389
FXUS62 KGSP 232150
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
550 PM EDT Mon Sep 23 2024

.SYNOPSIS...
A moist and cooler airmass will remain in place across the forecast
area through mid-week.  A tropical system is expected to strengthen
over the Gulf of Mexico, making landfall late Thursday and affecting
the Carolinas on Friday.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 545 PM EDT Monday: Low level stratocu continues eroding along
the edges but won`t fully be out of the NC Piedmont until the next
hour or two. Scattered showers and thunderstorms remain over the
western half of the forecast area this evening and are slowly
progressing eastward. Increased PoPs to likely where it`s currently
raining (based off the latest KGSP radar loops) and made some tweaks
to PoPs through the rest of the near term period based on the latest
high-res guidance. No other changes were needed.

Otherwise, a moderately unstable air mass remains in place this
evening with 20 to 35 kts of effective bulk shear. The shear will
help with storm organization helping create the potential for a few
strong storms, and perhaps an isolated severe storm or two, with
damaging winds as the main hazard. Storms should diminish later this
evening with isolated showers possible into the early overnight
hours. Expect low stratus and patchy fog to redevelop from NE to SW
across the area overnight. Lows will be around 10 degrees above
normal.

Tuesday should see a similar scenario as today with morning stratus
slowly eroding and lifting from SW to NE through the day. Moderate
instability with up to 35 kts of effective bulk shear expected as
well. There may be better DCAPE Tuesday afternoon though. Expect
good diurnal convective coverage during the afternoon with a few
severe thunderstorms. Highs will see a similar pattern as well with
cooler temps where the stratus burns off latest. Highs will be
around 5 degrees above normal.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 155 PM EDT Monday: By Tuesday evening, a potent upper low will
be in place over the Ozarks, with the western Carolina sandwiched
between its eastern periphery and a deteriorating ridge off the
Atlantic coast.  On the east side of a narrow baroclinic zone
stretching from the Mississippi Delta region up into the Tennessee
Valley, moisture will stream into the forecast are out of the
southwest, maintaining cloud skies and at least scattered afternoon
on Wednesday.  As low-level flow turns around out of the ESE late
Wednesday and into Thursday, moisture will gradually diminish,
resulting in a brief reprieve from the wet conditions. For what
showers do develop on Wednesday, some thunder will be able to mix
in, given 1000-1200 J/kg sbCAPE.  20kts or so of deep layer shear
may support some loosely-organized multicell clusters, and,
especially as dry air begins to build in aloft, perhaps some
downburst wind threat.

Thursday will feature the slow approach of the remnants of Potential
Tropical Cyclone 9.  Models are in good agreement at this time on
the low moving inland over the Florida Panhandle and tracking NNE
into central Georgia through late in the day Thursday.  The first
upper-level moisture associated with PTC 9 will arrive as early as
Thursday morning...but in terms of the deeper moisture needed to
spur heavy rain...that`s not depicted arriving earlier than late
Thursday afternoon, or possibly past the end of the short term
period Thursday night.  Thus, more appreciable rainfall doesn`t
appear likely through the end of the day Thursday.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 204 PM EDT Monday: Thursday night and the first part of Friday
will give us our taste of whatever impacts PTC 9 brings. Long-range
ensembles remain split into two camps.  The dominant group of LREF
members curve the post-tropical low westward as it interacts with
the preexisting upper low still churning over the Ozarks.  This
results in generally lower QPF response for the western Carolinas,
and weaker winds.  The secondary group of LREF members, however,
depicts either less interaction with the low, or a low shifted a
little farther eastward, bringing the remnant low directly over
northeast Georgia and the western Upstate.  Such a solution would
result in potentially significant rainfall across parts of the
Upstate.  Forecast soundings for the Upstate, from both the
operational GFS and GDPS, also feature low-level winds in excess of
45kts.  The 850mb charts are even more dire.  Although the
configuration won`t be favorable for mixing this wind down to the
surface in its entirety, it`s a hint that strong winds may also be a
problem Thursday night and Friday.  The current NHC forecast calls
for no more than a 10% chance of tropical-storm force winds for the
Upstate...and that seems quite reasonable at this time.

Friday night, whatever is left of the post-tropical low will either
deteriorate in a zone of unfavorable dynamics over central Virginia,
or be absorbed into the preexisting Ozark low (depending on whether
the eastern or western track pans out).  Behind it, an extensive
layer of dry air aloft is progged to push into the area from the
south, associated with weak ridging.  Despite this, weekend
temperatures look to stay in the low- to mid-70s.  The Ozark low,
meanwhile, should remain in place somewhere over the Deep South.
Consensus is, though, that it`ll remain far enough west as to
preclude any impacts for our area.

&&

.AVIATION /22Z MONDAY THROUGH SATURDAY/...
At KCLT and elsewhere: MVFR to IFR stratocu is eroding around the
edges and will scatter out at the SC sites shortly, then KHKY and
KCLT about 22Z. Convection ongoing across the mountains and should
move out across the area through the afternoon and early evening.
Have included TEMPO TSRA at all sites but timing may need to be
adjusted later. TSRA ends this evening but isolated SHRA could
continue into the early overnight. Guidance is in good agreement
that MVFR stratus redevelops from NE to SW overnight, drops to LIFR,
then lingers through the morning Tuesday as it rises back to MVFR.
Have gone with this in the TAFs. There will be some MVFR to possibly
IFR vsby as well. Expect mainly diurnal convection again on Tuesday.
E to SE wind this afternoon becomes light and variable overnight
then S to SE on Tuesday.

Outlook: A slowly approaching weak cold front stalls to our
northwest Wednesday. This will keep the chance for diurnal showers
and thunderstorms over the area. Fog/low stratus may develop each
morning, especially in the mountain valleys. Confidence is
increasing that a tropical cyclone, with attendant heavy rain and
possible strong winds will approach the area Thursday, move over or
near the area Friday, then move out of the area for Saturday.

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$

SYNOPSIS...MPR
NEAR TERM...AR/RWH
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...RWH