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FOUS30 KWBC 261523
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1123 AM EDT Thu Sep 26 2024

Day 1
Valid 16Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FROM THE FLORIDA
PANHANDLE THROUGH THE NORTH CAROLINA BLUE RIDGE...

...WIDESPREAD AND CATASTROPHIC FLASH FLOODING LIKELY...

...16Z Update...

The overperformance of the predecessor rain event (PRE) that is
still ongoing from Georgia north as of this writing has only
worsened the potential impacts from Helene as it approaches the Big
Bend region of the Florida Panhandle. Just in the last 24 hours, a
huge swath of rainfall of 3 inches or more has fallen in the High
Risk area and extending north into southwest Virginia and much of
southern West Virginia. All of this rain from the PRE will be in
addition to the very heavy rainfall produced by the circulation of
Helene itself.

Given these now much more favorable preexisting soil conditions and
full rivers, and in coordination with FFC/Peachtree City, GA;
GSP/Greer, SC; and RNK/Blacksburg, VA forecast offices, the
previous 2 High Risk areas were connected to each other and
expanded eastward with this latest update. The High Risk now
includes metro Atlanta, and still includes Tallahassee, FL, Asheville,
NC, and Greenville, SC. The Moderate Risk was expanded to include
metro Charlotte, NC, as well as Savannah, GA with this update as
well.

Part of the rationale for connecting the two High Risk areas
across central Georgia is the combined impact potential of strong
winds knocking out power, downed trees, and the potential for
blocked drainage from that debris when 4+ inches of rain falls
across the entire swath of the High Risk area. The greatest
certainty for widespread and catastrophic flash flooding impacts
will be in the mountains of the Carolinas. Strong southeasterly
flow into the southern Appalachians will result in enhanced
upslope. This will effectively squeeze out additional moisture,
which Helene will bring in spades, resulting in much higher
rainfall totals into the Carolinas. 24 hour totals in much of the
higher resolution guidance suggest 8-12 inches of rain between now
and Friday morning in this area. Given the sensitivity of this area
to heavy rainfall, that amount of rain on top of the 4 to 6 inches
(broadly, locally higher) that has already fallen. This will lead
to numerous flash floods with catastrophic flash flooding likely.

For the Moderate Risk area, the expansion of the Moderate along the
GA/SC border to the coast was primarily for the combination of
river and tidal flooding with high tide expected mid to late
afternoon. Training storms are developing and tracking north off
the coast of Florida and Georgia. These storms are on a beeline for
the Savannah and Beaufort areas, which is in addition to the 2-3
inches of rain that has already fallen in that area. Further, once
the circulation center of Helene gets closer to landfall this
evening, the high-resolution guidance is in good agreement that a
training band will form along the border with cells tracking
northwestward towards the center of circulation. Thus, greater
impacts are expected in those areas and the Moderate Risk was
expanded. Elsewhere the Moderate Risk was expanded east largely due
to the overperformance of the PRE, which has greatly increased the
flooding potential impacts into the Piedmont of the Carolinas.

The latest guidance has also shifted a bit to the north and east
with the westward branch of heavy rain tracking from southeast
Tennessee northwestward to southern Illinois. Thus, minor
trims/downgrades were made for Alabama, Mississippi, Arkansas, and
Missouri. Conversely, upgrades were made further to the north
across the Ohio Valley.

Finally, a strong and unrelated-to-Helene-wave of moisture is
currently tracking across the very dry New England. Given the start
of fall with leaf debris blocking drainage ditches and a reasonably
long time of up to 1 inch per hour rainfall rates, a Marginal Risk
was added.

Wegman

...Previous Discussion...

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


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