FOUS30 KWBC 260847
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
447 AM EDT Thu Sep 26 2024

Day 1
Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE
SOUTHERN APPALACHIANS AND A PART OF THE FLORIDA PANHANDLE...

Catastrophic and life-threatening flash flooding and urban
flooding is expected...including numerous major damaging landslides
in the terrain...today across portions of the southern
Appalachians into Friday morning due to the cumulative effects of
heavy rain that fell on Wednesday plus additional rainfall today.
Once Helene makes landfall...the storm is forecast to continue on a
north and then northwestward path by the overnight hours of
Thursday night/early Friday morning. This leaves a prolonged period
of strong upslope flow directed into the complex terrain of the
southern Appalachians. Considerable to locally catastrophic flash
flooding is also likely for northwestern and northern Florida and a
portion of the Southeast United States...with widespread minor to
moderate river flooding..given the combined effects of a stripe of
heavy to excessive rainfall that fell on Wednesday and Wednesday
night plus any rain that falls today /Thursday/.

Most of the model guidance was aligned closely with the latest NHC
track guidance...keeping their QPF amounts and placement in play
as a component for the Excessive Rainfall Outlook. The 26/05Z WPC
QPF had a widespread swath of 4 to 7 inches along the Helene's
path...with isolated maximum amounts in excess of a foot in the
mountainous terrain of the Appalachians in South Carolina and North
Carolina. Given decent run to run continuity...few changes were
needed to the on-going high risk area there.

Near the Florida panhandle coastline...expanded the High risk area
somewhat to the north and to the east based on the 12-hour MRMS
amounts showing spotty amounts 6 inches or 7 inches amounts already
fell as part of the PRE before the Day 1 period began. The
expectation was that the soils were already getting watter logged
before the arrival of Helene's center.

The development of a deep layer cyclone over portions of the
Mississippi valley will start to pull some of the tropical moisture
westward from Helene across parts of the Tennessee Valley.
Maintained the Slight Risk area here. Even though there has been
some disagreement in the guidance on the exact placement of the
axis but was close enough to result in only some minor adjustments
to the previously-issued Slight risk.

Bann


Day 2
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL IN THE SOUTHERN APPALACHIANS...

The flow of moisture laden air into the east face of the
Appalachians should be on-going given the latest NHC guidance. With
an additional 2 to 3+ inches of rain to overlap with the footprint
of 12 to 15 inches of rainfall in the previous 48
hours...maintained a Moderate risk even though those amounts
typically do not trigger a Moderate risk, As the center of the
system continues into the Tennessee Valley...additional rainfall
amounts to the east of the mountains should be tapering off.

Farther west...convection should start to become better rainfall
producers over portions of the Mississippi and Ohio/Tennessee
Valleys as the deeper moisture encounters steepening low- and mid-
level lapse rates near the center of the deep-layered
cyclone...which has started to work in tandem with what had been
Helene. Only minor adjustments to the previously-issued Slight risk
area were needed based on the latest guidance.

Bann


Day 3
Valid 12Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE EASTERN
OHIO VALLEY INTO PARTS OF THE MID ATLANTIC REGION...

Rainfall from the Mississippi Valley/Tennessee Valley will begin to
spread eastward with considerably less moisture to work with than
before. As a result...expected rainfall amounts do not appear to be
enough to result in significant problems. In addition...the track
over during the latter part of the day should be north of areas
soaked in the Days 1 and 2 period. Made some adjustments to the
orientation from the inherited ERO but the overall forecast
rationale has changed little.

Bann
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