Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 120004

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
804 PM EDT Fri Aug 11 2017

A moist cold front will drop southeast into the region and become
stationary over the weekend. Waves of low pressure will then ride
east along the front beginning Monday continuing the chance of
locally heavy rainfall. Drier air will mix in from the northeast by
the middle of next week.


As of 750 PM: A broad upper trough will dig slightly over the Great
Lakes and Ohio Valley, causing some height falls atop the CWFA thru
tonight. Meanwhile, a cold front pushes across the Midwest, and will
not reach our area until after daybreak Saturday. So a very moist
warm sector air mass remains in place. Overall flow is fairly weak,
but enough to provide some moisture flux into the area from the
southwest. Numerous/widespread convection is becoming increasingly
concentrated across the NC mtns and foothills this evening, and
while a few thunderstorms producing heavy rainfall are occurring
across the foothills, these cells are moving steadily along so as to
avoid any near term hydro concerns, while much of the additional
activity is not producing much in the way of troublesome rainfall
rates. Another area to watch over the next few hours will be out
southern tier, as coverage is increasing just south through SW of
the area, at least in part due to NW-moving remnant sea breeze
boundary. Intensity and coverage of convection has been increasing in
association with this boundary in the Midlands, and at least
scattered activity is likely to make its way into the urban corridor
along I-77 by late-evening. SBCAPE is generally being analyzed at
less then 1500 J/kg across the area this evening, and with the
diurnal cooling cycle already underway, it would appear that
whatever severe convective threat existed has passed. However, hydro
issues will remain a concern with any training and/or back building
heavy rain showers or thunderstorms, especially in urban areas.

Saturday, the upper trough begins to deamplify as it crosses the
Appalachians. Meanwhile, a cold front should push into the NC
mountains during the AFTN. There is a big question mark on how
unstable we will be able to get due to all the convective
overturning today into tonight. But if the models are right with
1000-2000 J/kg of SBCAPE. There may be a marginal severe threat, as
bulk shear remains elevated, especially across the northern third of
the CWFA. Drier air will start to filter in from the west, so heavy
rain threat should start to diminish. I took the SUPERBLEND pop as a
first guess, but if we don`t get as unstable as the models depict,
these pops may be overdone. Temps will be below normal under plenty
of cloud cover.


As of 230 PM EDT Friday: The pattern will remain intact through the
rest of the weekend and to start next week, with a broad upper trof
across the region that will have weak short wave trofs and ridges
moving through it. The entire period lacks a strong synoptic-scale
signal or forcing, so expect a diurnal nature to be slightly
more dominant. A weak boundary is expected to slip southeast on
Saturday evening which should allow for some nw flow drying and
diurnal weakening of convection, thus the precip probs drop off
to the slight chance range. Sunday still looks more uncertain as
we lack a lot of forcing and the strength of moist southerly low
level upslope flow is questionable. The guidance blends still
point toward a high chance/low likely probability so that will
be kept in the fcst. Precip chances were kept Sunday night in
spite of the drying out on the GFS, mainly out of continuity and
agreement with neighbors. On Monday, the models show a slightly
improved environment as a weak upper wave moves through. That
should provide a small extra boost, so precip prob will be kept
in the likely range, with some categorical near the Blue Ridge
Escarpment. The lack of real air mass change suggests a continued
low-end severe and flash flood threat through the period. Temps
will remain near, if not slightly below, normal as well.


As of 150 PM EDT Friday: The ulvl trof slowly lifts and moves east
thru the period while a ridge axis begins to advance and cross the
area by late Wed. At the sfc...not expecting much change in the
airmass as a wavy frontal zone remains parked along the srn
periphery of the FA. This bndry will likely waft north/south a bit
thru the first part of the period...before becoming suppressed south
by llvl ridging early Thu. Tue and Wed look to be fairly active with
sct -shra/tstms each afternoon as sbCAPE values reach moderate
levels. Some measure of ulvl lift and bulk shear will be present
mainly across the nrn zones. Thus...the better chance for stg/svr
tstms shud be across nrn half of the FA...with more general thunder
south. On Thu...the aforementioned high pressure begins to build in
with the parent high center tracking east across the mid/Atl region
and off the coastline. This will enable a wedge of high pressure to
develop over the area...which looks to become less defined and more
insitu/like as the period progresses. Plenty of h85 Atl moisture
flux will be in play to expect a reinforcing moist wedge. A distinct
instability gradient is noted in the guidance which gives the wedge
favored wrn/srn zones the best chance for tstm development both Thu
and Fri afternoons. Max temps will be rather steady state a few
degrees below normal thru the period...with mins remaining a couple
degrees above normal due to a high moisture environ.


At KCLT and elsewhere: Numerous showers and storms are becoming
increasingly focused across the mtns and foothills this evening, thus
the best chances for convection and restrictions in the very near
term will be at KHKY/KAVL, where tempos are included for TSRA (at
KHKY) or SHRA (at KAVL). However, showers could redevelop at just
about anytime near the other terminals, and most get another period
of tempos for SHRA during late evening through the early part of the
overnight, while VCSH is carried everywhere through much of the
night. The other concern will be timing the development of low
stratus and BR, which seems a virtual certainty in the extremely
moist environs, with most terminals seeing dewpoint depressions of
less than 5 degrees already this evening. In fact, feel there is a
pretty good chance of IFR conditions developing at the Upstate SC
terminals and KAVL during the first six hours of the period, and
tempos have been included for such. Otherwise, all terminals should
settle into IFR/LIFR conditions btw 06Z and 09Z, with visby ranging
from 1-3SM at times. Conditions will gradually improve Saturday
morning, with VFR anticipated by early afternoon. More convection is
expected during the afternoon, and all terminals receive accordingly
receive a PROB30.

Outlook: A moist and unsettled pattern will dominate through the
weekend into early next week.  With that, expect possible shra/tsra
associated restrictions each day, while low clouds and fog remain
a possibility each morning with the best chances in the mtn valleys
as well as for sites that experienced recent rainfall.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  98%     Med   74%     Med   74%     High 100%
KGSP       High  87%     Med   73%     High  82%     High 100%
KAVL       Low   56%     Med   72%     High  84%     High 100%
KHKY       High  89%     Med   75%     High  84%     High 100%
KGMU       Med   79%     Med   79%     High  82%     High 100%
KAND       High  92%     Med   63%     High  90%     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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