Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210522

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
122 AM EDT Thu Sep 21 2017

A weak trough will move east of the area tonight and settle along
the southeast coast on Thursday. A gradual increase in more moist,
easterly flow is expected through the latter part of the work week.
Drier and stronger high pressure will settle across the region over
the weekend and likely persist through early next week, as Tropical
Systems Jose and Maria remain off the east coast.


As of 120 AM EDT Thursday:  A few patchy llv cu remain spread about
the region early this morning, while ground fog has started to
develop in the mtn valleys as expected.  Further fog development
outside of the mtns isn`t out of the question, but should stay
rather isolated with the best chances for locales that experienced
rainfall last evening.  Otherwise, no sig changes needed/made to
the fcst with this update.

Previous Discussion: Convection has completely waned within the
CWFA, but there are a couple of isolated storms to the NE, which
look to be on a dying trend. The latest SPC mesoanalysis page shows
lingering CAPE of around 1000 J/kg, but with plenty of CIN. The
weakest CIN is in the northern NC mountains, where the GOES RGB
nighttime product shows some ridgetop cu. So cannot rule out a
stray shower over the next couple of hours, but not enough to
warrant any precip mention. So going with a dry forecast for the
rest of the night. Otherwise, expect valley fog to develop again,
which could be dense late. Temps look on track, with mins a couple
categories above normal.

A sharp ridge of high pressure will build over the inland parts
of the eastern CONUS on Thursday, while a shortwave will remain
on our southeast flank. Though the high will bring N to NE flow,
thermal advection is basically neutral, so max temps will remain
6 to 8 degrees above normal. Lapse rates won`t be quite as good in
the wake of the shortwave, but respectable instability still will
develop. Models mainly key on terrain as an initiation mechanism,
leading to chance range PoPs over the mountains and slight chance
in the Piedmont. Profiles feature a bit more moisture aloft but
sfc-midlevel theta-E lapses are progged to be sufficient for a tiny,
but nonzero, threat of damaging winds.


As of 155 PM EDT Wednesday: An upper level trough axis will settle
along the southeast coastline Friday and linger through Saturday, as
a 590 dm ridge axis sets up from the Great Lakes to the MS Valley.
Low level easterly flow continues Friday into Saturday which should
help to focus the highest chance PoPs along the eastern slopes of
the mountains. Thus anticipate mainly diurnal, scattered ridge
top/eastern mountain convection both days with weak forcing and
limited moisture. Drier air moves into the NC Piedmont on this flow
Saturday limiting precip chances there, with better chances
remaining over the mountains, western Upstate and NE GA. SBCAPE
values should steadily recover to 1000 to 1500 J/kg across the
mountains to support a scattered thunder mention. Instability will
be lower elsewhere but high enough for thunder. Highs will be nearly
steady around 5 degrees above normal both days. With lows around 5
degrees above normal Friday morning falling a couple of degrees
Saturday morning.


As of 155 PM EDT Wednesday: the extended fcst picks up at 00z on
Sunday with persistent and highly amplified upper ridging in place
over the Eastern CONUS and steep upper trofing over the west. The
trof will gradually slide eastward thru the period and will begin to
flatten the ridge by the end of the period. At the sfc, very broad
high pressure will be in place to start the period. The remnants of
TS Jose will still be lingering off the New England Coast while TS
Maria is currently forecast to be due east of the Bahamas by early
Sunday. Maria is still expected to remain far enough off the east
coast to not have significant impacts on our fcst area. We will
remain under broad high pressure thru day 7 with predominately dry,
N to NELY low-lvl flow expected to persist. A cold front is expected
to approach the fcst area just beyond the fcst period early Thurs.
As for the sensible fcst, no major changes were needed. The period
should be mostly dry with temperatures starting out about a category
above normal and warming a few more degrees by the end of the period.


At KCLT and Elsewhere: VFR conditions expected at all sites tonight
aside for at KAVL where mtn valleys fog is expected once again,
and thus MVFR/IFR restrictions are included.  Otherwise, light/calm
nly flow under a few low/mid cu and high cirrus will dominate.
Winds will increase slightly through the morning hours on Thursday
at all sites with nly flow expected in the 4-6kts range regionwide
amidst developing llv cu and high cirrus.  Aside for a prob30 at
KAVL and vcts at KHKY accounting for mtn convection, all remaining
sites are wx free through the period.

Outlook: Chances for diurnally favored showers and thunderstorms
remain in the fcst thru Friday. Restrictions from early morning
fog across the northern NC Piedmont and foothills, as well as in
the mtn valleys will also be a concern. Drier weather is expected
to return for the weekend.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     Med   75%     High 100%     High 100%
KGSP       High 100%     Med   75%     High 100%     High 100%
KAVL       High 100%     Low   30%     High 100%     High 100%
KHKY       High 100%     Med   75%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     Med   75%     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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