Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210951

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
551 AM EDT Mon Aug 21 2017

Typical late summer conditions will persist ahead of an approaching
cold front, which will reach our area Wednesday. A large region of
surface high pressure will then gradually build into the area late
this week through the weekend, bringing unseasonably cool and dry


As of 5:50am EDT Monday: Radar is currently clear with some
areas of patchy fog as we await the total eclipse this afternoon
which will briefly drop temperatures a few degrees.

General synoptic situation has weak zonal flow with an exiting
shortwave to the north and a Bermuda high to the southeast.  Very
little changes with this pattern today with the next upstream
shortwave holding-off until Tuesday.  Flow around weak Bermuda high
will generate some light southeasterly flow today which will help to
maintain some moisture, but afternoon instability will be very
marginal and there is no forcing outside of orographic effects.
Models pop some clouds and showers over the mountains this afternoon
which should remain sub-severe.  Temperatures will remain hot and
humidity for another couple days, with highs 6 or so degrees above
normal and heat indices in the mid to upper 90s in piedmont areas.
Cloudiest areas will be over the mountains and adjacent areas to the
east where anvil cirrus may propagate should any convection develop.


As of 320 AM Monday: Heights will fall across the eastern Conus
through the short term, as a strong short wave trough sweeps across
the northern Great Lakes/northern New England. This will introduce a
cold front into the southern Appalachians and surrounding areas
during the Wed/Wed night time frame. In the interim, Tues looks to
be a standard late summer day across the forecast area, with
seasonal levels of heat/humidity/instability supporting scattered
coverage of deep convection, initially across the high terrain
during early/mid afternoon, but gradually expanding into the
valleys/foothills/Piedmont from mid-afternoon through early evening.
30-40 pops are advertised for the mountain and NC foothills zones,
with slight chances across the remainder of the area.

Pops will be on the incline Wednesday, as the cold front and
attendant deep moisture likely push into the forecast area by
afternoon. Mid-level lapse rates will remain weak, but steeper than
has been seen in recent days, and this should yield an afternoon of
at least moderate sbCAPE values. Meanwhile, the increasing height
gradient aloft will result in improving shear parameters, although
deep layer shear will likely peak at less than 30 kts. Nevertheless,
scattered to numerous coverage of convection, along with elevated
potential for some degree of organization will likely yield a
low-end severe thunderstorm threat Wed PM. Coverage of convection
should gradually diminish Wed night, as drier/cooler air gradually
oozes into the area in the wake of the cold front.


As of 335 AM Monday: The upper trough will be well-established
across the eastern Conus early in the medium range, and will be slow
to relinquish its influence through the period. Confluent mid-level
flow into the base of the trough will support sprawling surface high
pressure that will impact much of the East Coast through the period.
The only real controversy should exist early in the period regarding
how quickly to remove pops in the wake of the short term cold front.
Most of the global model guidance quickly develops an easterly flow
regime across our area, with lingering low level moisture possibly
resulting in considerable cloud cover as well as precipitation
potential late in the work week. This scenario is especially
prevalent in the latest GFS guidance, while other global model
guidance is quite muted with any qpf response after Thu morning. The
official forecast will indicate token small pops Thursday, tapering
down to less than slight chances by Friday.

Otherwise, conditions will dry/clear out into the weekend, with
anomalously low thickness values supporting temps 5-10 degrees below
climo, while increasingly dry air will act to limit the potential
for diurnal convection through the period.


At KCLT and elsewhere: A few isolated showers have died-out across
the region over the last couple hours.  Overnight should be mostly
clear with light winds. Winds are rather variable at KCLT this
evening, but guidance insists it should turn back out of the SE by
start of TAF period. Otherwise, mountain valley fog is expected, as
moisture has increased since yesterday. The Piedmont should be VFR,
but there may be some patchy MVFR BR or HZ near daybreak. It
looks like a cu field should develop around 16-17z, mainly FEW-SCT,
but possibly some BKN in the NC mountains. Isolated showers and
tstms possible in the AFTN, again mainly in the mtns. No mention
will be carried in the TAFs at this time, due to the low coverage.
Winds will start out VRB or favoring ENE, then switching out of the
SE by midday, picking up to around 5-8 kts.

Outlook: Chances for diurnal convection and morning mtn valley
fog/low stratus gradually increase during through midweek, with the
best chances expected Wednesday, as a cold front pushes into the
area. Chances for restrictions and diurnal convection may diminish
again during late week.

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High  91%     High 100%     High 100%     High 100%
KGSP       High  91%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High  91%
KHKY       High  91%     High 100%     High 100%     High 100%
KGMU       High  91%     High 100%     High 100%     High 100%
KAND       High  91%     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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