711
FXUS62 KGSP 260025
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
825 PM EDT Wed Sep 25 2024

.SYNOPSIS...
Tropical Cyclone Helene is forecast to make landfall over the
Florida Panhandle late Thursday, with the remnant circulation moving
quickly north bringing gusty winds and heavy rainfall to our area
Thursday night into Friday.  Expect a calmer weekend with
temperatures near seasonal normals.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 755 pm...A wide band of rain continues to stream northward in
a plume of deep tropical moisture to the east of a stalled frontal
boundary over east TN. Thus, our Predecessor Rain Event (PRE)
was well underway. The eastern edge of the rain band has moved
very little, and it is along this eastern edge where there has
been enough overlap of shear, buoyancy, and forcing to keep deep
convection going. In addition, the shear was supporting occasional
rotating storms, and that may continue for the next several hours
according to the 22Z run of the HRRR. There will also be a severe
storm threat from metro CLT up across the NW Piedmont, which retains
1500-2000 J/kg of sfc-based CAPE and deep layer shear of 40-45 kt. A
brief tornado would not be out of the question, after what we have
already seen today. That being said, the latest guidance keeps the
primary rain band generally to the west of a line from Abbeville
to Spartanburg to Forest City and Lenoir through the evening.

The decision was made on the 5 pm Advisory for Helene to upgrade
to a Tropical Storm Warning based on coordination with neighboring
offices and a compelling trend in the consensus of the guidance
regarding the wind gust potential, which mostly ramps up Thursday
evening. Confidence was high enough to make the upgrade at 5 pm to
give the most lead time, rather than wait until the late evening or
early morning cycles when prep time would have been lost. Otherwise,
all signs point to a significant event across the western Carolinas
and northeast Georgia. Although a subtle westward shift in the
track is noted, Helene will be a large and fast-moving storm,
so the shift doesn`t really matter. We still expect a very rare
and significant amount of rainfall and some areas across the mtns
and adjacent zones will see catastrophic and damaging flooding by
the later near term. Once the rainfall gets above ten inches of
rain or more, we often see numerous reports of landslides across
the high terrain.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 130 PM EDT Wednesday: ***MAJOR RAINFALL EVENT FOR THE
SOUTHERN APPALACHIANS AND VICINITY WITH POTENTIAL FOR WIDESPREAD
FLASH FLOODING***

The PRE will be coming to an end by the beginning portions of
the forecast period as the initial tropical rain bands from
Hurricane Helene begins to move in from the south as Helene will
be in the midst of making landfall as a major hurricane near
the Big Bend of Florida Thursday evening. Large amounts of
rainfall will have already fallen across the Blue Ridge
Escarpment, southeast facing slopes, western Upstate South
Carolina, and northeast Georgia. Sandwiched between a closed
upper low over the mid-Mississippi Valley and an upper
anticyclone over the western Atlantic will inadvertently sling
shot Helene into the Southeast Thursday night before the storm
takes on a more western track as the closed upper low overtakes
the track by Friday afternoon. The heaviest rainfall rates will
be within the tropical rainbands and with already rising
streams, creeks, and rivers, expect for significant flash
flooding to take place in locations that will get hammered by
the PRE in the near-term forecast. The areas of most concern
will be the southern Appalachians and adjacent foothills as
flash flooding becomes widespread, with locally severe and life-
threatening flooding likely. The rainfall is expected to taper
off by Friday afternoon as Helene becomes absorbed by the closed
upper low and drier air filters in. Before everything is all
said and done, 5-10" of rainfall will be possible in locations
west of HWY 321 and 10-15+" on the southeastern facing slope
along the Blue Ridge. Still expect moderate to major flooding on
some main stem rivers including: French Broad River Basin,
Pigeon River and Tuckasegee River Basin, and Saluda and Broad
River Basins. These areas are forecasted to see river floods
that rival 1995 (TC Jerry) and 2004. With the amount of rain and
heavy rainfall rates, it`s likely that numerous landslides will
occur over the mountains, especially the Blue Ridge Escarpments
and result in a couple of large, damaging debris flows/slope
failures as well.

