Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 230555
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
155 AM EDT Wed Aug 23 2017

.SYNOPSIS...
Summer like conditions will prevail into Wednesday when a cold front
sweeps through the region from the west allowing for increasing
precipitation chances.  High pressure will build into the region
late in the week yielding drier and cooler conditions through the
weekend.  Ongoing tropical activity will make for a variable
forecast leading into next week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 140 AM EDT: the line of convection east of Nashville has
steadily weakened over the past few hours and is not expected
to have any significant impact on our fcst area thru the overnight
and early morning.

Otherwise, an upper trough and associated cold front will steadily
push eastward tonight through Wednesday. Although the upstream
convection should weaken, mid and high level debris clouds may
increase and this might keep mountain valley and foothill fog a bit
less dense than observed this morning.

For later today, it`s difficult to tell exactly when the front will
transition from the Blue Ridge to the usual lee trough position, but
it`s looking like around 00z Thursday. Ahead of the front, there may
be a fair amount of cloud cover from overnight convection that wafts
into the area as the flow veers more out of the west. The operational
models do not show anything more than 1000 J/kg in the mountains, while
the piedmont gets into 1500-2000 J/kg range. Shear will be remain weak.
So overall, PoPs were lowered slightly from previous forecasts. The
severe threat looks low, as all the better ingredients remain north
and east of the area. Highs will be near to slightly above normal.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...
As of 2:30 PM EDT Tuesday:  A cold front will ease through our
forecast area (FA) Wednesday night into early Thursday morning.
Meanwhile the upper air pattern has become more northwesterly,
therefore we have sped up the FROPA just a tad.

It still looks like parts of the I77 corridor, and down into the
upstate of South Carolina, will have the best shot of showers and
thunderstorms Wednesday night (mainly first half). Depending if
these areas receive prior storms, lingering instability Wednesday
evening, along with precipitable water values between 1.75 and 2.00
inches, and convergence along the frontal boundary, could lead to
some strong storms. Shear values however remain rather marginal.

Interestingly an inverted trough pokes up from northeast Georgia and
the mountains of western SC, into the southern mountains of western
NC Thursday into Thursday evening. Along this inverted trough we
expect an axis of low level convergence. Model guidance was showing
enough moisture for some random showers. A quick look at instability
also shows a corridor of elevated values, but for the most part we
plan to leave any thunder out of the equation.

Elsewhere dry conditions should prevail, although we will watch our
far southern piedmont counties closely, for any precipitation trying
to sneak back in.

High Pressure should build into much of the region Friday with dry
conditions, although a sliver of moisture remains over the
mountains.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 145 PM EDT Tuesday: The medium range fcst period kicks off on
Friday evening amidst a broad/deep yet departing longwave upper trof
across the eastern CONUS, while a sharp ridge builds westward atop
the Plains.  At the surface, high pressure centered beneath max
confluence on the descending branch of the aforementioned upper trof
will extend across much of the OH/TN Valleys, eastward atop the
Appalachians and into the mid Atlantic seaboard.  With that,
cooler/drier air looks to slide into the region by way of a damming
wedge on Saturday morning with below climo pops favored, the highest
of which residing over the terrain where subsidence is weakest.  At
that point much uncertainty arises in the fcst given two distinct
systems of tropical origin.  As of now, the first of these systems
is the remnants of TC Harley which is expected to rapidly intensify
over the western Gulf, possibly regaining TS/HC status by Saturday
with various model sources favoring landfall along the TX coast.
The second system is currently a weak area of low pressure off the
southeastern FL shores, which is also expected to slowly gain
strength with time, possibly moving northward and approaching
northeast FL by Saturday.

Sunday into midweek, the latest GFS seems to have come into form
with the 00z ECMWF as 06z GFS runs favored additional tropical
moisture advection into the region leading to broad scale isentropic
lift and thus showers, which combined with improving easterly llv
could lead to upsloping as well.  However, these two solutions now
tend to favor intensification of the Atlantic system as it slides
northward along the GA and Carolinas coastlines Monday through
Tuesday.  Meanwhile, the TC Harvey looks to have slowed along the
TX/LA coastline all the while potentially producing significant
rainfall, however with little/no impact across CWFA through the
period.  All said, given the expected track of the Atlantic system,
would expect the high pressure wedge to be reinforced during this
time frame keeping PWATs rather low, generally under 1.5 inches.
Thus, will carry token slight/chance pops through the remainder of
the period, increasing to widespread chances Tuesday evening into
Wednesday.  Temperatures through the period will remain generally a
few degrees below normal across the high terrain, possibly as much
as 6-10 degrees below normal over the low terrain where wedging and
thus abundant sky cover could prevail.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: the line of convection east of Nashville
has steadily weakened over the past few hours and is not expected
to have any significant impact on our fcst area going forward.
Mountain valley stratus and fog are likely again overnight and into
the morning, but will probably be less dense due to the encroaching
mid/high level debris clouds. Showers and embedded thunderstorms
are expected to redevelop over the region from the west as the
boundary moves slowly across the fcst area today/tonight. Thus,
I kept TEMPOs for TSRA at KAVL and PROB30 TSRA at the foothill and
piedmont sites through the afternoon and well into the evening.
Convective activity should be done for most locations by late tonight,
however a few lingering showers could persist into the overnight.
Winds are expected to turn NWLY and become gusty at KAVL after
roughly 21Z, but the fropa could be a bit more delayed toward the
end of the period for the foothills/piedmont sites before finally
transitioning into a piedmont trof setup.

Outlook: The front should push through the region by early Thursday
and allow drier air to work in from the north. Thus, the probability
of restrictions and diurnal convection should diminish Thursday and
Friday. Low level moisture may make a return toward the Blue Ridge
over the weekend, however.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       High  97%     High  94%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       Low   34%     Med   74%     High 100%     High 100%
KHKY       High  89%     High  94%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  97%     High  94%     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...CDG
NEAR TERM...HG/JPT
SHORT TERM...TS
LONG TERM...CDG
AVIATION...JPT


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