National Weather Service Raw Text Product

290 
FXUS62 KGSP 260303
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1103 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 1055 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) continues, with the rain starting to soak in and run off,
already raising some streams into flood. The eastern edge of the
rain band is sort of being re-oriented on a line from Abbeville to
Spartanburg to Morganton, but that might not move much for the next
several hours. There remains some overlap on the eastern edge with
an air mass over the western Piedmont that retains 500-1000 J/kg
of sfc-based CAPE and deep layer shear of 30-40 kt. Another brief
tornado would not be out of the question, after what we have already
seen today, but the trend continues slowly downward in terms of
the severe weather threat. That being said, the flash flood threat
continues to ramp up as the rain continues to fall, mainly over the
foothills and along/near the Blue Ridge Escarpment. We are eyeing
another large area of rain and thunderstorms over central GA moving
northward in the conveyor belt. This could be the precip that puts
more streams into flood in the early morning hours. Several more
warnings should be expected overnight.

The new Advisory for Helene will keep the Tropical Storm Warnings
unchanged, while the Flood Watch also remains unchanged based on
the most recent QPF guidance. The 00Z CAMs are filtering in, and
the HRRR remains compelling with its wind gust forecast bringing
a swath of wind gusts into the region, but still not until after
00Z Friday. In other words, the wind gust potential still does
not ramp up until Thursday evening. Otherwise, all signs point to
a significant event across the western Carolinas and northeast
Georgia. Although another subtle westward shift in the track
is noted, Helene will be a large and fast-moving storm, so the
westward 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. Once
the rainfall gets above ten inches of rain or more, we often see
numerous reports of landslides across the high terrain, and the
threat for landslides and debris flows will increase dramatically
Thursday night.

&&

.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 predecessor rainfall event 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 /03Z 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