Area Forecast Discussion
Issued by NWS Greer, SC

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS62 KGSP 211447

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1047 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 1040 AM EDT Monday:  Latest GOES16 visible satellite indicates
mostly sunny skies across much of northeast GA and the western
Carolinas this morning as any residual mtn valley ground fog should
erode rapidly.  With that, said visb sat does indicate building
CU across the higher peaks and ridgetops at this time thanks to
increasing instability and sely/sly upslope flow.  Outside of
the terrain, starting to see modest CU development across the
SC Midlands southward toward the coast adjacent a quasistationary
frontal axis.  Not much has changed with regard to the previous fcst
as latest near term guidance favors shra/tsra development across
higher peaks and ridgetops of the mtns, especially near/along the
southern facing slopes.  As for the I85 corridor, still looking
dry, however with the possibility of llv CU development beneath
some thin high cirrus.  All in all, conditions for viewing the
eclipse still look good.

Previous Discussion:  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: Some patches of so far not very thick fog
will be around for the next few hours.  Weather pattern continues
stable with mild zonal flow and a Bermuda high off the coast. This
afternoon, fair-weather cumulus clouds are anticipated most places
with some convective showers possible over the mountains.  Winds are
expected to be light and variable with a southeasterly bias as flow
around the Bermuda high gains a modicum of organization.  Except for
some isolated patches of fog this morning, and some possible
convective showers over the mountains this afternoon, conditions
should be good VFR.

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...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High  95%
KAVL       High 100%     High 100%     High 100%     High  90%
KHKY       High 100%     High 100%     High 100%     High  95%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     High  95%

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:




AVIATION...WJM is the U.S. government's official web portal to all federal, state and local government web resources and services.