Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 131806
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
106 PM EST Sat Jan 13 2018

.SYNOPSIS...
Cold and dry high pressure will settle in from the north through the
weekend. Another cold front will cross the region Tuesday through
Tuesday night, ushering in even colder air from mid to late next
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1250 PM: High pressure is building into the region from the
NW, which has resulted in clearing downstream of the mountains. Low
stratocu and snowfall continues in the upslope areas near the
Tenn border. Plentiful moisture exists upstream; while winds will
slowly taper off this afternoon and evening, there is not much
reason to expect the snow to shut off as quickly as we had once
forecast. Webcams from Avery County indicated decent snow was still
falling around 1230 PM. Thus I updated PoP to hold on to a -SHSN
mention even longer than I fcst at the previous update, with hi-res
models showing light precip continuing well into tonight. Additional
accumulation is not expected to worsen any existing impacts.

Beyond the mountain snowfall there is little to talk about in
the near term. CAA should maintain some degree of mixing tonight,
and in light of a decent pressure gradient persisting overnight,
a weak breeze is expected to persist. That looks to do little to
keep the mountains from dipping into the lower teens (perhaps even
single digits north), and the Piedmont mostly in the upper teens
or lower 20s. Slightly below-zero wind chills will result at high
mtn elevations, though not reaching advisory criteria. Despite
sunshine Sunday, highs are expected 12-14 degrees below normal.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
As of 230 AM EST Saturday: The first part of next week looks quiet
as the western Carolinas should be near the bottom of a broad upper
trof Sunday and Monday. A short wave ridge superimposed on the trof
will support a cold continental high that moves from the Midwest on
Sunday to Quebec on Monday, all the while ridging down across our
region. The end result will be fair weather and well below normal
temperatures...not quite to the "cold wave" threshold like two weeks
ago...but reminiscent of that. The high center will move away Monday
night as the next cold front approaches from the west. Whether
or not any clouds/precip reach us by daybreak Tuesday may depend
on the nature of the upper trof. If it remains open like the GFS,
we might have a small chance of precip reaching the mountains by
12Z Tuesday, but very little chance if the upper system closes
off a low as in the ECMWF. At any rate, the interesting part of
the fcst happens after 12Z Tuesday.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
As of 230 AM EST Saturday: New 00Z guidance has created a bit of a
pickle in the details of the medium range forecast. Models are in
decent agreement to start the period Tuesday morning, with a closed
upper low over the Great Lakes, upper troughing amplifying over the
eastern CONUS, and a cold front trailing a Great Lakes surface low
as it marches eastward towards the forecast area. Temperatures will
be close to normal on Tuesday, though very cold air will move into
the southeast Tuesday night as heights fall and an arctic surface
high centered over the Plains edges in. Cold air will be in place
Tuesday night through Wednesday morning, and the forecast question
that remains is whether or not any moisture associated with the
advancing surface cold front will be able to cross the Appalachians
and spill into the SC Upstate and NC Piedmont/Foothills. The GFS,
carrying a more open upper wave, has virtually no moisture able to
break containment from the Appalachians, and results in no QPF
response Tuesday night and Wednesday outside of some NW flow snow in
the NC mountains. The ECMWF, however, has remained stubborn in its
depiction of the upper low closing off very close to our forecast
area and carrying with it sufficient moisture to squeeze out some
light snow showers across almost the entire forecast area early
Wednesday morning.

The public forecast therefore displays a compromise between the two
solutions. Pops spread across the entire area early Wednesday, and
the potential for some light snow is reflected in the grids. Pops,
however, were kept at chance or lower as a nod to the extremely high
uncertainty in this forecast, and accums less than an inch for the
time being. This forecast is guaranteed to change (there either will
be moisture, or there won`t), so keeping an eye on future updates is
strongly advised. The balance of the medium range is very quiet as
the cold front passes Wednesday ushering dry air behind it, and
temperatures will gradually rebound through the end of the next work
week as heights rise aloft over the eastern CONUS.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: A dry and cold airmass will continue to
build into the area through Sunday. Winds remain rather gusty this
afternoon, particularly over the NC mountains and KAVL, where NW
upslope flow has also produced an MVFR cloud deck. Models do not
have a good handle on the status of the clouds, so confidence is
low as to how long this deck will persist. Winds do appear to have
begun diminishing as models indicated would occur, so the deck is
expected to scatter by around sunset. Somewhat breezy conditions
continue tonight due to CAA and resultant mixing, but not likely
enough to cause trouble. Directions will gradually veer as high
becomes centered N of the region. A passing weak disturbance will
bring in thin cirrus tomorrow.

Outlook: VFR and cold conditions are expected to prevail through
early next week. A fast-moving system may bring a brief shot of
moisture Tuesday night into Wednesday.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...Carroll
NEAR TERM...Wimberley
SHORT TERM...PM
LONG TERM...Carroll
AVIATION...Wimberley


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