693
FOUS30 KWBC 251544
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1144 AM EDT Wed Sep 25 2024

Day 1
Valid 16Z Wed Sep 25 2024 - 12Z Thu Sep 26 2024

...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
NORTHEAST GEORGIA THROUGH FAR WESTERN SOUTH CAROLINA AND SOUTHWEST
NORTH CAROLINA...

..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE
SOUTHERN APPALACHIANS..

Deep layer south to south-southwest flow will allow for the
advection of increasingly moist, unstable air to funnel poleward
ahead of Helene thanks to a closed upper low/trough centered over
the Mid-Mississippi Valley. Afternoon destabilization within the
budding warm sector along with sufficient left-exit region (LER)
jet dynamics from the trough will translate to more widespread
convective coverage across the Southeastern U.S into the
Central/Southern Appalachians as we move into the late-afternoon
and evening time frame. Areas across the Escarpment located over
far Northeast GA into the Western SC/NC border have seen 2" to as
much as 5" of rainfall in the last 24-36 hrs leading to a priming
of the top soil layer as indicated by the latest NASA SPoRT soil
moisture percentiles approaching 60-70% over the areas affected.
Recent CAMs and ensemble output have increased the QPF signal
across the Southeast with the axis of heaviest rainfall bisecting
Eastern AL up through Western GA into North GA and the Southern
Apps/Smokies bordering TN/SC/NC. HREF blended mean QPF indicates
widespread 3-6" of rainfall from this predecessor rainfall event
(PRE) that correlates with the strong upper forcing and deep
tropical connection to north of Helene as it wanders into the
Eastern Gulf. Probability fields are also very aggressive with the
signature for enhanced rainfall rates and totals with the EAS
signal for 3" settling at 50-80% within the Escarpment area into
Northeast GA. Neighborhood probabilities of >5" between 70-90+% and
>8" between 20-35% are more than sufficient for a very impactful
event expecting to unfold across the complex terrain in-of the
Southern Appalachians. Given the signals expressed via the hi-res
ensemble, along with ample support from the various ML output for
the heaviest rainfall, as well as coordination with the local
Greenville-Spartanburg WFO, a targeted High Risk was added for the
D1 period.

The MDT risk was expanded a bit further southwest and west based on
the latest trends with the axis of heavier QPF a touch further
west within the 12z deterministic and adjusted HREF mean QPF
output. This is likely due to the deeper trough axis to the west
and slower progression of the overall upper pattern placing the
best LER jet dynamics a bit more in the interior than what was
progged the last 24 hrs. The MDT still encompasses much of Central
and Northern GA, but does now expand down into a small area of
Eastern AL and Eastern TN along the Great Smoky Mountains. SLGT
risks still align within the Apalachicola region in the Panhandle,
as well as the Western coast of FL as the outer bands of now
Hurricane Helene begin approaching the area(s) overnight with
locally heavy rainfall (>3"/hr) in any banding structures.

Kleebauer

Day 2
Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

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

Given the increasing confidence about the overlap of the axis of
a predecessor rainfall event on Day 1 period and the amount of
rainfall expected as the center of Helene approaches the southern
Appalachians during the latter part of the Day 2 period...there has
been a growing number of models generating close to 10 or 11
inches of rainfall for the 2-days combined in the complex terrain.
That raises concerns about impactful flooding becoming widespread
with considerable and potentially life-threatening flash and urban
flooding as well as areas of significant river flooding over
portions of Florida, the Southeast U.S. and the southern
Appalachians. Widespread minor to moderate river flooding is likely
and major river flooding is possible. Landslides are possible in
areas of steep terrain in the southern Appalachians given these
rainfall amounts. This prompted the upgrade to a high risk over
portions of northwestern Georgia into far western South Carolina
that was mainly confined to the terrain. What effectively amounts
to a high-end moderate risk surrounds the high that extends along
much of the track as Helene makes landfall along the Florida
panhandle coastline around 27/00Z (give or take a couple of hours)
based on the latest NHC guidance. In terms of model guidance...the
tracks tended to be clustered well resulting in NHC guidance being
close to most model runs. For the Day 2 period...that resulted in
the highest rainfall totals being close to each other where the
orographic forcing was consistent. A second axis of excessive
rainfall branched off to the northwest from Helene`s track towards
a deep-layered low developing over the southern portion of the
Mississippi Valley as moisture gets entrained into that system
leading to scattered convection that could produce locally heavy
amounts. Mainly confined the Slight risk to the Tennessee Valley
and only a Marginal risk extending back towards the core of the
upper system.


Bann

Day 3
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024

...INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE UPSLOPE REGION INTO THE SOUTHERN APPALACHIANS...

Based on latest NHC track guidance...Helene should be in northern
Georgia and tracking on a more northwestward course during the day.
This suggests the flow of deep moisture should be on-going to the
terrain of western Carolinas into southwest Virginia. Latest model
guidance shows several inches of rainfall in addition to what falls
in the predecessor 48 hours...felt a focused Moderate risk placed
where there was best model consensus was warranted. It was also a
multi-day rainfall total farther south along the NHC track in the
eastern Tennessee Valley that was the main reason to keep a Slight
Risk where model QPF be a little too light to support one. That
portion of the Slight Risk area extends westward to the deep-
layered low where steepening low level lapse rate and the
additional moisture streaming off of Helene could lead to some
showers and thunderstorms capable of producing locally heavy
downpours.


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