Westminster, South Carolina Weather Forecast Discussion

396
FXUS62 KGSP 230554
AFDGSP
Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
154 AM EDT Tue Sep 23 2025
.SYNOPSIS...
Scattered showers and thunderstorms, mainly over the mountains,
through Wednesday. A cold front reaches the area with more showers
and thunderstorms possible by Friday. Temperatures remain above
normal ahead of the front before cooling into the weekend with drier
weather.
&&
.NEAR TERM /THROUGH TONIGHT/...
As of 100 AM Tue: Shortwave moving thru the middle Ohio Valley
is driving bands of garden variety convection across eastern
KY/TN, but steering flow should keep the extant activity west of
the Appalachian spine thru daybreak. ML/MU CAPE is considerably
higher in that area than it is in our mountains; a small PoP will
persist early this morning along the TN border for the off chance
something survives that far. Debris clouds, and the fact dewpoints
mixed out a bit lower yesterday than the previous couple of days,
suggest mountain valley fog will be less extensive than on recent
mornings, though still likely close to daybreak.
RAP, as well as larger scale models, depict the main vort max
associated with the shortwave passing the mountains by late morning,
which could help explain the limited QPF/convective response from
CAMs today. However, some less distinct vort maxes will trail the
main wave, and notably the 12km NAM seems to latch onto a loosely
organized line forming in TN with an apparent MCV, which reaches the
northern mountains 15-18z. Other CAMs reflect more typical diurnal
mountain initiation although they favor the northern mountains,
possibly a nod to forcing from a remnant MCV like feature. Lapse
rates do marginally improve today via the cooler air aloft brought
by the shortwave, so a chance range PoP is forecast for most of
the mountains. Not entirely out of the question a few cells will
be able to move off the northern Escarpment and into the NW NC
Piedmont so will monitor CAMs for possible changes to PoPs later
this morning. Surface temperatures trend 2-4 degrees warmer today
across the board.
Heights fall slightly in the mid-MS and lower OH Valleys tonight
as next shortwave moves into the central Plains. Convection is
expected to wane more easily tonight in East TN without the boost
from a shortwave as seen this morning. WAA will persist there,
however, and into some of our mountains for that matter, so small
PoPs are again forecast near the TN border for the possibility of
stray convective initiation tonight or early Wed. Mountain valley
fog likely to return once again.
&&
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 120 AM EDT Tuesday: Picking up on Wednesday, an upper low
across the Midwest sticks around and slowly moves NE. Meanwhile,
guidance has a positively tilted trough elongated over the central
CONUS that amplifies through Thursday night. Ahead of a cold front
approaching toward the end of the period, more widespread showers
and thunderstorms are expected. Looking at the parameters, short
term model soundings show some instability with 20-25kt bulk shear.
The one caveat is the positive tilt to the trough, which could keep
the lid on more severe weather as the better forcing shifts well
north of the CWA. At this time, the Storm Prediction Center keeps
the area in a general thunder pattern. As with any short or long
term forecast, this could change, but the confidence is increasing
that any widespread severe is becoming less likely. As the front
nears, expect PoP chances to ramp up Wednesday through the end of
the period, with categorical (76%-80%) across the mountains and
eventually 60-75% elsewhere on Thursday into Thursday night. As far
as QPF response, the probability of receiving more than 1 inch of
rainfall is 15-20%. This is mainly for the far western NC mountains
as the chance decreases rapidly east of the mountains. This could
change, but as of now, model guidance is not depicting too much
concern for rainfall totals. Will continue to monitor. As for
temperatures, expect above normal temps ahead of the front with
relief to follow.
&&
.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 125 AM EDT Tuesday: By Friday, the cold front continues to
push toward the CWA, bringing showers and thunderstorms. Aloft, long
term guidance suggests the upper low weakens and gets reabsorbed
into the general flow, keeping the better DPVA far to the north of
the CWA. At this time, the front looks to cross the area Friday and
into Friday night, ushering in drier air by the start of the
weekend. Timing for the front is still not exactly certain this far
out. Friday, at this time, looks to have PoPs ramping down with
chance (55-60%) for the majority of the area and dipping to slight
chance (30-40%) by Saturday. Winds turn northerly for the remainder
of the extended period, reinforcing the drier air from the north.
After Saturday, model guidance gets sketchy. At the surface, the GFS
wants to keep a persistent area of high pressure that eventually
slides eastward toward the coast suppressing the majority of rain
chances. Once the front comes through, temperatures are trending to
be much cooler and more conducive for Fall.
&&
.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Light, largely calm winds early this morning
with FEW-SCT clouds above FL100 spreading in from convection west
of the Appalachians. The presence of this cloud cover looks to limit
the magnitude of valley fog restrictions, but not completely inhibit
fog/stratus formation. Also, KAVL managed to mix out a bit more
yesterday afternoon and precip was less widespread. Went with a more
typical 09z onset of IFR, now only in TEMPO. Otherwise VFR. Diurnal
cu will break out at 035-050 late morning to midday, while winds
remain largely SSW to SW for the day. Mountain convection looks a
little too isolated to warrant a PROB30 at KAVL but a VCSH mention
is still carried. Can`t completely rule out a cell near KHKY as well
but too unlikely to mention there. Winds turn light again tonight;
mountain valley fog probably will return in some form after 06z
Wed with higher cloud decks looking less of a factor overnight.
Outlook: Mountain shower and thunderstorm chances return again Wed,
then fog and stratus possible in valleys Wed night. Better chances
for restrictions with a front Thursday and Thursday night. Low
confidence as to flight conds Fri-Sat with guidance split between
dry high pressure and an upper low producing showers and promoting
moist onshore flow.
&&
.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.
&&
$$
SYNOPSIS...CP
NEAR TERM...Wimberley
SHORT TERM...CP
LONG TERM...CP
AVIATION...Wimberley

NWS GSP Office Area Forecast Discussion