FOUS30 KWBC 230815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Mon Sep 23 2024

Day 1
Valid 12Z Mon Sep 23 2024 - 12Z Tue Sep 24 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-MISSISSIPPI VALLEY EASTWARD TO THE SOUTHERN APPALACHIANS...

Precipitation over the southern Plains will continue to spread
north ahead of a positively-tilted upper shortwave trough drifting
east through the central Plains. Deepening moisture (PWs ~1.75-2
inches) supported by southwesterly inflow, and large-scale ascent 
generated by the upper shortwave, are expected to produce a broad 
area of precipitation spreading northeast early in the period from 
the Ozarks through the lower Missouri and mid Mississippi valleys. 
There is the potential for a stripe of locally heavy amounts, with 
the 00Z HREF showing high neighborhood probabilities for 
accumulations of 3 inches or more from central Missouri eastward 
through the St. Louis metro. However, limited instability should 
help to keep rainfall rates in check through the morning hours. A 
better chance for heavy rainfall rates may develop later in the day
as trailing energy and daytime heating contribute to storms 
redeveloping near the Ozarks and tracking northeast through the mid
Mississippi Valley and into the lower Ohio Valley later this 
afternoon into the evening.

Moisture spreading further east is expected to fuel showers and
storms developing later today across the central and southern
Appalachians. Some of the hi-res guidance continue to show some
potential for isolated heavy amounts across the region, with the 
greatest threat centered over western North Carolina, where the 
HREF is showing high neighborhood probabilities for accumulations 
of 2 inches or more.

Pereira


Day 2
Valid 12Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024

..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY AND THE CENTRAL TO SOUTHERN APPALACHIANS...

The shortwave trough moving across the mid Mississippi Valley on
Day 1 will begin to interact with a northern stream trough
amplifying over the northern Plains early in the period. Models
show the upper pattern quickly amplifying over the Mississippi
Valley on Tuesday. At the surface, low pressure is expected to 
track northeast from the mid Mississippi Valley to the Great Lakes,
with a trailing cold front extending back through the Ohio Valley.
An axis of deeper moisture (PWs ~1.50-1.75 inches) coinciding with
deep southeasterly flow ahead of the upper trough will fuel 
showers and storms along the low track and ahead of the trailing 
cold front. Models do not present a signal for widespread heavy 
amounts but show the potential for locally heavy amounts, 
including for portions of the central to southern Appalachians, 
where relative soil moisture is slightly higher and flash flood 
guidance values lower.

Pereira


Day 3
Valid 12Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST, TENNESSEE VALLEY, AND THE SOUTHERN APPALACHIANS...

Although disagreements on the track continue, the consensus of the
models show a powerful tropical cyclone tracking north through the
eastern Gulf of Mexico during this period. Apart from the faster 
Canadian solutions, which bring heavy amounts into the Florida 
Panhandle, most of the deterministic models and their ensemble 
means show moderate to heavy rains brushing the coast, but the 
heaviest amounts remaining out over the open waters through 12Z 
Thu. Therefore, just a Marginal was maintained from South Florida 
and the Keys northward along the Florida Gulf Coast through the 
Panhandle. However, should more guidance begin to trend toward a
faster solution, future upgrades may be necessary.

Despite their disagreements with the cyclone track, the models are
presenting a growing signal for heavy amounts developing well to 
the north. The cold front pushing east into the Ohio, Tennessee, 
and lower Mississippi Valley prior to the period is forecast to 
slow across the region, providing a focus for moisture ushered in 
by strong southeasterly to easterly flow ahead of the system. With 
some longitudinal disagreement, the GFS and ECMWF show PWs 
increasing to 1.75-2 inches along an axis of strong low level 
convergence extending north from the northern Gulf Coast into the 
Tennessee Valley. Meanwhile, an upper low that will begin to 
develop over the mid Mississippi Valley on Wednesday is expected to
drop south, enhancing lift across the region. While there is some 
west-east disagreement on its placement, most of the 00Z 
deterministic runs showed a stripe of 2-4 inch accumulations 
extending northeastward from northern Alabama into the southern 
Appalachians.

Pereira
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