Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 221842

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
242 PM EDT Fri Sep 22 2017

Drier and stronger high pressure will develop over the weekend and
persist through at least the middle of next week. In the meantime,
Tropical Cyclones Jose and Maria will remain off the east coast.


As of 220 PM: A somewhat blocked pattern persists over most of
the eastern CONUS; an anticyclone extends from Texas to the Great
Lakes, but is undercut by a weak upper low over the Southeast. At
the sfc, high pressure centered to our north continues to drive
a light northeasterly flow into the CWFA. Warm and humid sfc
conditions combined with modest lapse rates aloft will result
in aftn instability. The easterly flow should kick off scattered
showers/storms over the mountain ridges, with more isolated cells
developing over mainly the SC/GA zones, where dewpoints are a bit
higher and subsidence slightly weaker. Dry profiles and weak shear
are similar to those seen the past couple of days, when a couple of
storms managed to produce damaging winds and severe hail. Slightly
lesser coverage is fcst today, though the isolated severe threat
still exists.

The pattern progresses a tiny bit tonight, as the high starts
to push east into the NE CONUS, and the low retrogrades slightly
toward the central Gulf Coast. Vort lobes on the periphery of the
low may bring some patches of high clouds through the area, but
skies should be mostly clear. Crossover temps are expected to be
reached in some areas of the Piedmont and so patchy fog is expected;
the mountain valleys should fog easily particularly where rain
falls nearby. Min temps will be similar to the previous morning.

The increased influence of the ridge should allow for deeper mixing
and slightly drier sfc conditions that accordingly will reduce
instability. PoPs hence are lower and limited to the NC mountains
and far NE GA. Max temps will be largely unchanged: in the upper
80s across the Piedmont, and lower 80s in the mountain valleys.


As of 150 PM Friday: Upper ridging builds south into the area from
the Great Lakes and mid-Atlantic on Sunday and remains through
Monday although it does weaken. Surface high pressure follows a
similar pattern while Maria moves north well off the GA and SC
coast. The result will be a dry northeasterly flow keeping stable
air over the area. Despite the NE flow, thickness levels remain high
keeping highs around 5 degrees above normal each day. Fair skies and
the drier air mass will allow temps to fall overnight, but lows also
remain around 5 degrees above normal each morning.


As of 215 PM EDT Friday: the extended fcst picks up at 00z on
Tuesday with steep upper ridging in place across the Eastern CONUS
and deep trofing over the west. Over the next few days, the upper
trof will move eastward and begin to suppress the ridge as the
remnants of Tropical Cyclone Maria remain off the Carolina Coast. By
the end of the period next Friday, the upper trof axis is expected
to be over the region or just to our west. At the sfc, broad high
pressure will persist over the region thru most of the period. As
previously mentioned, TC Maria is expected to remain just off the
East Coast and not have a significant impact on the fcst area. A
fairly robust looking cold front will approach the CWFA from the NW
by early Thurs and likely move thru the CWFA by the end of the
period on Fri. As for the sensible wx, no major changes were made to
the fcst with mostly dry conditions expected. Temps will remain well
above climatology thru most of the period with a noticeable cool
down by day 7.


At KCLT and Elsewhere: High pressure and summerlike conditions will
be the norm again this period. Easterly/upslope low-level flow will
drive mountain convection this aftn, which has prompted VCTS mention
at KAVL. Humid sfc conditions also will lead to instability over
the Upstate sites, though SHRA/TSRA are expected to be too isolated
to warrant TAF mention. Low VFR cu and precip chances dissipate
at sunset. Winds will be light and VRB tonight by and large, but
should prefer N to NE. Crossover temps are expected to be reached in
portions of the area tonight, which will lead to fog formation and
MVFR vsbys for KHKY/KAND, and IFR in the mtn valleys and at KAVL.

Outlook: Precip chances will wane over the weekend and early
next week. With clear skies and calm conditions, 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  91%     High 100%
KAVL       High 100%     High 100%     Low   46%     High  81%
KHKY       High 100%     High  97%     High  89%     High  94%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  94%     High  94%

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:




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