Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 100231
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
931 PM EST Fri Dec 9 2016

.SYNOPSIS...
Cold high pressure will remain in place through the weekend. A cold
front will approach the Western Carolinas late Sunday and move
through the area on Monday, increasing rain chances for early next
week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 930 PM EST: Sat pix show clear skies across the region and
this should continue through the rest of the night. Winds have not
dropped off as quickly as expected, so temps have not fallen as
quickly either. Still expect winds to diminish through the night, so
temps should fall to forecast lows. Winds are a little stronger
across the higher ridges leading to some below zero wind chill
values. Although single digit wind chills are expected in some
mountain locations overnight, any advisory level wind chills will be
above 5000 feet elevation, so no wind chill advisory will be issued.

Otherwise, a passing upper level shortwave will move off the eastern
seaboard this evening. Upper flow behind the wave will become
increasingly zonal tonight through tomorrow. At the surface, strong
and cold high pressure currently centered over the confluence of the
OH/MS rivers will build east to the Appalachians tonight through
Saturday. A very cold night is expected as low level thicknesses
bottom out overhead and winds go light as the pressure gradient
relaxes everywhere but the highest peaks. Expect mountain mins well
down in the teens, with teens to lower 20s east of the mountains,
but most locations well above record mins for the date.

Weak westerly downsloping flow will continue at 850 mb through
Saturday. A modest upward bump in max temps is expected. A round of
mid level moisture will pass north of the area in the increasingly
zonal flow, but with little impact on sky cover through Saturday.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
At 2 PM Friday: on Saturday night nearly zonal flow will exist over
the CONUS, and this pattern will exit into Monday. The bulk of upper
level vorticity will be channeled well to the north of our area,
with the possible exception of Monday, when the GFS shows a modes
advection vort lobe crossing the Southern Appalachians.

At the surface, on Saturday night a surface ridge will be crossing
the Southern Appalachians. The ridge center moves offshore on
Sunday, but maintains a foothold over the Carolinas and Georgia. The
models show shallow low layer Atlantic moisture moving inland from
the Gulf Stream to the Blue Ridge on Sunday. The moist layer becomes
deeper on Sunday night as moisture from the Gulf of Mexico arrives
ahead of a cold front, supporting precipitation production. If a few
mountain valleys get cold enough, some freezing rain is possible
early Monday morning. Precipitation will increase on Monday as the
front arrives, decreasing from the west in the afternoon as the
front moves to the eastern portion of our area.

Temperatures will start out well below normal due to the wedge of
cold air initially over our area, but will warm to slightly above
normal as the wedge breaks down, and cloud cover increases ahead of
and along the front.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 245 PM EST Friday:  The medium range fcst period initializes
on Monday evening as a cold front slides through the region.
Broad troffing aloft looks to support at least modest caa behind the
front, however expecting any lingering post fropa upslope precip
to remain in liquid form through Tuesday morning.  There still
remains some uncertainty withing model solutions as it pertains
to the onset time of any upglide induced precip on Tuesday into
Wednesday as transient high pressure moves east atop the Mid
Atlantic and the aforementioned front looks to stall along the
I20 corridor.  As such, the fcst will feature continued low end
slight/chance pops, increasing a bit into Wednesday as the next
system approaches from the west given weak wave development along
the old stalled front per the GFS which would amplify any upglide.
It should be noted that the ECMWF features a much stronger surface
high moving in behind the front, thereby allowing for further
southern movement of the stalled front, which all in all leads to
no upglide and thus a dry fcst.

These discrepancies within the op guidance continue as the next
cold front approaches/passes on Wednesday night into Thursday
with the GFS being much wetter than its European counterpart,
with the former yielding measurable snowfall along portions of
the NC high terrain Thursday morning/afternoon as northwesterly
upsloping prevails.  Favored this GFS solution with the official
fcst, however kept pops below likely levels as confidence is rather
low at this point in ptypes as well as QPF/SnowAmt`s.

Meanwhile another shortwave digging across the northwest will
dive through the Rockies into the Plains, while cold high pressure
settles across the central/southern Appalachians.  The disagreement
between op guidance continues further late in the period as the
ECMWF features surface wave development over the MS Delta region,
while another/colder reinforcing surface front surges through
the OH valley into the TN valley.  Nevertheless, the GFS is much
faster with this front and thereby keeps much if not all of the
QPF south of the region, with the exception of another bout of
northwest flow snowfall along the TN line.  However, the ECMWF is
very aggressive with the wave development, and thus overpowers the
intruding front yielding widespread upglide induced precip atop
a cold airmass yielding vast ptype issues.  All said, given these
continued discrepancies, confidence is very low in the out periods.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: VFR conditions will continue through the
terminal forecast period as cold and dry surface high pressure
builds eastward from the OH/MS rivers to the Appalachians. Light
winds N to NE through the night and morning switch to SW for the
afternoon. Expect continued light NW to N flow in the French Broad
valley near KAVL through the end of the period. May see some cirrus
move in during the afternoon. Otherwise, clear skies until then.

Outlook: Dry and cool high pressure will move from the Appalachians
off the east coast Sunday. Moisture will return ahead of an
approaching cold front Sunday night through Monday with mainly a
cold rain and associated restrictions expected. Conditions remain
unsettled through mid week as the passing front stalls just to the
south of the area.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...LEV
NEAR TERM...HG/RWH
SHORT TERM...JAT
LONG TERM...CDG
AVIATION...RWH


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