Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200249

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1049 PM EDT Wed Jul 19 2017

An upper level ridge will build over our region from the west
through the end of the week. This pattern will give the Carolinas
and northeast Georgia some of the hottest weather so far this season
along with showers and storms mainly restricted to the mountains. A
broad upper level trough develops this weekend and into next week
which lowers temperatures.


As of 1030 PM EDT Wednesday: No changes to the grids for the late
evening update other than minor tweaks to hourly trends.

Otherwise, a mid/upper level low pressure center continues to spin
over the eastern NC piedmont and coastal plain this afternoon. This
feature will continue to drift southeast over the coastal waters of
the Carolinas before retrograding west across coastal Georgia and
northern Florida through Thursday. West of this low, the convection-
allowing models have very sparse thunderstorm coverage across the
region this afternoon and evening with limited triggering. However,
isolated to scattered thunderstorm coverage remains possible in weak
triggering along the high terrain and also in the better instability
near pre-existing boundaries over the lower piedmont. With sbCAPE
values running 2000 to 3000 J/kg in the southeast half of the area,
and with decent surface to mid level theta e lapses still in place,
a few storms could be strong to severe through early evening. Expect
profiles to stabilize quickly with the loss of heating through late
evening. Another round of mainly mountain valley fog and low clouds
are expected overnight.

Even less convective coverage is expected on Thursday as mid and
upper level ridging noses more strongly over the region and produces
warmer temperatures aloft. Steering flow will remain very light up
the column through the period, so locally heavy rainfall will also
be possible under any showers/thunderstorms that manage to develop.
With 850 mb temps rising another degree C or so through Thursday
afternoon, plenty of mid 90s max temperatures are expected.
Fortunately, dewpoint mixing should cap off heat index values at
around 100 degrees or less.


As of 200 PM Wed: As the upper ridge builds east, heights continue to
rise through Friday, but begin to fall slightly on Saturday as the
next trough digs over the northern Plains. Return flow around the
Bermuda High will continue unabated, as will hot and humid
conditions. Heat indices, per this cycle of guidance, are likely to
exceed 100 but not 105. Nonetheless that may be enough to warrant
caution in sensitive groups and a Heat Advisory could be warranted
at some point.

Both afternoons, the midlevel temps resulting from the warm flow are
too warm to permit much, if any, Piedmont convection. However, the
models feature some development over the high terrain both days,
particularly Saturday. Weak wind profiles imply pulse storms.
Despite the humid conditions at the surface, the mid to upper levels
are quite dry and damaging winds cannot be ruled out with the
stronger cells either day.


As of 100 PM Wed: Heights begin to fall over the Eastern states
Saturday night as a clipper shortwave moves into the upper
Mississippi Valley region. This wave will move along an existing
stationary boundary, initiate cyclogenesis over the eastern Great
Lakes, and push a cold front gradually southeastward. Slightly higher
PoPs appear warranted Sunday afternoon with the suppression of the
ridge. The approach and arrival of the front then will elevate precip
chances Monday and Tuesday across the forecast area. The front is
should initiate a cooling trend and drop temps and dewpoints back to
near normal late in the period. PWAT values most likely will be 1 or
more standard deviations above normal along and ahead of the front,
but our upper level flow does not increase much due to the better
upper support remaining to our north. These findings suggest we will
trade our weekend pulse storms and associated wind concerns for the
threat of slow-moving storms producing heavy rainfall early next


At KCLT and elsewhere: Only change to the KCLT TAF for the
03z AMD is to drop off the Cu and go with just cirrus. Otherwise,
VFR expected overnight for most TAFs. Expect LIFR to VLIFR
conditions to develop again at KAVL and other mountain valleys, and
MVFR fog not out of the question at KAND. Winds will trend light
southerly overnight, with brief veering to NW around daybreak, but
with mixing and diurnal tightening of the lee trough gradient,
should back to the SW again by midday. Warming temperatures aloft
will lead to very little in the way of convection Thursday, so no
PROB30s at this time.

Outlook: Drier air will likely continue to limit convective coverage
and the extent of low clouds and fog through Friday. Chances for
diurnal convection begin a steady increase on Saturday, with this
trend continuing into early next week ahead of an approaching cold
front on Monday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High  91%     High 100%     High 100%
KAVL       High  95%     Low   43%     High  88%     High 100%
KHKY       High 100%     High  87%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     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:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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