Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180757

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
357 AM EDT Fri Aug 18 2017

Moist and unsettled weather will persist across the forecast area
through the tonight. Some drying is favored over the weekend, with
a return to moist and active weather by the middle of next week.


Heights will fall across the region through the near term, as a
rather potent short wave trough digs from the Corn Belt toward the
Ohio Valley through tonight. Associated frontal zone will move into
the forecast area by this afternoon, providing the main focus for
convective development later today. In the interim, a remnant pre-
frontal convective band has evolved into an area of decaying showers
across East TN this morning. Based upon radar trends, it`s
debatable as to how much of this will be left by the time it
enters western NC btw 6 and 7 am. In fact, we only felt good about
advertising 30-40 pops across the high terrain this morning, and
CAMs guidance is in reasonably good agreement that the band will
dissipate over the southern Appalachians later this morning.

In terms of redevelopment this afternoon, the main point of
contention is what impact morning debris clouds will have on
afternoon destabilization. In general, guidance depicts moderate
levels of sbCAPE this afternoon (around 2000 J/kg), which should be
sufficient to initiate at least scattered convection along the front
this afternoon, as depicted by a consensus of guidance sources in
the vicinity of the Blue Ridge between 18-21Z. Pops will be
advertised in the 30-50 percent range across much of the area, with
the highest pops favoring western NC, where better upper support is
forecast. Shear remains weak (deep layer shear of 15-20 kts; 0-3km
shear of around 10 kts), so convective organization does not appear
likely and any severe convective potential should be limited to the
pulse variety.

Convection will diminish through the evening as drier/more stable
air filters into the region in the wake of the front. After another
day of above-normal maxes, the slightly drier air should allow
tonight`s mins to drip closer to climo.


As of 210 AM Friday: A strong s/w trof will cross the ern CONUS thru
the period. The best energy will remain well to the north of the FA
however as a sub/trop high noses into the SE region. Soundings show
a good amount of deep subs developing Sat persisting thru the day
Sun as the sub/trop ridge couples with a Canadian sfc high. Low
level moisture and flow will be meager...yet enuf for the
development of few/sct Cu. It will be hard to get much going
convec-wise each afternoon with overall neg forcing and mlvl warming
off-setting good llvl LRs. There may be some areas that break the
cap and a few deeper -shra or tstms could be locally possible.
Yet...think the best chance for -shra/tstms will be across the nrn
mtns due to mtn top convg near the passing h5 trof axis...and
perhaps across the far srn zones in proximity of a lingering broad
sfc trof. A subtle drying trend was made to the PoP grids mainly on
Sat. Max/min temps will be able to reach a cat or so above normal in
this environ.


As of 240 AM Friday: Not too many changes were made to the going
fcst. Mon still looks to be firmly under the control of a strong
sub/trop ridge with little flow thru the layer. This will create
strong sub and hold a 1021 mb sfc high off the Carolina coast.
Meanwhile...a sfc bndry will remain trapped to the south of the
fcst area. The models continue to show a slow northward progression
with the moist llvl convg zone and in fact fire up some convec
across the Midlands and cent GA arnd 16z. Soundings over the Upstate
however show little in the way of convec potential and mainly
few/sct Cu leading up to the solar eclipse. There could be some
moist adv into the area from any eventual storms to the south...but
the flow is so weak that it would likely take until the later
afternoon for increased mid/high level clouds to materialize over
the FA. The depth and coverage of these advective clouds is still
fairly uncertain attm and a lot will depend on the strength and
placement of the Atl ridge by Mon morning. Suffice to say...the fcst
will remain on the optimistic side as far as good viewing conds for
the mid day eclipse.

Otherwise...the deeper afternoon convec will remain mostly limited
across NC mtns Mon afternoon and again on Tue with some isol non/mtn
storms developing in strong thermals. A little better non/mtn
coverage may be had Tue/Wed afternoon as a pre/frontal trof works
in and stalls just east of the escarpment. Late Wed into Thu could
be rather active as deep layered shear increases ahead of a frontal
zone and the pre/frontal airmass becomes quite unstable. Continued
warm and muggy conds will continue thru the period with perhaps some
lower theta/e air mixing in by Thu afternoon/evening.


At KCLT and elsewhere: Main concern in the very near term is the
potential for fog and/or low stratus, mainly at KAVL and KHKY, with
KAVL already reporting LIFR conditions as of 0530Z. Expect mainly
VLIFR, with periodic VLIFR conditions to persist at KAVL through at
least 12Z, while tempo LIFR conditions are forecast at KHKY from
10-12Z. VFR is forecast to persist through the period at the other
terminals, but would not rule out some patchy low stratus briefly
impacting the Piedmont terminals around sunrise. A weak cold front
is expected to bring scattered convection to the area during the
afternoon. Although KAVL and KHKY have the best chance of seeing TS
in their vicinity, chances are high enough that a PROB30 is
warranted at all terminals during the afternoon and early evening.
Winds should generally be light SW through much of the period,
before turning to the NW by Friday evening in the wave of the front.

Outlook: Drier air is expected to result in an inactive weekend,
with very little chance of diurnal convection, and probably only
patchy early AM mtn valley fog/low stratus Sat and Sunday. A more
typical late summer pattern returns next week, with isolated/
scattered afternoon/evening storms expected, along with better
chances for patchy fog/low stratus, especially in the mtn valleys.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  93%     High 100%     High 100%     High 100%
KGSP       High  93%     High 100%     High 100%     High 100%
KAVL       Low   30%     High 100%     High 100%     High  93%
KHKY       Med   60%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  93%     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:




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