Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 280012
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
812 PM EDT Thu Jul 27 2017

.SYNOPSIS...
A strong cold front will cross the region from the west Friday
through Friday night. Cooler and drier air will arrive across the
western Carolinas and northeast Georgia through the weekend and
linger through the early part of next week. More typical summertime
conditions will return about the middle of next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 745 PM: T-storms continue over mainly the Upstate attm,
though occasional minor redevelopment has been seen recently over
the NC mtns. Though instability will wane a bit following sunset,
a steady supply of cirrus will continue to filter in, ejected from
the storms near the cold front to our north. This cloud cover may
serve to keep the low levels warmer and maintain some elevated
CAPE. So despite the refined PoPs allowing the current activity to
diminish, I allow a chance to return in many areas later tonight
as eddies advect in from that upstream activity. I adjusted temps
slightly but with dewpoints so high, and the cloud cover, I think
I will need to adjust them higher with next update.

18z NAM has bucked the trend of late and shows less of a
stabilizing effect Friday due to the morning cirrus and possibly
light stratiform precip. Our confidence has increased that at
least the NC Piedmont, where model consensus has been highest for
significant instability, will be at risk for severe wx.

While the NAM and the GFS more or less dissipate this area of
precip across the southern Appalachians Friday morning, at least
the GFS sweeps the remnant cold pool into our forecast area such
that it depicts virtually no sbCAPE across much of the forecast
area by 18Z. While this is plausible, it`s by no means a given, and
it does create a considerable amount of uncertainty in tomorrow`s
forecast. The uptick in instability seen in the most recent NAM is
decreasing the uncertainty in that it lends confidence to other
solutions still showing high SBCAPE values in our east. However
it increases the uncertainty in how far west the advertised SPC
Slight Risk should be extended when Day 2 becomes Day 1 tonight.

At any rate, the potential for abundant buoyancy is a bit
disconcerting considering the anomalously strong mid/upper level
flow, with attendant deep layer forcing and low level & deep
layer shear of 25-35 kts, as one would typically expect a very
active severe weather day with such ingredients in late July. The
combination of upper support, along with lee side/pre-frontal
trough and perhaps outflow boundary(s) associated with the morning
convection should support scattered to numerous showers and storms
across the area by afternoon. The potential for a few significant
severe storms still cannot be ruled out. Tomorrow`s highs should
be a couple of degrees (if not more) below climo under expected
widespread mid/high clouds.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 225 PM EDT Thursday: The best upper-level forcing rounding the
slowly deepening eastern trough will likely cross our piedmont
Friday evening before swinging rapidly east to the coast overnight.
An attendant sharp cold front will also cross the region through the
nighttime hours. PoPs will thus remain fairly high during the
evening hours before drier air starts arriving from the northwest
overnight post-fropa. However, the best severe convective threat
should have passed east of the area by evening - unless any strong
redevelopment occurs along the immediate frontal zone. Showers may
linger along the northern mountains in moist low level upslope flow
well into Saturday morning.

A closed, upper-level low pressure system will then migrate from the
central Appalachians to the Virginia Tidewater region Saturday
through Saturday night. The surface cold front will depart southeast
of the region and drier air will work in. The main exception is that
a trailing vorticity lobe rounding the upper low will swing down
across the western Carolinas Saturday night, reinforcing moisture in
the 850 to 700 mb layer. This will lead to some return cloudiness,
but probably no associated showers.

Deep layer northerly flow will continue Sunday through Sunday night
on the back side of the deep eastern coastal trough. Periods of mid
level moisture could permit slightly higher cloud cover at times,
but with no attendant showers expected. Anticipate maximum
temperatures about 1 to 2 categories below climo by Sunday
afternoon.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 225 PM EDT Thursday: Reinforcing shortwave energy arriving
from the upper Midwest will keep a mean trough axis positioned over
the eastern U.S. Monday through Wednesday. Mainly dry surface high
pressure will sprawl over to the north of the region through this
same period. The airmass will slowly modify, with isolated
convective coverage returning to the southeast mountains late Monday
and then more typical diurnal mountain ridge top coverage Tuesday
through Thursday. Some modest height recovery is expected on
Thursday with max temperatures finally returning to near seasonal
normals.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
At KCLT and elsewhere: A few SHRA/TSRA will continue this evening
mainly over the mountains and Upstate. With abundant moisture and
increasingly perturbed upper level flow downstream of trough and
cold front, it is not out of the question that occasional showers
will redevelop overnight. Hence VCSH/VCTS is used liberally in
the first few hrs of the TAF and then returns in some places by
daybreak. Cirrus will continue to blanket the area and limit
radiation, so only the mountain valleys are expected to see
restrictions. Falling heights and the nearing cold front could
permit robust storms to develop Friday aftn...if we are able
to destabilize. Over the past few runs, guidance has mainly
indicated that remnant cloud cover and stratiform precip would
drift in from TN and keep us stable during the day. While this
still looks possible over the mountains and western Upstate,
over at least KCLT and KHKY trends suggest above-climatological
values of SBCAPE will develop. Stronger winds aloft imply a severe
wx risk with any storms that form in that environment, mainly in
the form of damaging winds. Hence PROB30s for those sites include
a gusty wind mention. Elsewhere the severe risk is smaller though
SCT storms may still form.

Outlook:Drier air will filter in from the north behind a passing
cold front this weekend. Chances for morning fog and stratus
will also be possible, Saturday morning, mainly in the mountain
valleys. A typical summer-time pattern is expected to return by
the middle of next week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  99%     High 100%     High 100%
KGSP       High 100%     High 100%     High  99%     High 100%
KAVL       High  88%     Med   78%     High  88%     High  93%
KHKY       High 100%     High  83%     High  91%     High 100%
KGMU       High 100%     High  99%     High  97%     High 100%
KAND       High  99%     High  99%     High  99%     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...HG
NEAR TERM...JDL/Wimberley
SHORT TERM...HG
LONG TERM...HG
AVIATION...Wimberley


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