357
FXUS62 KGSP 251736
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
136 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 TONIGHT/...
1000 AM Update...An upper trof axis has setup west of the FA and is
instigating an active PRE along a sfc frontal zone. Showers and
thunderstorms have developed and will continue to train along the
front into the sw/rn NC mtns an nrn NE GA. So far, new rainfall amts
have approached an inch across wrn Graham Co with little impact as
far as flooding is concerned. Good confidence is had in this pattern
holding rather steady state into the afternoon and evening, which
will increase the threat of flash flooding over the aforementioned
areas.

The WPC has a Day 2 Moderate Risk area that includes our western
zones and we have a Flood Watch in effect for that same area. In
addition, our area will be under the gun again this aftn/evening for
strong to severe convection with some organization possible due to
the continued high shear environment. These storms will be capable
of producing high rainfall rates and pose a risk of localized
flooding outside/east of the PRE area mentioned above. All this
precip is expected to occur before the rainfall associated with TC
Helene begins moving into our area on Thurs. Thus, more widespread
flooding is likely with this tropical system. Please remain weather
aware over the next few days, the potential for major to
catastrophic flooding over parts of our fcst area is looking more
likely.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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 THROUGH TUESDAY/...
As of 325 am EDT Wednesday: Upper low is forecast to be centered
somewhere over the Ohio Valley at the start of the extended, and
should begin wobbling east over the weekend, as a series of strong
short wave brush the northern part of the Conus. We should have an
opportunity to dry out over the weekend, as precip chances are
expected to be limited to spotty, mainly diurnal showers. Precip
chances ramp up a bit (although remaining of the token chance
variety) early in the work week as the upper low opens up and ejects
over the region. Any additional rainfall amounts should (hopefully)
be light during this time frame. Temperatures are forecast to be
close to normal through the period.

&&

.AVIATION /18Z 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...JPT
NEAR TERM...JPT/SBK
SHORT TERM...JDL
LONG TERM...JDL
AVIATION...SBK