Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 080246

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
946 PM EST Wed Dec 7 2016

A reinforcing cold front will move through the area overnight.
In its wake, cold high pressure will gradually spread across the
region, bringing the coldest temperatures so far this season.
Another cold front will approach the Western Carolinas late Sunday
and move through the area early Monday with drier high pressure
in its wake.


As of 945 PM EST Wednesday: Low clouds remain over all of the area
but the Upper Savannah River Valley. In these locations, fog is
forming due to the moist ground and better radiational conditions.
The fog is patchy dense, and could linger through the night.
However, it is unclear if that will happen with the approach of the
cold front. Winds are expected to increase which could lead to
better mixing and less fog, but increased potential for low clouds.
For now, have included fog for this area and kept it in place
through the night, with low clouds elsewhere. The GFS has continued
the precip potential across NC with the frontal system, but the NAM
and mesoscale guidance shows little to no precip. Have left the PoP
as is, but do not expect any significant precipitation, even with
the wintry mix potential. Temp trends look on track.

The upper pattern is expected to slowly evolve over the next 24
hours as a large closed low moves across eastern Canada bringing the
axis of the long wave trof in from the west. An old weak boundary
strung out near the mtns will be re-invigorated to some extent as a
large cold continental high drops down into the nrn Plains and
increases the pressure and temperature gradient. Meanwhile, a weak
sheared short wave and weak upper divergence will move over the top
of this feature. The models suggest that some light precip might
develop along the boundary after midnight, but particularly around
daybreak, mainly across the mtns. The previous fcst went along with
that idea so little change is necessary in that regard. The timing
is such that if any precip falls, there could be a brief window of
wintry p-types mainly at high elevations. Fcst soundings suggest
this could be a mix of valley rain, high elevation snow, and
mid-slope freezing rain. Either way, it will not amount to much. In
the first place, the chance is only about 20 pct. Secondly, amts
will be no more than a few hundredths. Temps will remain above
freezing in most places away from the TN border. The front will push
southeast on Thursday with drier air spreading across the region as
the high builds in. Downslope east of the Blue Ridge and sunny sky
should afford us one more day with near normal temps.


At 2 PM Wednesday: On Thursday night a positively tilted low
amplitude upper trough will extend from Quebec to Mexico. This
feature slowly progresses to the Eastern Seaboard and the Gulf of
Mexico by Friday evening. Channeled vorticity rounding the base of
the trough will pass north of our area on Friday. The upper pattern
deamplifies such that by Saturday nearly zonal flow extends across
the CONUS.

At the surface, on Thursday night dry air will move into our area in
the wake of a departing cold front. Although model guidance does
show a shallow moist layer near the TN border at around 875 MB
overnight, confidence is low that any snowfall would result. A tight
pressure gradient behind the front will allow winds to only slowly
diminish through Friday. A surface high will cross the Southern
Appalachians late on Saturday. Temperatures will run well below
normal in the cooler air mass behind the front.


As of 245 PM EST Wednesday:  The medium range fcst period kicks
off on Saturday evening amidst zonal flow aloft across much of
the eastern CONUS, while the surface pattern is to be highlighted
by a broad/cold high pressure centered over the southern/central
Appalachians.  Pattern evolution through the period will feature
amplification of the synoptic pattern by way of a digging H5
shortwave into the central plains states, which will drive a
surface cold front into/through the MS river valley into Sunday.
Guidance is rather discontinuous with regards to timing of the
frontal passage through northeast GA and the western Carolinas
with ECMWF being slower and warmer than the GFS.  Nevertheless,
wintry precip is possible in the form of high elevation snowfall
and/or sheltered valley icing into Monday morning, with perhaps
better chances for mtn snowfall being on Monday night if the ECMWF
verifies.  Cool/dry high pressure to prevail behind this system,
into Wednesday/Thursday before another cold front pushes into the
region. As with the aforementioned system, the op guidance is still
out of phase thus timing is yet to be determined, however there
seems to be some convergence in that another round of wintry precip
is possible late in the week as an arctic airmass surges southward.


At KCLT and elsewhere: MVFR will develop at the NC sites this
evening and linger through the night. Will likely see some MVFR fog
as well. At the SC sites, cigs will generally be MVFR, but there
will be breaks from time to time this evening. Some MVFR fog is also
possible. As the cold front moves into the late tonight or early
Thursday, mixing begins and clouds will lift then scatter out. The
passage of the front and a weak upper wave may help to develop some
light precip, mainly over the mtns and foothills, around daybreak.
This could produce some wintry precip at KAVL, but the chance is too
low to mention. Southerly winds this evening become light and
variable overnight. Behind the front, winds turn NW and increase in
speed with gusts likely most locations.

Outlook: Drier and much colder conditions continue through the
weekend. Another front will approach the area Sunday afternoon
bringing a chance of precipitation and restrictions.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  91%     High  86%     High  95%     High 100%
KGSP       High  86%     High  84%     High 100%     High 100%
KAVL       Med   76%     Med   66%     High 100%     High 100%
KHKY       Med   78%     High  83%     High 100%     High 100%
KGMU       High  84%     Med   79%     High 100%     High 100%
KAND       Low   58%     High  94%     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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