Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 211735

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
135 PM 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 130 PM EDT Monday:  Although deep ridging prevails this
afternoon across the region, iso/sct shra have developed as
instability peaks with heating.  Giving warm temps aloft, am not
expecting any deep convection, however enough instability is present
to warrant slight/chance pops across portions of the area, maximized
along the highest peaks and ridgetops of western NC.  Latest GOES16
visb does indicate modest cu development across much of the area,
with a minima of sky cover just north of the I85 corridor.  Am still
expecting some mixing to occur, which combined with totality could
yield localized and short term cooling and scting of low cu.
Otherwise, convection across the region will diminish after sunset
with loss of heating, leading into a dry evening.  Guidance
continues to favor abundant BL moisture across the region on Tuesday
morning, with patchy fog possible for any regions that experience
convection this afternoon, yet with the best chances in the mtn
valleys as was the case this morning.  Moving into Tuesday, the
upper ridge looks to weaken a bit as a trof approaches from the
west.  With that, guidance favors convection across the high terrain
once again based on a diurnal trend, with propagation into the
fthills/piedmont possible late in the day.  Thus widespread chance
pops were featured over the mtns, with slight chances spread across
the remainder of the fcst area.  Temperatures through the period
will remain a few degrees above normal.


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.


KCLT and Elsewhere: VFR through the period at all sites aside for
at KAVL where mtn valley fog is expected near daybreak on Tuesday.
Otherwise, deep ridging prevails across the region which should keep
convection at bay at most terminals, with sct/num shra expected over
the mtns this afternoon.  Therefore, did include vcsh at KAVL for
now, however wouldn`t be suprised if a shra/tsra amd is needed at
some point this afternoon.  All other sites are dry with llv cu and
current light/vrb flow veering southerly almost immediately.  As
stated, added fm group for IFR fog at KAVL tomorrow morning, with
all other sites remaining VFR through the period.

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...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  83%
KAVL       High 100%     High 100%     High  83%     High  87%
KHKY       High 100%     High 100%     High 100%     High  83%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High  83%

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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