Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 111431

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
931 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 930 AM Monday...Minor adjustments made to temperatures to
reflect latest observations. Otherwise, latest near term
forecast on track. With abundant sunshine across the area this
morning, temperatures are gradual to warm as expected,
especially across the mountains where snow cover remains on the
ground from this past weekend.

A clipper system associated with a vigorous shortwave that will
swing thru the Great Lakes and Ohio Valley today can be seen
over the Upper Midwest. Flow ahead of this wave will back to
more due westerly this afternoon, 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 normal).

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 enough 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: A clipper system will dive through the Great
Lakes and Ohio Valley today into tonight. Before it arrives, skies
should be clear with light SW winds (NW at KAVL). Later this AFTN
into tonight, a batch of mainly high clouds will stream over the
area and SW winds will increase slightly. Low-level moisture will
start banking up against the NC mountains in northwesterly flow just
before daybreak, with some low VFR or possibly MVFR cigs going up
the valley to KAVL. Restrictions at KAVL, if any develop, are after
12z Tuesday.

Outlook: A brief NW flow snow event is expected Tuesday and Tuesday
night, then another one 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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