FOUS30 KWBC 241659
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1259 PM EDT Tue Sep 24 2024

Day 1
Valid 16Z Tue Sep 24 2024 - 12Z Wed Sep 25 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
VIRGINIA/NORTH CAROLINA PIEDMONT NEAR THE STATE LINE...

...16Z Update...

...VA/NC border...

In coordination with RNK/Blacksburg, VA; RAH/Raleigh, NC; and
AKQ/Wakefield, VA forecast offices, a Slight Risk area was
introduced with this update along the VA/NC border in the Piedmont
area. Tonight, much of the guidance, including the 12Z HRRR, HREF,
and both ARWs are suggesting an area of training convection will
develop. The heavy rain will be forced by a stationary surface
trough over the area combined with a local maximum of atmospheric
moisture with PWATs over 1.75 inches. A strong and deep upper level
trough over the Mississippi Valley will greatly increase the upper
level divergence to its east. This too will greatly add lifting
support to the storms as they form with the convective maximum
after sunset this evening. HREF probabilities are up to 30% of
exceeding FFGs in the western part of the Slight risk area, as well
as over 80% of exceeding 3 inches of rain in the neighborhood
probabilities and almost 50% of exceeding 5 inches through 
tonight.

The factors working against heavy rain and flash flooding are some
antecedent dry conditions, with at least average soil moisture
across this area, and marginal instability generally around 500 to
1,000 J/kg. The instability will limit to some extent the
widespread coverage of the heavy rain, but the excellent forcing
may be able to make up for that. Portions of the Slight Risk area
saw heavy rainfall last week, which at least has kept those area
from having totally dry soils...but average soil moisture can
sometimes work against flash flooding as some clays can be
hydrophobic when they're really dry, resulting in extra runoff.
Such is not expected to be the case here.

...Midwest...

Elsewhere across the Upper Ohio and Tennessee Valleys expect
continued off and on showers and storms, but general struggles with
organization. Very dry soils north of the Ohio River should also
generally limit the flooding threat...whereas along the west facing
slopes of the Appalachians, localized upslope may enhance it a bit.
Regardless any flash flooding in these areas should be confined to
urban and flood-prone locations.

Wegman

...Previous Discussion...

One area of showers and thunderstorms that produced spotty moderate
to heavy rainfall overnight along a quasi-stationary front
overnight should be weakening as it reaches portions of the Mid-
Atlantic region by morning. However...shortwave energy in the
northern stream will be developing a closed low that deepens with
time as it tracks from the Upper Midwest into the Mid-Mississippi
Valley later tonight. This backs the flow aloft and supports some
convection with locally heavy rainfall rates in a diffluent flow
regime over the eastern Ohio Valley as well as thunderstorms ahead
of the low track and ahead of a trailing cold front. Model guidance
continues to show the potential for locally heavy amounts while not
presenting a signal for widespread heavy rainfall
totals...including portions of the central and southern
Appalachians where relative soil moisture is slightly higher and
where flash flood guidance was lower.

Bann


Day 2
Valid 12Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AND EASTERN TENNESSEE VALLEY...

Only minor adjustments were needed to the previously issued Slight
risk area from portions of Alabama and Georgia into southeast
Tennessee...while a targeted Slight Risk area was introduced along
the coastline of the Florida panhandle. It appears that a
predecessor rainfall event will take shape somewhere in the
Southeast US as moisture streams northward from the Gulf of Mexico
ahead of Potential Tropical Cyclone Nine and interacts with a cold
front dropping southward from the Tennessee Valley. There is still
some disagreement among the various models with respect to where
the axis may set up but the area covered by the Slight risk has the
best overlap of different solutions so only minor adjustments were
needed to what is effectively a high-end Slight Risk area. Along
the coast...model QPF has increased along portions of the Florida
coastline to the point where a Slight seems warranted.

Bann

Day 3
Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AT POTENTIAL TROPICAL CYCLONE NINE
MOVES INLAND...

There should be an increasing threat of widespread and potentially
significant rainfall as Potential Tropical Cyclone Nine approaches
the coast and eventually makes landfall sometime around 27/00Z
based on the latest guidance from the National Hurricane Center.
Moderate to heavy rainfall may develop across portions of Florida
panhandle into the Southeast US well before PTC Nine make landfall
as strong transport of Gulf moisture interacts with deepening mid-
and upper-layer closed low over the central/southern Mississippi
Valley. Primary changes were to extend the Moderate risk area
northward along the NHC path of PTC Nine where the guidance had
shown an increase in rainfall amounts...with the Moderate risk now
into the southern Appalachians where terrain effects look to result
in locally enhanced rainfall amounts.

With an unusually deep low best seen in the mid- and upper-levels
located to the west...at least some of the moisture being drawn
inland by Nine will begin to get drawn westward over portions of
the Tennessee Valley and into the Mississippi Valley. As a
result...part of the Slight risk area from the Southeast US gets
pulled westward into the Tennessee Valley around the mid- and
upper-level low. Given the model spread and the poor run to run
consistency...refrained from taking the Slight risk area too far
westward at this point.

Bann
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