With the fast track north to the area that Helene will encompass,
the frictional force that usually derails any tropical cyclone`s
wind fields won`t have enough time to totally wipe out the strong
wind field associated with Helene. In this case, wind gusts of
40-50, some 60-70 mph will be possible in the western portions
of the CFWA Thursday night into Friday morning. In this case,
we have issued a Tropical Storm Watch for the North Carolina
mountains/foothills, Upstate South Carolina, and northeast
Georgia. The impacts will be significant from winds as soils will
already be super saturated and any strong wind gusts will easily
knock down numerous trees and lead to numerous power outages
across the locations in the Tropical Storm Watch. The tornado
threat will be the lowest concern of the big three that come with
any tropical system, but definitely nonzero, especially with the
initial rainbands that work into the region Thursday evening into
the overnight period. The biggest threat will be in locations along
and south of the I-85 corridor as better instability will become
established in this region and will be in the favorable front-right
quadrant of Helene. Most instability will diminish overnight as
more stable northeasterly flow filters in thanks to Helene. Most
model guidance lifts Helene out of the region by Friday afternoon
with conditions quickly improving. Flooding will be ongoing Friday
afternoon and evening, while mainstem river flooding may persist
into the weekend.

Saturday will be much improved and provides relief from the impacts
that will occur Thursday and Friday, with a lot of locations
cleaning up after the storm. The closed upper low is expected to
engulf Helene, but should gradually push east. This could lead
to a few showers in the mountains Saturday afternoon, but expect
for rainfall to remain light in comparison to the rainfall from
Thursday and Friday. Temperatures will be near-normal Saturday as
well in part to the lower heights and any lingering cloud cover.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 245 PM EDT Wednesday: Closed upper low is expected to engulf
the remnants of Helene, while gradually pushing across the Ohio and
Tennessee Valleys over the weekend before setting shop over the
Mid-Atlantic and Carolinas by the start of the new workweek. In
this case, showers will return back into the forecast for Sunday
and Monday as the upper low crosses the area and a moisture starved
front will swing across the region by midweek. Signs of true Fall
weather from the model guidance is evident beyond D6. Temperatures
will remain near-normal through much of the period as elevated
cloud cover and PoPs remain in the forecast.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: A main band of rain and thunderstorms has
made very little eastward progress this afternoon and evening,
as predicted, thus most terminals remain VFR at issuance time,
with the exception of KAVL. Unfortunately, it`s all downhill
from here. An area of TSRA will pass just east of KCLT over the
next hour, otherwise, we should make it thru the evening VFR there
until lower clouds develop after midnight/04Z. Elsewhere, a gradual
eastward creep in the rain band will bring the other terminals into
the restrictions associated with storms by 03Z at the latest, and
probably even before that. Ceilings drop overnight as more upslope
forcing arrives and deep tropical moisture streams up from the
south, down through MVFR to IFR through the pre-dawn hours. Once
there, we should stay down for the count in the LIFR/IFR range,
with frequent heavier showers moving through and winds becoming
gusty from the SE after daybreak. Lots of details are being left
out for brevity for Thursday.

Outlook: Helene will bring widespread low CIGs, heavy rain, strong
winds, and low level wind shear Thurs nite into Fri before moving
out of the area late Fri into Sat. Isolated to sct mainly diurnal
convection will be possible thru the weekend.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Flood Watch through Friday afternoon for GAZ010-017-018-026-
     028-029.
     Tropical Storm Warning for GAZ010-017-018-026-028-029.
NC...Flood Watch through Friday afternoon for NCZ033-035-048>053-
     056-058-059-062>065-068-069-501>510.
     Tropical Storm Warning for NCZ033-048>053-056-058-059-062>065-
     068>071-082-501>510.
SC...Flood Watch through Friday afternoon for SCZ008-010>013-019-
     101>109.
     Tropical Storm Warning for SCZ008>014-019-101>109.
&&

$$

SYNOPSIS...DEO
NEAR TERM...PM
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...PM