Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 110831

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
331 AM EST Mon Dec 11 2017

High pressure will persist across the region today. However,
a strong cold front will approach the region tonight. Mountain
snow chances will return Tuesday the cold front moves through
the area. Dry high pressure will builds back in Wednesday with
temperatures dropping well below normal again, but the temperature
should warm back toward normal later in the week.


As of 300 AM EST Monday: An impressive swirl can be seen on water
vapor imagery diving southeast into the Upper Midwest. This is
associated with a vigorous shortwave that will swing thru the Great
Lakes and Ohio Valley today. Flow ahead of this wave will back to
more due westerly this AFTN, allowing thicknesses to increase and
bring a nice rebound in temps. Skies should be sunny and winds
generally light out of the SW. Melting snow cover north and west of
I-85 may limit max temps somewhat, but overall it should be about
8-10 degrees warmer today than yesterday (near to slightly below

Tonight, as the aforementioned shortwave dives into the base of the
mean longwave trough axis, an associated Alberta Clipper will bring
a generally dry cold front thru the mountains during the pre-dawn
hours. Low-level moisture behind the front will bank up against the
TN/NC border slopes, as flow switches from west-southwest to
northwest by the end of the period. So PoPs start to ramp up into
the slight CHC to low-end CHC range in the usual NW flow upslope
areas. Temps will bottom out one or two categories above normal
ahead of the front, with increasing mid and high clouds.


As of 245 AM Monday: The passage of a sharp short wave trof
across the region early Tuesday will be the main story for
the short range as it will bring another blast of cold air for
mid-week. The wave will bring a shot of mid-level forcing that
will support the production of snow showers over the mtns after
daybreak. Following its passage, the cold NW flow takes over with
moisture  deep enought to tap the dendritic growth zone in the mid-
to late-morning hours. Weak sfc-based instability over northeast TN
will also contribute to the production of snow showers on the NC
border into the afternoon. Will raise precip prob into the likely
range in the usual NW flow area. All told, there is the potential
for accumulating snow, but an important limiting factor will be
duration. The moisture starts to pull out Tuesday evening and the
event will wind down in the early morning hours on Wednesday. We
are looking at the potential for 1-3 inches in some spots near the
TN border, which could require an Advisory. Wind will also be a
problem with gusts in the 30-40 mph range above 3500 feet Tuesday
and Tuesday night, which may also require an Advisory. Temps will
plummet to the single digits early Wednesday morning over parts of
the mtns, so a Wind Chill Advisory may also be needed. However, no
Advisories will be issued at this time because the event is still
more than 24 hours in the future, so prefer to let the day shift
have one more look at it. Outside the mtns, apart from Tuesday
being a breezy day behind the cold front, and Wednesday having
temps more than ten degrees below normal, it should be uneventful.


As of 315 AM EST Monday: No significant changes were made during
the medium range owing to low confidence in the model guidance
and the relative lack of consistency. The GFS still supports the
idea that another strong short wave will approach from the west on
Friday with a shot of moisture that brings snow shower activity
back to the mountains near the TN border. However, the new ECMWF
downplays that potential, instead, moving the bulk of the upper
wave and moisture farther south. The GFS then develops a stronger
response to the passage of the wave across the area east of the
mountains on Friday. Meanwhile, the ECMWF keeps the precip to our
east on Friday. For now, we will keep continuity and maintain the
chance of snow on the TN border mainly on Friday and wait for more
of a trend in the guidance as to what might happen elsewhere as
this wave passes.

The upper pattern is fcst to be progressive and the models
show deamplification over the east for the early part of the
weekend that would result in a rebound of temperatures at least
on Saturday. However, some energy in the srn stream could bring
precip back to the region on Sunday if the GFS is correct. The
ECMWF is more progressive with that srn stream energy and would
keep light precip closer to the Gulf Coast. With such a discrepancy
in the model guidance, confidence is too low to push the fcst in
one direction or the other, so we will leave it alone.


At KCLT and Elsewhere: Very quiet with VFR conditions expected for
the next 24 hours at all stations. Dry westerly to northwesterly
flow will persist atop the region, ahead of a clipper system that is
diving through the Upper Midwest. Generally light west-southwest
winds expected thru the period, increasing slightly this AFTN thru
this evening. At KAVL, winds will be out of the NW until mid-AFTN,
then switch out of the SW.

Outlook: Dry conditions expected at all sites through the early part
of the week. Another NW flow snow event could take place Tuesday and
Tuesday night, then again Thursday night and Friday, with some brief
restrictions possible at KAVL.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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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