Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 240243

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1043 PM EDT Sat Sep 23 2017

Broad high pressure will continue to build through the weekend and
persist through the middle of next week. In the meantime, Tropical
Cyclones Jose and Maria will remain off the east coast.


As of 1030 PM: A couple showers still bubbling up across the central
French Broad Valley, but should dissipate shortly. Otherwise,
patches of stratocu and cirrus still over the area, which should
generally decrease in coverage as we head into the overnight hours.
Dewpts are fairly elevated, and winds are decoupling, so I expect
plenty of fog in the major mountain valleys and patchy fog around
the usual fog-prone areas of the piedmont. Temps will bottom out a
couple categories above normal.

Fairly benign weather pattern will continue through Sunday as a
large eastern CONUS upper ridge makes gradual eastward progress
Surface high over the northeast CONUS with ridge running down the
Southern Appalachians changes little and will keep the area in light
northeasterly flow. Slight warming at mid-levels will limit
instability, so no showers are forecast for Sunday.

Temperatures have been running above guidance and model blends.
Current forecast was arrived-at by adding a couple degrees to broad
model blend.  This gives temperatures 5 to 10 degrees above normal
today and tomorrow.

Hurricane Maria continues to spin in the Atlantic, and is moving
down the axis of minimum deep layer shear towards the NC Outer
Banks. For Upstate SC and surrounding areas, Maria will do little
more than slightly enhance the northeasterly surface flow on Sunday.


As of 215 PM Saturday: Dry and unseasonably warm weather will
persist through the short term, as a highly amplified upper ridge
tightens its grip on the eastern Conus. The air mass is expected to
remain dry and stable early in the week, as a surface ridge
strengthens with the upper anticyclone gradually spilling east of
the northern/central Appalachians. That said, the official forecast
track of Tropical Cyclone Maria has slowly edged westward through
time. While there is good agreement that this increases the pressure
gradient between Maria and the surface ridge, possibly leading to
breezy conditions by Tuesday, there is disagreement on whether low
level moisture increases in this flow. If the moisture does increase,
then the atmosphere could become unstable enough for isolated
convection over the area. For now, have kept the forecast dry due to
the uncertainty. Also uncertain is the amount of any high clouds
that would move in from the east around the periphery of the
cyclone. Again, due to the uncertainty, did not lower the highs as
much as a model blend would suggest, but did drop them a degree or
so across the I-77 corridor. For now, this keeps highs and lows 5 to
10 degrees above normal.


As of 205 PM EDT Saturday: the extended fcst picks up at 00z on
Wednesday with the persistent upper ridge beginning to weaken as
a northern stream upper trof moves up and over the ridge from the
west. As we move into Thurs and Fri, the upper trof axis will
broaden and amplify over the Great Lakes and is expected to dig
farther southward and approach the fcst area by the end of the
period on Sat. At the sfc, high pressure will still be lingering
over the region on Wed as TC Maria remains just off the Carolina
Coast. Compared to previous fcsts, the trend has been to bring
Maria a bit closer to the Coast on Wed, and then move her farther
offshore later on Thurs. This newer scenario will likely result in
a less impressive overall fropa on Thurs. In the wake of the front,
cooler high pressure will overspread the region on Fri and linger
well into the weekend. As for the sensible wx, mostly dry conditions
are still anticipated, although the long-range models are showing
more moist NLY flow on Wed as Maria gets closer to the Carolina
Coast. Temps will remain well above climatology thru Thurs and
cool to below normal by Sat.


At KCLT and Elsewhere: Not much change for the 00z TAFs. A few
showers lingering north of KAVL for first hour or so of TAF period,
but otherwise should be quiet. Cu field will dissipate, then
mountain valley fog will develop overnight as it has the last few
nights. An upper ridge will persist just north of the area, keeping
a steady and light northeasterly flow atop the area the period. The
air mass will be more stable tomorrow, so only a fair wx cu field
under areas of cirrus expected. Winds stay out of the NE tomorrow.

Outlook: Dry conditions expected for the first half of the week.
Under mostly clear skies and calm conditions, fog and/or low stratus
are likely in the mtn valleys each of the next few mornings.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  95%     Low   44%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High 100%

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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