FOUS30 KWBC 050824
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
324 AM EST Tue Nov 5 2024

Day 1
Valid 12Z Tue Nov 05 2024 - 12Z Wed Nov 06 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
LOWER TO MID MISSISSIPPI AND LOWER OHIO VALLEYS...

A cold front tracking east across the middle of the country is
expected to stall out generally along the Mississippi and Ohio
Rivers late tonight. Southerly flow off the Gulf will continue
pumping plentiful moisture into the front. Meanwhile, in the upper
levels, a large longwave trough over the west will establish itself
as a positively tilted trough by tonight as it digs southwestward
over the Southwest. Before it does so, the last potent shortwave of
the "old" trough will race northeastward into the Great Lakes as
the leading edge of the trough becomes a roughly straight line
southwesterly from the Southwest right through into eastern 
Canada.

The potent shortwave and associated surface low will both race
northeastward through the period. While they're over the upper
Mississippi Valley, they will move the front currently stalled from
Texas through Michigan eastward a bit to align with the Slight and
Marginal Risk areas along the Mississippi and Ohio Rivers. This
will give the hard hit northwestern Arkansas region a break as the
heaviest rain shifts east. However, by tonight, both features will
be well into the Great Lakes and Canada, so the forcing pushing
them east will quickly diminish, resulting in another stall-out of
the front. With continuous Gulf moisture, another round of training
thunderstorms is expected up the Mississippi and lower Ohio Rivers
tonight. Given both the prior history of this front stalling as
well as urban concerns in the Memphis area especially, a Slight
Risk area was introduced with this update. 

Despite the stalled out front keeping the rainfall largely confined
to a narrow corridor, the upper level forcing quickly weakening
tonight should hold rates and coverage in check much better tonight
as compared to the ongoing storms over Texas, Arkansas and Missouri
right now. Thus, forecast rainfall amounts are considerably lower
today and tonight than yesterday. The Slight is considered a
"lower-end Slight" with any flash flooding impacts expected to be
largely confined to low-lying flood prone areas and urban areas.
Like areas further west, this area has also been very dry prior to
this rainfall, so it will still take a prolonged period of heavy
rainfall to result in flash flooding.

Wegman


Day 2
Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
GEORGIA AND SOUTH CAROLINA...

A large plume of deep tropical moisture will overspread all of
Florida and move into Georgia and South Carolina on Wednesday and
Wednesday night. This same plume is home to T.S. Rafael down in the
Caribbean. PWATs will increase to as high as 2.25 inches, meaning
the storms expected to form will have plenty of moisture to convert
to heavy rainfall. The front over the Ohio Valley is unlikely to
play too much of a role in the storms over Georgia and South
Carolina. Rather, the guidance suggests a secondary front will from
at the leading edge of the moisture plume that will likely be
enhanced by the right entrance region of the upper level jet 
streak, as well as possibly some upslope component (especially in 
South Carolina). Regardless of the primary lifting mechanism, there
is good agreement in the guidance for an area of 3 to 5 inches of
rain (though likely much higher amounts will be embedded) in the
Slight Risk area, which is a small increase in amounts from the
previous forecast.

As with areas further west the past several days, this area has
also been incredibly dry lately, with many areas not having seen
appreciable rain in a month. For the swampy and sandy areas, this
will mean it will take a lot of rain before runoff converts to
flooding. Further north across interior SC and GA, the clay soils
are likely quite hydrophobic due to the recent dry weather, which
will notably increase runoff once the heavy rain gets started.
Thus, locally increased impacts are possible. An internal higher-
end Slight area was introduced for the peak rainfall area of east-
central Georgia and west central South Carolina for this potential.

Since this plume is home to T.S. Rafael, this would be considered a
predecessor rain event (PRE) if Rafael were to head towards the Big
Bend and GA/SC. However, since the latest forecasts keep Rafael
tracking westward in the Gulf this rainfall event is not considered
a PRE. 

Wegman


Day 3
Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST AS WELL AS FOR PORTIONS OF THE SOUTHERN PLAINS...

...Southeast...

The cold front over the Appalachians that will drive the Day
2/Wednesday rainfall event will move southward to coastal South
Carolina and southern Georgia Thursday and Thursday night. The
southerly flow of deep tropical moisture ahead of the front will
abate somewhat in favor of a more neutral easterly flow. This will
keep periods of heavy rain going...but both less widespread, less
persistent, and less heavily. The core of heaviest rain will shift
to the coast of SC/GA and north FL, but since most of this area is
likely to not have seen substantive amounts of rain on Wednesday,
and the coastal swamps can usually handle a lot of rainfall with
few problems, have opted to keep the area in a Marginal Risk for
this update. However, recent model trends have been for the
heaviest rain to increase with time as the CAMs move into this
period of time. Should that happen a Slight Risk will likely be
necessary.

...Southern Plains...

A potent upper level low will begin to eject out of the Southwest
and into the southern Plains on Thursday. A strong cold front will
develop as a result with plentiful snow into much of northeastern
New Mexico. This area was trimmed from the Marginal Risk with this
update. For the rest of the Marginal Risk area, some Gulf moisture
will be captured and advected northward across north Texas and
Oklahoma, but amounts will be somewhat limited. With only broad
forcing from an upper level jet streak but few coherent features
away from the strong cold front, the rainfall footprint will be
broad and disorganized. This area has seen plentiful rainfall over
the past few days, so the additional rainfall, while not much, may
still cause isolated instances of flash flooding.

Wegman

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