Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 182334

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
734 PM EDT Fri Aug 18 2017

A weak cold front will drop into the area tonight, but with
some drying expected over the weekend as the front sags south. The
front will stall just south of the area early next week, but the
warm and moist airmass will remain in place. Another front will
approach the area by mid-week.


As of 730 PM: Updated forecast to concentrate remaining precip
chances over the wrn Piedmont of NC and northern Upstate, and
removed much of the precip chance elsewhere. Sfc-based CAPE and
DCAPE values remain sufficient across the area where storms are
firing such that an isolated severe storm will not be ruled out
through mid-evening. Temps were adjusted for storm outflows.

Convection will diminish through the late evening as drier/more
stable air filters into the region in the wake of the front.
Lingering low level moisture will keep a chance of fog or low clouds
even with the increasing NW wind. Lows will be near normal mountains
and above normal elsewhere.

Dry high pressure builds over the area Saturday keeping a lid on any
convection. Thickness values will be lower leading to highs a few
degrees cooler than previous days. Dew point values, while still
seasonal, will be lower as well. This should keep heat index values
below the century mark.


As of 145 PM EDT Friday: The short term will begin with a sharp
upper trough stretching down the Appalachians, continuing to push
east. This upper trough is actually behind the main front (which
will be located off the Eastern Seaboard). The trough will damp
rapidly during the day with shortwave ridging building in across the
Great Lakes behind it on Sunday; meanwhile the Atlantic upper ridge
will be pushing west, into the Southeast, with weak near-zonal flow
aloft over our area. Some convection not out of the question Sunday
afternoon especially in the mountains, just due to topography, but
otherwise the moist airmass will not quite have returned behind the
front (winds will be swinging around during the day from NE to SE),
so while the warmth really will not have abated, the dewpoints will
still be on the dry side, resulting in heat indices only a degree or
two warmer than actual temps.

This changes on Monday as the winds swing around to the south, and
the sultry airmass underneath the upper ridge begins to build back
to the north. Luckily for those wanting to view the eclipse across
our forecast area, the remnants of the front will remain stalled to
the south, more over the coast. Diurnal convection just from
reaching convective temperature can be expected, but generally
across extreme southern zones, with another max possible across the
mountains. This leaves a portion of the NE GA/Upstate SC/western NC
Piedmont in a relative minimum, and actually this update now has
<15% pops for Monday afternoon in these areas. Sky cover continues
to improve as well, though expect that bubbling fair-weather Cu will
be the case during the afternoon. That said, still a bit of an
unknown how *exactly* the eclipse will affect temperatures/diurnal
heating and thus clouds/convection. We do show a reduction in
temperatures in the path of totality from 2-3p, springing back up
afterwards, but since the partial eclipse will be from 1-4p, that
uncertainty remains. One thing we do know is with the moisture
returning from the southerly winds, dewpoints will be on the
increase and so will heat indices. Assuming temperatures in the
lower-to-mid 90s across southern zones, forecast dewpoints result in
heat indices approaching 100. While not heat advisory criteria, a
lot of people (a LOT) will be outside on Monday, so residents and
visitors should be aware of heat safety precautions and above all
stay well-hydrated.


As of 215 PM EDT Friday: The upper ridge will still be dominating
our area on Tuesday, but with a tropical weakness pushing onto the
LA/TX Gulf Coast during the day, and a positively tilted upper
trough dipping from Canada into the Great Lakes/Mid MS Valley. The
front associated with the latter will be pushing toward the OH
Valley by the end of the day Tuesday. We remain in a warm and moist
airmass with diurnally-driven convection for Tuesday afternoon, with
overall very little change from Monday (um, except no eclipse that
is). The front will push toward the Appalachians overnight, with
precip expected to move into the NC mountains by daybreak Wednesday.
The forcing trough will continue to dig as the front pushes through
during the day. Cloud cover ahead of the front may prevent a
substantial increase in instability especially just adjacent to the
mountains, but the Piedmont should destabilize decently (1500-
2000J/kg). Add to this the minimal but non-negligible deep-layer
shear of ~30kt right along the front and it`s possible that we could
see strong to isolated severe convection as the front moves through.

Behind the front, timing of the upper trough axis itself varies in
the global models, with the ECMWF faster than the GFS. The latter
brings some upper support across the mountains Thursday afternoon
which could possibly lead to some convection, but the bigger story
will be the airmass change as high pressure over the Great Lakes
will cross the Appalachians and tries to dam down the Eastern
Seaboard. Max temperatures Thursday should be a good 4-6 degrees
lower than those on Wednesday, with another 3-5 degree reduction
possible on Friday as the strong upper trough remains in place.


At KCLT: Colliding outflow boundaries nearby could result in
new deep convection near the air field, so a TEMPO was kept for
TSRA for the first two hours of the fcst. Activity to the west
could survive and reach the airfield toward the end of that time
period. The NW outflow appears to be the stronger of the two as
it has pushed farther south and flipped the wind at KCLT around
to the NW for the time being. Think we might go back to light
SW for a period of time overnight, but eventually this will come
back to light N behind the cold front. The TAF includes a slight
vis restriction in the pre-dawn hours with the expectation of
a shower/storm developing close by this evening. This becomes
less likely if the showers fail to develop. Saturday should be
relatively quiet with few clouds. Wind should come back around to
NW as a lee trof starts to develop.

Elsewhere: Have kept a VCSH over the Upstate to start with. Wind
still SW in many areas, but this should come around to NW then N
behind the front. Drier low level air will be slow to fill in,
so some questions remain about fog or low cloud restrictions
overnight. Have kept MVFR at KAVL for now as all guidance agrees on
some restrictions there. KHKY the next best chance, but confidence
too low to include for now. Winds turn N to NE for Saturday morning
with some Cu developing in the lingering low level moisture.

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 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...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  83%     Med   60%     High 100%
KHKY       High 100%     High 100%     Med   66%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  83%     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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