Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191837

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
237 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 230 PM EDT: 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: Very isolated coverage in the weak triggering
should keep most thunderstorms away from the TAF sites through early
evening. Thus, expect mainly FEW to SCT VFR cumulus clouds with
light winds. Mixing should keep winds north of west at KAVL, KHKY,
and KCLT, with some adjustment to WSW in parts of the Upstate. A
trend toward light southerly is expected overnight, with winds
returning light WSW to WNW for Thursday. Expect fog and low clouds
once again mainly in the mountain river valleys around daybreak.
Thunderstorm coverage will be even less on Thursday with warmer
temperatures aloft.

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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  97%     Low   33%     Med   73%
KHKY       High 100%     High 100%     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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