FOUS30 KWBC 090050
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
750 PM EST Sun Dec 8 2024

Day 1
Valid 01Z Mon Dec 09 2024 - 12Z Mon Dec 09 2024

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

...01Z Update...

The inherited Marginal was trimmed on the western side where the
rain has ended or will end soon. Elsewhere no changes were made.
Rates have stayed below 1 inch per hour so far. Any flash flooding
through Monday morning will be very isolated.

Wegman

...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
THE CENTRAL GULF COAST...

...2030Z Update...

In coordination with LIX/Slidell, LA forecast office, the inherited
Slight Risk was expanded south to include the rest of the
Mississippi Delta as well as the towns along Bayous Lafourche and
Terrebonne. The focus for the heaviest rain on Monday is likely to
include the New Orleans Metro and points south. The storms will
track east-northeastward from Louisiana through the western Florida
Panhandle. Since many of these areas are urban and therefore more
prone to flooding, the potential for some convection, even if
instability is limited, is still enough to forecast widely
scattered instances of flash flooding. The surrounding Marginal
Risk was left largely unchanged.

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 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 A PORTION OF
THE SOUTHEAST FROM SOUTHEASTERN ALABAMA TO WESTERN NORTH CAROLINA...

...2030Z Update...

The Slight Risk area inherited from the previous forecast was
nudged a bit to the north with this update to now include portions
of northeastern Georgia, western South Carolina and southwestern
North Carolina with this update. Meanwhile the Slight was removed
out of the Florida Panhandle.

A strong upper level trough and attendant cold front will swing
eastward across the Southeast on Tuesday and especially Tuesday
night. The greatest forcing with the front and southerly flow ahead
of it will likely be focused into Georgia and the Carolinas...with
less forcing further south along the Florida Panhandle. While
convection will still be present along the Gulf Coast, it will
become better organized as it moves inland into northern Georgia
and the Carolinas, hence the expansion of the Slight Risk into
those areas. Despite the long stretch of very dry weather into the
Carolinas, 1-3 inches of rain in a short period of time could still
cause flooding issues, particularly in urbanized areas and small
creeks and streams which fill up quickly.

Wegman

...Previous Discussion...

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!5SZlJqqpkConkVhBR8X9t_Kn5AWuq_Euj9KRTE_t9lSH=
utH7tgQMm51umoaf9HLGwh4F0lyxb5sFwj8y3TPRXbgXvI8$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5SZlJqqpkConkVhBR8X9t_Kn5AWuq_Euj9KRTE_t9lSH=
utH7tgQMm51umoaf9HLGwh4F0lyxb5sFwj8y3TPRtL1mvsA$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5SZlJqqpkConkVhBR8X9t_Kn5AWuq_Euj9KRTE_t9lSH=
utH7tgQMm51umoaf9HLGwh4F0lyxb5sFwj8y3TPR2hNynDA$=20

=3D =3D =3D
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