Area Forecast Discussion
Issued by NWS Greer, SC

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359
FXUS62 KGSP 231811
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
211 PM EDT Wed Aug 23 2017

.SYNOPSIS...
A cold front will move through the region today, allowing for
increasing precipitation chances. High pressure building in behind
the front will introduce an extended period of unseasonably cool
conditions that is expected to last through at least early next
week. Although active tropical weather is anticipated near the
Southeast early next week, this is unlikely to have any impact on
the weather over our area for at least the next 7 days.

&&

.NEAR TERM /THROUGH THURSDAY/...
As of 210 PM EDT Wednesday: Closed upper low just southeast of James
Bay will continue to lift NE through the period, though the upper
trough axis will remain in place down the Appalachians. Surface low
and attendant cold front (which is right over our forecast area at
AFD time) will continue to slide toward the coast, pushing into the
ocean overnight. Along and ahead of the front, convection has
developed across the Piedmont. sbCAPEs are generally between 1000-
2000 J/kg, with best dCAPEs to our southeast ahead of the front.
Deep layer shear is minimal at best, though seeing some 25-30kt
effective bulk shear right along the front. SPC Marginal Risk
continues to just barely grab extreme eastern Union County NC, with
general thunder elsewhere. All this to say that gusty winds and of
course lightning/heavy rain certainly possible, and cannot totally
rule out a brief damaging wind gust through the afternoon hours.

Canadian high pressure moves in behind the front, ushering in a very
pleasant airmass change. Flow aloft as the trough axis passes the
spine of the Appalachians will shift to NW with a bit of an upslope
component, so cannot rule out some isolated showers across the
mountains tomorrow, and some guidance depicts some lingering showers
across the Piedmont, but the bigger story will be high temperatures
which will drop a handful of degrees below seasonal normals, with
dewpoints tomorrow afternoon a good 10 degrees lower than today,
setting up a nice trend as we move into the short term.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
As of 130 PM Wednesday: Northern CONUS high pressure will remain
dominant over our area Thursday night and Friday, with an upper
trough axis along the Eastern Seaboard. Initially northerly flow
will begin to veer as the pattern progresses ever so slightly
heading into Saturday. Moderately cool and dry conditions
will accompany the high, keeping convective potential small in
conjunction with low-level subsidence. Maxes about a category below
normal are expected. Nonetheless model consensus indicates slight
destabilization over our southwestern mountains Friday afternoon,
so an isolated shower is not out of the question. The more easterly
fetch leads to slightly higher dewpoints and more widespread SBCAPE
Saturday, and a stronger model response for precip along the Blue
Ridge. Thus PoPs expand a bit then. Increased cloud cover however
appears likely to keep temps even cooler, about two categories
below normal.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
As of 140 PM EDT Wednesday:  The medium range period kicks off on
Saturday evening amidst a rather active synoptic setup.  The pattern
will be highlighted by a departing longwave trof over the northeast,
while a pair of tropical systems dominate across the lower latitudes
of the CONUS.  The first and most distinct still looks to be what
is now TD Harvey, which by Saturday night is progged to be slowly
churning inland over eastern TX.  The second disturbance currently
has a low chance of becoming a TC as it sits over the FL Keys,
eastward toward the Bahamas.  Closer to home, a strong 1028MB
high pressure centered over Ontario/Quebec will be stretching
southward across the eastern CONUS along the east coast in lee of
the southern Appalachians.  With that, the fcst initializes with
decreasing diurnal pops along the high terrain, with further drying
expected on Sunday as the wedge builds across the region leading
to below climo pops outside of the mtns, near/above climo in.

Additional drying continues into Monday as an approaching upper
trof over the upper Midwest amplifies shortwave ridging along the
Appalachians, which will turn reinforce the wedge of high pressure.
In addition, guidance favors the south FL system referenced above
to move northeast offshore of the Carolina coastlines by Monday
morning.  Flow around said system will also work to enhance the
southern extent of the wedge and associated dry air.  By Tuesday
guidance tends to diverge a bit more with regards to timing of
both tropical systems and the approaching upper trof and associated
surface front.  The GFS is a bit slower with TC Harvey`s northward
migration as it favors a deeper upper trof and a slow yet robust
surface front.  The ECMWF on the other hand has a less amplified
upper trof over the OH Valley by Tuesday night, with the front well
northward and possibly stalled thanks to a more northern position
of Harvey.  All in all, conditions look surprisingly dry Tuesday
through Wednesday as well across northeast GA and the western
Carolinas aside for possible mtn convection and stratiform upglide
precipitation along the periphery of the wedge front.  Therefore
pops will remain below climo levels Monday through Wednesday.
Temperatures will remain below climo as well given abundant sky
cover thanks to the persistence of the wedged airmass.

&&

.AVIATION /18Z WEDNESDAY THROUGH MONDAY/...
At KCLT and Elsewhere: Cold front moving across the area has already
switched winds around to NW at CLT and other NC sites, with the SC
sites soon to follow. Scattered convection developing will spread SE
across the area, and have continued TEMPO TSRA for all sites but
KAVL (which at best should only see SHRA) for the next few hours.
Convection will clear behind the front overnight, with the
possibility of some mountain valley fog/low stratus as usual. W to
WNW winds to start the period (up-valley and occasionally gusty at
KAVL) will continue to swing around to NE by the end of the period.

Outlook: Drier air will move in behind the front on Thursday, with
only some isolated mountain showers expected in the afternoon. Low
level moisture may make a return toward the Blue Ridge over the
weekend.

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 100%     High 100%
KAVL       High 100%     High  97%     High  93%     High  90%
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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...JDL
NEAR TERM...TDP
SHORT TERM...Wimberley
LONG TERM...CDG
AVIATION...TDP



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