Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191820

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
220 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 200 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 south east 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 little to no
triggering apparent. 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 255 AM EDT Wednesday: the short-term fcst picks up at 12z
on Thursday with very broad upper ridging in place over most of
the CONUS and a lingering upper trof centered over the Carolina
Coast. Over the next 24 hrs or so, the trof becomes separated from
the mean flow and spins off a weak H5 low that proceeds to drift
SW towards the Gulf of Mexico. By the end of the period early Sat,
what`s left of the low is centered over the northern gulf coast
with flat upper ridging in place over the fcst area. At the sfc,
broad high pressure will be centered to our north with the Bermuda
High in place offshore. The overall synoptic pattern is not expected
to change much thru the period with weak high pressure persisting
to our north and some degree of weak lee trofing expected over the
CWFA each day. Thursday is expected to be drier as we`ll likely
see more of a downslope component across the fcst area. PoPs will
return to more climatological values for Friday as winds take on
a more SLY component and lee trofing looks more developed. Temps
will be well above normal thru the period with heat index values
reaching triple digits over the lower elevations each day. At this
point, I don`t expect them to reach heat advisory criteria.


As of 235 AM EDT Wednesday: the extended fcst picks up at 12z on
Saturday with flat upper ridging over most of the CONUS and what`s
left of a weak H5 low that lingers over the northern Gulf Coast.
By late Sunday, the upper trof over Central Canada begins to amplify
and dig down over the Great Lakes. The long range guidance moves
the trof axis to our north by late Monday or early Tuesday depending
on which model you believe. The trof begins to move off the New
England Coast by the end of the period late Tues/early Wed. The
ECMWF is currently more progressive with the pattern and moves the
trof eastward about 18 to 24 hrs faster than the GFS or Canadian.
The GFS and Canadian also develop a broader trof overall and the
Canadian even tries to spin off a large H5 low by the end of the
period. At the sfc, the Bermuda High will be in place to our east
with light SLY flow over the fcst area and some degree of weak lee
trofing. On Sunday, the models deepen a low over the Eastern Great
Lakes and move its associated cold front towards the Western Carolinas.
As mentioned above, the ECMWF is faster with the system and has the
front move thru the CWFA on Monday while the GFS is about 18 to 24
hrs slower with the fropa. Both models have drier high pressure
moving in behind the fropa to end the period. As for the sensible
fcst we can expect diurnally-driven climo PoPs for Sat with higher
PoPs for Sunday, Monday, and Tuesday as the front impacts the CWFA.
Fcst temps have trended down a bit since yesterday, but they are
still expected to start out a good 5 to 7 degrees above climatology
on Sat and then steadily moderate thru the rest of the period. By
early next week, they will likely return to near normal.


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.

Outlook: Drier air will likely continue to limit convective coverage
and the extent of and 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:




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