942
FXUS62 KGSP 251851
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
251 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/...
A well developed Precursor Rain Event (PRE) has developed from srn
AL northeast across ne/ern TN. This has band of precip has become
more organized in the past couple hours and has filled in across and
upstream of the far wrn portions of FA. Rainfall rates will be quite
high as this line will not move much thru the near term period and
expect issues with flash flooding stream/urban to develop this
evening and continue thru the overnight. Areas east of the mtns will
have a harder time developing convection as mlvl LRs remain a little
low, however, a few cells will have the potential to become severe
as bouncy increases and interacts with high levels of bulk shear.
Also, updraft helicity has increased and a tornado or two will be
possible thru the evening.

A Tropical Storm Watch has been issued for the wrn 2/3rds of the FA
as tropical storm conds will likely begin developing just after the
near term, or Thu night. Also, rainfall amts have gone up with the
latest models showing an increase of QPF as the trop system
interacts with the PRE environment and additional upslope forcing.
Thus, the going Flood Watch has been expanded east by a county or
two. This system will produce very rare and significant amts of
rainfall and isol areas across the mtns and adjacent zones will see
catastrophic and damaging flooding by the later near term.
Landslides across the high terrain will also be probable.

&&

.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 /19Z WEDNESDAY THROUGH MONDAY/...
At KCLT and elsewhere: The TAF set will start out mostly VFR with
MVFR lingering at KHKY for a few more hrs this afternoon. Precip and
thunder chances outside the the mtns will be tricky, but enuf
destabilization will likely develop sct showers with isol thunder at
least, so went with a TEMPO thunder all sites later this afternoon
into the evening. Flight conds will begin to deteriorate overnight
with IFR conds, mainly due to CIGs, likely at all sites before
daybreak. High moisture levels persist Thu and will expect little
improvement in CIGs thru the day. Winds remain se/ly outside the
mtns and more varying at KAVL with low-end gusts all sites by the
late period.

Outlook: A tropical cyclone will bring low CIGs, heavy rain, and
strong winds Thurs 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 Watch 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 Watch for NCZ033-048>053-058-059-062>065-068-
     501>510.
SC...Flood Watch through Friday afternoon for SCZ008-010>013-019-
     101>109.
     Tropical Storm Watch for SCZ008-010>013-019-101>109.
&&

$$

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