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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 251046
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
646 AM EDT Sun Sep 25 2022

.SYNOPSIS...
Scattered showers and isolated thunderstorms are expected today. Dry
high pressure builds in through Wednesday with a slow cooling trend.
Tropical cyclone Ian moves north into the Gulf of Mexico affecting
our weather next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
As of 645 AM...A patch of light rain has entered the western
zones over the last couple hours, but is weakening. Otherwise,
there is plenty of thick mid clouds and continued mixing to keep
temps a little elevated in spots. Definitely starting out warmer
than this time yesterday. Only changes with this update were to
the aviation grids for the 12z TAFs.

An upper long-wave trough will continue to deepen across the Ohio
Valley to the Southern Appalachians today, as a series of strong
shortwaves ride thru the trough axis. This trough will bring an
attendant cold front across the TN Valley this morning, and into
the NC mountains this aftn. The digging trough will help increase
bulk shear (0-6 km shear in the 50-60 kt range). But the fairly
narrow band of deeper moisture will struggle to produce much
deep convection as the front pushes into the forecast area. This
is thanks to a westerly flow, limiting moisture transport and
causing downslope-aided CIN east of the mountains. A lack of good
instability seems to be the limiting factor on thunder chances and
any severe wx threat. The new Day 1 convective outlook has expanded
the marginal risk to include all the NC mountains and adjacent
foothills. For what it`s worth, the last few runs of the HRRR
have been bringing in 500-800 J/kg of sbCAPE into the mountains,
and producing a rather solid line of convection that crosses the
northern 2/3rd of the forecast area this aftn, exiting east early
this evening. A few damaging wind gusts look possible today, if that
verifies. However, the GFS/ECMWF/Canadian have more paltry CAPE,
and the other CAMs show much weaker/less organized convection. For
now, going with mainly chc PoPs and isolated thunder chcs. Temps
will be near normal under partly to mostly cloudy skies.

The cold front will exit to our southeast this evening, ushering
in drier air and clearing skies. The low-level flow behind the
front will not be that strong and generally across the mountain
valleys. So mountain valley fog looks likely. Lows will be near
normal.
-- End Changed Discussion --

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 220 AM EDT Sunday: Quiet weather expected for the short term
period as a series of short waves move through the the long wave
trough over the eastern CONUS but no significant forcing over our
area. Mild and dry high pressure filters in from the north through
the period as well. Breezy and dry conditions are expected with a
slow cooling trend. Highs will be near to slightly below normal
Monday dropping to around 5 degrees below normal for Tuesday. Lows
will be around 10 degrees below normal Monday night then a degree or
two cooler Tuesday night.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 245 AM EDT Sunday: The upper trough moves east of the area
Wednesday with surface high pressure setting up in a cold air
damming configuration. The dry forecast continues with highs and
lows around 10 degrees below normal.

All eyes turn to Ian as it will dominate the rest of the forecast.
All of the operations global guidance shows Ian moving north into the
weakness in the upper flow behind the departing trough. Questions
remain on the exact track as Ian moves from the Gulf, over or near
FL, then into the southeastern US. Minor differences in the track
could make major changes in the to the observed weather over our
area. Therefore, have continued to use the model blend for the
forecast. This means a dry Thursday with PoP increasing into the
chance range for Friday and Saturday. Highs will be on either side
of 10 degrees below normal each day, with lows rising to near normal
by the end of the period.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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At KCLT and Elsewhere: A patch of light rain is moving east into
the Upstate, but is expected to dissipate before reaching any
of the TAF sites. Plenty of mid clouds expected this morning. A
cold front will push into the area this aftn, and could trigger
scattered SHRA and possibly isolated TSRA. KAVL has the highest
chance of some TS, so will add VCTS/PROB30 there. But confidence
is still too low for TS at the other TAF sites. But PoPs are high
enough to warrant PROB30 for SHRA this aftn, exiting east early
evening. Wind should stay SW, with some occasional gusts expected
this aftn. Wind will become light this evening and begin to shift
from SW to NW toward the end of the 06z TAF period. The mountain
valleys may fog up late tonight, so will add some reduced VSBY at
KAVL. But not expecting fog elsewhere.

Outlook: Dry high pressure will return on Monday and dominate the
weather across the region through midweek. Aviation interests will
need to monitor the progress of Ian for possible impacts later in
the week.
-- End Changed Discussion --

&&

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

&&

$$
SYNOPSIS...RWH
NEAR TERM...ARK
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...ARK