FOUS30 KWBC 240826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Tue Sep 24 2024

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

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO/Tennessee VALLEYS AND ADJACENT PORTIONS OF THE MID-ATLANTIC ...

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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