Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211838
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
238 PM EDT Thu Sep 21 2017

.SYNOPSIS...
A moist easterly flow will be developing on Friday followed by
drier and stronger high pressure this weekend. Tropical Systems Jose
and Maria will remain off the east coast.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 225 PM: The sfc pattern remains somewhat stagnant, with high
pressure extending from eastern Canada down the Appalachian chain,
while Tropical Storm Jose remains over the western Atlantic. The
high is associated with a sharp upper ridge. A shortwave trough is
present over the Carolinas, with the axis of best DPVA straddling
the region now, advecting south. Diurnal instability nonetheless
will drive scattered showers and t-storms over the mountains
this aftn. Expect these cells to drift south over time with weak
northerly steering flow. Models do indicate some expansion of
coverage in the southern SC/GA zones later this aftn presumably
as the DPVA exerts its influence. DCAPEs are marginally indicative
of a severe wind threat, though sfc-midlevel delta-theta-E values
are not as remarkable; values are not as high as yesterday when
we did manage a couple severe wind/hail events. I won`t totally
rule one out today but the chance appears lower.

Cloud cover and PoPs will diminish with the end of diurnal heating
this evening. The high pressure will bring increasingly easterly
flow tonight but there is not enough low level moisture progged
to suggest nocturnal stratus will result. Mins will remain muggy
and well above normal. Mountain valley fog looks likely to result
once again, with patchy Piedmont fog, though the latter is not
expected to be any more widespread than it was this morning.

Friday will be another day dominated by the incumbent weak high
pressure, featuring above-normal sfc temps. Thicknesses will be
slightly reduced, however, suggesting maxes 2-3 degrees cooler
than Thursday. Still, given recent model performance and plentiful
sunshine, I went toward the warm end of guidance. Modest instability
is progged, and the easterly flow will help initiate storms over
the higher terrain; weak subsidence most likely will cap off any
activity over the NW NC Piedmont. A slight chance PoP still is
warranted over the Upstate and NE GA. Marginally dry profiles
imply an isolated severe storm could develop.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 205 PM EDT Thursday: A weak upper low over the Carolinas will
sink south to the FL panhandle on Saturday as ridging strengthens
over the Great Lakes and mid-Atlantic. The center of the ridge moves
east on Sunday and a ridge axis builds south toward the Carolinas.
At the surface, weak high pressure slowly builds to the north then
noses southeast into the area through the period. This sets up weak
northeasterly flow across the area. Drier low level air moves in
from the NE on this flow and subsidence develops as Maria moves
northward well off shore of GA and SC. Instability diminishes
through the period with the drying and subsidence. Expect some
lingering isolated convection across the mountains Saturday where
moisture is best and LFC levels will be low enough for convection to
develop. Dry conditions expected elsewhere on Saturday and all areas
Sunday. Highs and lows will be around 5 degrees above normal
Saturday then maybe drop a degree or two on Sunday.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 100 PM EDT Thursday: Dry and warm conditions will continue
into the start of the new workweek with ridging lingering atop the
southern Appalachians.  Temperatures will change little Monday and
Tuesday featuring readings 5 to 7 deg f above climo.  By Wednesday,
even though the ridge center atop the NE conus gets pinched off, the
SE conus remains in a subsidence regime between the Northern Plains/
Rocky Mtn l/vw trough and TC Maria.  At this point, there seems to
be enough s/wv ridging lingering acrs the SE Conus into the new day
7 for the fcst to remain dry.  The streak of above normal
temperatures are expected to linger as well through the end of the
period.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
At KCLT and Elsewhere: Little real change this period compared to
recent days. A shortwave trough is over the area at the start of
the period, and will slightly enhance convective initiation over the
Upstate, in addition to terrain effects which are already producing
SHRA/TSRA over the mountains. TEMPO TSRA at KAVL, with VCSH at
KAND. The trough will gradually lose influence over the area as
it moves south and east tonight, with convection waning following
the loss of heating. Profiles remain rather dry and little cloud
cover is expected. There appears no reason not to expect valley
fog yet again. KAVL`s chances will be enhanced if showers occur at
or invof the field this aftn. NE winds will veer a bit overnight
as high pressure exerts increasing influence from the north.

Outlook: Chances for diurnally favored showers and thunderstorms
remain in the fcst again Friday, but precip chances are a bit lower
over the weekend and early next week. Fog and/or low stratus are
likely to return in the mtn valleys each of the next few mornings.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High  97%     High  94%
KAVL       High 100%     High  97%     High  88%     High  90%
KHKY       High 100%     High 100%     High  97%     High  97%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  97%     High  97%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...CSH
NEAR TERM...Wimberley
SHORT TERM...RWH
LONG TERM...CSH
AVIATION...Wimberley


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