Forecast Discussion for GSP NWS Office
FXUS62 KGSP 041855

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
255 PM EDT Wed Oct 4 2023

Dry high pressure will remain in place across the region through
Thursday. A cold front will approach from the northwest on Friday
and move east of the area on Saturday. Rainfall amounts will be very
limited with the frontal passage. Temperatures will remain above
normal ahead of the front, becoming below normal this weekend
following the frontal passage. Dry conditions will continue early
next week.


As of 215 PM EDT Wednesday: Stout diurnal cu field currently
roaming the sky across the CFWA as mixing within the boundary
layer maximizes. Rex Block still in place along the East Coast
with surface ridging continuing to nose in from the northeast,
which will help place well-above normal temperatures across most
locations this afternoon/evening. The Rex Blocking pattern will
begin to breakdown overnight into Thursday as a digging longwave
upper trough propagates across the north-central CONUS during the
period with an attendant cold front. The airmass won`t change much
during the near-term, but the parameters for an airmass change will
be coming together. The cold front is shown stretching from the
Gulf Coast into the OH/TN Valleys, and all the way to the Hudson
Bay by the end of the period Thursday. The moisture return ahead
of the front is relatively weak and pre-frontal DPVA isn`t potent
at all. With that being said, subtle height falls will begin to
take place and should help allow temperatures to drop a degree or
two for Thursday compared to today. Still expect a weak nocturnal
subsidence inversion with light moisture trapped underneath, which
could lead to another night of stratocu. Also, another round of
mountain valley fog will be in store as long as it`s not fully
disrupted by widespread nocturnal stratocu. Overnight lows will
be a few ticks above normal.


As of 144 PM EDT Thursday: An intense upper jet diving out of the
Canadian Prairies will continue to carve out a potent trough that
will dig across the Midwest and Ohio Valley tomorrow night into
Friday. Ahead of this feature, a lead shortwave trough will be
sliding across the Deep South and into the Southern Appalachians
with an associated surface cold front pushing across Middle
Tennessee and into the Cumberland Plateau. A ribbon of 1.5" PWATs
pooled ahead of the front will encroach on the mountains tomorrow
night into Friday where a few isolated to scattered showers will be
possible, especially along the Tennessee border. As this initial
front moves into the mountains it will be quickly overtaken by a
much stronger cold front racing into the area in associated with the
previously mentioned potent trough as it takes on a negative tilt
and lifts through the Mid-Atlantic.

This much stronger front will quickly sweep across the area Friday
night into Saturday morning. The environment east of the mountains
will not be conducive for precipitation as moisture return will be
negligible ahead of the boundary owing to northerly low-level flow
as a coastal low slides north well offshore. Antecedent dry air
ahead of the rapidly advancing front will have little time to modify
with scant boundary layer moisture present on forecast soundings.
Some moistening of the column may be realized with strong forcing
possibly squeezing out a few isolated showers east of the mountains,
but this will be an exception and not the ruled with the majority of
the area experiencing a dry frontal passage. Any isolated showers
should quickly come to an end Saturday morning as PWATs crash to
0.25" in the face of strong dry advection and downsloping.

Temperatures will start off on the mild side tomorrow night with
lows running 5-10 degrees above average as cloud cover increases and
the boundary layer remains mixed. Warm high temperatures will
continue on Friday with temperatures near or just above average.
Cold advection quickly ramps up behind the front Friday night into
Saturday with 850mb temperatures crashing to 3-6 C by Saturday
evening. Friday night lows will be at or a few degrees below average
while Saturday highs will be 10-15 degrees below average.


As of 229 PM EDT Wednesday: The coldest temperatures so far this
fall will be gracing the area Saturday night through Sunday night.
Strong cold advection will usher in a chilly early fall airmass with
temperatures running 10-15 degrees below normal. The boundary layer
will likely remain mixed Saturday night, but temperatures are still
forecast to fall into the low to upper 30s across the mountains and
low to mid 40s farther east. Sheltered valleys across the southwest
mountains have the greatest chance of seeing frost if winds are able
to relax enough. Continued cold advection will keep highs on the
cool side Saturday despite plenty of sunshine. Mountain valleys may
reach the 60 degree mark with higher elevations remaining in the
50s. Highs outside of the mountains should range in the mid to upper
60s. The coldest night of the period will likely be Sunday night as
surface high pressure slides across the Gulf Coast states and into
the Southern Appalachians. This will allow for very efficient
radiational cooling of the dry airmass with light to calm winds
beneath mostly clear skies. Temperatures Monday morning will be in
the low to mid 30s across the mountains with upper 30s to low 40s
elsewhere. The highest mountain tops and ridgelines will likely see
sub-freezing temperatures. Patchy to widespread frost is also
expected across the mountains.

The airmass will quickly begin to modify on Monday as low-level flow
backs and brings an end to cold advection as a closed upper low
wobbles across southeast Canada. 850mb temperatures quickly warm in
a quasi-zonal flow regime with further deep mixing of the dry
airmass. This will allow for large diurnal temperature swings with
chilly temptress Monday morning quickly warming into the 60s to
low 70s. This would be a 30-35 degree temperature swing across much
of the area, which isn`t too uncommon with a dry fall airmass.
Further airmass modification will occur through early next week with
temperatures back into the low to upper 70s by Wednesday.


At KCLT and elsewhere: VFR conditions expected through the forecast
period with the exception of mountain valley fog/low stratus
potential at KAVL daybreak Thursday. Winds are mainly light and
variable, but are forecasted to toggle to a east-southeasterly
component during peak heating. A FEW/SCT field of fair weather cu
and/or stratocu will continue to roam the sky through sunset, but
will remain VFR. A weak nocturnal subsidence inversion is being
depicted per model guidance once again overnight and could lead to
sporadic stratocu (040-060). The environment is not as ideal as last
night, but could still impact TAFs. Otherwise, another round of
mountain valley fog/low stratus will occur just before daybreak
Thursday as winds go light and variable at all terminals.

Outlook: Persistent high pressure is expected to keep VFR conditions
in place through the workweek. The main exception is the potential
for mountain valley fog around each morning around daybreak. A cold
front may bring showers and associated restrictions to the mountains
Friday into Saturday.





NWS GSP Office Area Forecast Discussion