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FOUS30 KWBC 232029
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Mon Sep 23 2024

Day 1
Valid 16Z 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...

...16z Update...
Marginal risk generally looks in good shape. Recent rainfall has
lowered FFG some over southern MO and do expect we will see
another round of convection here this afternoon. This will likely
drive an isolated flash flood risk, although the expectation is
that cells will be progressive enough to limit a more widespread
flood threat, but will need to keep an eye on trends.

The Marginal risk was expanded a bit northward over portions of IL
and IN. Convection today should stay south, however there appears
to be a risk of convection pushing north into these areas later
tonight into Tuesday morning as the stronger forcing digs to the
west driving a northward push to the convective threat.

Chenard

...Previous Discussion...

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 MID-ATLANTIC...

20z Update:
The Marginal risk was expanded with this update. The northwestern
extent was expanded near the expected low track. We should have
convection ongoing at 12z Tuesday over portions of IL into the OH
valley, and this may be producing a localized flash flood risk.
Then do expect scattered convection to redevelop during the
afternoon hours within the warm sector across portions of IN and
OH.

Th risk was also expanded to the southeast across more of VA and
NC. Convection will likely move into this region Tuesday afternoon
and evening. These cells should initially be quick moving, however
there is some concern cells could grow upscale and backbuild for a
period of time near the backdoor front in the vicinity. If this
occurs then a smaller corridor of more organized flash flooding
could evolve. For now will carry a Marginal risk to cover the
expected isolated flash flood threat...but will need to monitor
trends in the new HREF. If confidence in a more concentrated area
of organized convection increases then a targeted Slight risk may
be needed.

...Previous Discussion...

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...

20z Update:
The Slight risk over portions of AL, northern GA and southeast TN
looks in good shape. We very well could be looking at a PRE event
ahead of Helene as moisture associated with the tropical system
interacts with a front and upper jet. Overall the synoptic setup
looks quite favorable for excessive rainfall and global
deterministic model QPF is as high as 4-6" over portions of the
area. Given the convective nature of the QPF, it seems plausible
that rainfall totals could even exceed these values on a localized
basis. There is still some uncertainty on the exact location of
this higher rainfall axis. Todays model runs have shifted a bit
east, focusing more from eastern AL into northern GA and southeast
TN. The Slight risk remains a bit broad to account for this
uncertainty, but it does seem to cover the areas at greatest risk.
The ingredients in place and overall setup does suggest a MDT risk
could eventually be needed. However this depends on exactly how
Helene ends up interacting with the approaching trough/closed low
and where exactly this interaction happens. So while confidence in
a flash flood risk is increasing, think a higher end Slight risk is
the best option for now.

If Helene speeds up then a Slight risk may be needed ahead of
landfall along portions of the FL Panhandle. Although at this time
it looks like any rain Wed into Wed night is more preconditioning
ahead of Thursday, which will have a bigger flood risk. Thus kept
this area within a Marginal for now.

Chenard

...Previous Discussion...

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

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt