesting widely scattered instances of flooding (at best).

Churchill


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!-c7w-f9kJIrVmm-qOEY4lW8FWXYk9gb4wMVMyeVupXkx=
TeINDyTHyTDMxNFjUjNnz7bnGpwgO50mjdoOyRFsqLwIFyk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-c7w-f9kJIrVmm-qOEY4lW8FWXYk9gb4wMVMyeVupXkx=
TeINDyTHyTDMxNFjUjNnz7bnGpwgO50mjdoOyRFstr-G73M$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-c7w-f9kJIrVmm-qOEY4lW8FWXYk9gb4wMVMyeVupXkx=
TeINDyTHyTDMxNFjUjNnz7bnGpwgO50mjdoOyRFsFQmEpW0$=20

=3D =3D =3D
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