Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 080548
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1248 AM EST Thu Dec 8 2016

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TODAY/...
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.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
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.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
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.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: MVFR and IFR restrictions in place again this
morning and will monitor expansion of restrictions as necessary for
TAF amendments and potential for Dense Fog Advisory. Should see
improvement again after sunrise as dry air filters in. Light SW to W
winds early this morning will shift NW and increase to 10-15kt with
some low-end gusts (higher at KAVL). Should see a mid-level deck
push across the area during the day, but clearing filtering in this
afternoon. NW winds will drop off a little late tonight but
generally remain 5-10kt.

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...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       Low   56%     High  83%     High 100%     High 100%
KGSP       Med   69%     Med   61%     High 100%     High 100%
KAVL       High  81%     Med   61%     High 100%     High  83%
KHKY       Med   69%     Low   30%     High 100%     High  94%
KGMU       High  81%     Med   61%     High 100%     High  94%
KAND       High  81%     Med   72%     Med   69%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...JPT
NEAR TERM...PM/RWH
SHORT TERM...JAT
LONG TERM...CDG
AVIATION...TDP


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