Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 081948

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
248 PM EST Thu Dec 8 2016

Cold high pressure will gradually spread across the region, bringing
the coldest temperatures so far this season. A cold front will
approach the Western Carolinas late Sunday and move through the area
on Monday with high pressure returning in its wake.


As of 200 PM EST: Increasing, post-frontal northwesterly winds are
ushering in the leading edge of the coldest airmass of the season
across our forecast area. Aloft, a broad trough axis remains west of
the mountain chain late this afternoon and IR imagery shows clouds
thickening overhead east of the trough as the upper jet axis
sharpens along the southern Appalachians. The upper jetlet will
gradually pass off to the southeast overnight as mid/high level
clouds diminish. Shallow moisture at low levels will intermittently
bank up against the NC mountains in NW flow to produce some shallow
stratocumulus in the upslope preferred areas. Anticipate moisture
remaining too shallow for any low end snow PoPs.

The main issue overnight will be brisk to windy conditions in the
high terrain as the NW flow 850 mb jet ramps up to 35 kt 06z to 12z.
This will combine with very chilly min temps in the teens to push
northern mountain wind chills down to the zero to minus 5 range. The
higher peaks will likely bottom out at minus 5 to minus 10 degrees.
Since most of the affected counties will not see minus 5 or colder
values, an SPS for the cold weather will be issued in lieu of a Wind
Chill Advisory.

The shallow upslope moisture and pressure gradient in the NW flow
will gradually diminish/relax through the day on Friday. Some weak
lee troughing will likely develop during the day as downsloping
continues. Temperatures will likely not recover above freezing in
many NC mountain locations and lower to middle 40s maxes are likely
east of the mountains Friday afternoon.


At 2 PM Thursday: On Friday night a low amplitude upper trough will
will be along the Eastern Seaboard, while a low amplitude ridge will
be over the Western USA. The pattern deamplifies such that by
Saturday nearly zonal flow will exist across the CONUS, which will
persist into Sunday.

At the surface, on Friday night high pressure over the Central MS
River Valley will be spreading east across the Southern and Central
Appalachians. A decreasing pressure gradient will allow winds to
gradually decrease, especially east of the mountains. The surface
high will cross the mountains on Saturday, reaching the East Coast
on Sunday. Southerly winds will increase at high mountain elevations
on Saturday night and Sunday as the pressure gradient tightens on
the back side of the surface high. Atlantic moisture moving inland
on the south side of the high, and moisture from the Gulf of Mexico
spreading north, will impact our area on Sunday, when precipitation
is expected to develop. Temperatures will run well below normal as
the cooler air mass arrives, but will be warm enough on Sunday that
precipitation would be liquid rain.


As of 245 PM EST Thursday:  The medium range fcst period kicks off
on Sunday evening amidst a weak and short lived hybrid CAD event
across the Mid Atlantic, while a region of H5 height falls
approaches from the west atop a surface cyclone and associated cold
front.  Pattern evolution through the period will feature increasing
pops into Monday morning as upglide commences ahead of the front,
with pops ramping up further to likely levels across the mtns, with
chances east as the sfc trof/front pass through the day.  Sheltered
valley subfreezing temperatures near daybreak could lead to a very
brief period of fzra across the northern mtns, changing to all rain
after sunrise.  Furthermore, weak caa through the column behind the
front into early Tuesday morning could yield a brief round of sn
showers for the same region, with all liquid favored elsewhere.

Although the front will have passed on Tuesday, fast onset of weak
upglide atop the transitioning surface ridge will warrant
continuation of at least slight/chance pops regionwide per the
ECMWF, which is much more aggressive than the GFS with regards to
onset timing.  Nevertheless, another frontal complex will approach
the region into Wednesday with good model consensus on prefrontal
upglide with temperatures supportive of wintry precip near daybreak
across the high terrain.  Once again, ptypes will change phase back
to all liquid through the day on Wednesday, before frontal passage
leads to modest caa and falling temperatures through the profile
into Thursday morning.  This could support another round of sn
showers across the high terrain, possibly into the Piedmont.  Beyond
that cold/dry high pressure will persist with regional temperatures
around a category below normal.


At KCLT and elsewhere: High clouds will remain expansive across the
terminal forecast area as an upper jetlet moves over the mountains.
The clouds will diminish from the west overnight as the jet axis
slips east. A few stratocumulus could develop in NW upslope flow
near the TN border, but these will likely not reach KAVL. Anticipate
low ends gusts across the region through late afternoon with mixing
and cold advection, with the highest gusts into the mid 20s at KAVL.
KAVL gusts could well linger through the end of the period Friday as
cold advection continues. Otherwise, VFR conditions are expected

Outlook: Drier and much colder conditions will continue through most
of the weekend. Another front will approach the area Sunday
afternoon, with uncertain chances of precipitation and restrictions.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     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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