Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 132332
AFDGSP

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

.SYNOPSIS...
Cold and dry high pressure will move from the Great Lakes tonight to
New England Sunday night.  Another cold front will cross the region
Tuesday through Tuesday night, ushering in even colder air
Wednesday into Thursday.

&&

.NEAR TERM /THROUGH SUNDAY/...
500 PM EDT Update...Made downward temp adjs across the mtn valleys
where llvl CAA remains strong holding values 3-5 degrees below the
fcst curve. Other than some far wrn NC mtn upslope clouds...a very
dry atmos will persist into the evening. Gusty conds will begin to
weaken over the next couple hrs as well.

As of 215 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, though the cloud cover is now beginning to dissipate as
winds start to decline.  There is not much reason to expect the
snow to shut off as quickly as we had once forecast. Webcams from
Avery County still show some flurries, though not as "intense"
as was shown a couple hours ago. Hi-res models show light precip
continuing well into tonight, and I have reflected that to some
degree by keeping an isolated-scattered -SHSN mention until after
00z in the most favored areas. 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
given the expected pressure gradient, 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
should be 12-14 degrees below normal.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
As of 230 PM EST Saturday: A short wave rounding the base of the
trough over the eastern CONUS crosses the area Monday. This will
bring an increase in high cloudiness but no precip. Temps remain
well below normal.

An upper low over the Great Lakes opens up and moves slowly east
Tuesday as a short wave rounds the low and digs a trough across the
OH and MS River valleys. The short wave and upper trough remain
positively tilted to neutral at worst as they cross the area
Wednesday. The 12Z ECMWF, GFS, and Canadian models all show this
trend which is a significant change from their runs 24 hours ago
when they showed a closed low crossing over or very close to the
area. The models still show the associated cold front moving into
the mountains late Tuesday then crossing the area Tuesday night.
They also retain the wave developing on the front in the lee of the
mountains over NC and the trailing inverted surface wave over the
area behind the front into Wednesday. However, they have all trended
much drier with this system. There is less low level instability and
forcing given the open wave. There is some deep moisture but it
moves through quite quickly as well given the open wave. There is
still decent Q-vector convergence, so there should be enough mid
level forcing for precip to develop across the mountains on Tuesday
which moves out of the mountains and across the area Tuesday night.
Best PoP outside of the mountains will be across the NC Piedmont
where the mid level forcing and low level instability and forcing
can phase late Tuesday night and early Wednesday. That said, even
there QPF will be light with the weaker forcing quicker movement.
This lower trend is shown across the board on all guidance. For
example, the member of the GEFS with the highest snowfall amounts
has fallen from over a foot yesterday to 6 inches across the NW NC
Piedmont. The mean snowfall has also taken a tumble to around an
inch with significant clustering below even the 0.5 inch amount in
the operational GFS. All other locations are lower than that, even
across the NC Mountains. Therefore, will retain the chance PoP for
the NC Mountains Tuesday afternoon and all areas Tuesday night with
a quick transition east of the area Wednesday morning as some precip
will fall. For now, have snow amounts maxing out around an inch
outside of the mountains, mainly over the NC Piedmont and up to 2
inches in the favored high elevation locations of the NC Mountains.
NE GA and the Upstate look more in the dusting to maybe 0.5 inch
range.

Temps ahead of the front Tuesday will warm to near normal outside of
the mountains and around 5 degrees below normal across the mountains
but fall to well below normal Wednesday behind the front as the
arctic air mass moves in. Breezy conditions develop ahead of the
front and breezy to windy conditions area expected behind the front.
This could lead to wind chill values less than 5 below zero across
the higher elevations of the NC Mountains Tuesday night and
Wednesday morning.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
As of 2:45 PM EST Saturday: The long wave upper trough of low
pressure will be east of the region Wednesday night. Skies should be
clear, or clearing, with cold air pouring back into the region.
Winds are expected to remain fresh from the northwest. As a result
this will be the coldest night of this part of the forecast cycle,
with single digits and teens on tap. Meanwhile, the fresh winds
should contribute to wind chill values of zero to 15 below in the
mountains, and single digits to lower teens elsewhere.

The flow aloft should remain rather fast with the aforementioned
upper trough quickly deamplifying Thursday into Friday. This is in
response to a packet of energy aloft moving from the four corners in
the CONUS to Texas. At the same time surface high pressure will be
repositioning from the Missouri Valley, to an elongated position
from the western Atlantic back into the Carolinas, northeast Georgia
and the Tennessee Valley. The upshot is for quiet weather during the
latter half of the work week. with a very slow moderation in
temperatures.

The longer range guidance begins to diverge from Friday into
Saturday. The operational GFS scoots the upper system along the
lower Gulf Coast states (along with a surface reflection). It begins
the isentropic upglide pattern Saturday and beyond. The Canadian is
on the dry side. The new European, although not as bullish, does
trend toward some light precipitation (liquid and small chance)
Saturday afternoon in parts of northeast Georgia, the upstate South
Carolina mountains and southwest mountains of North Carolina. We
will follow that trend for small POPS in those locations.

Otherwise there will be a gradually increase in mid and high level
cloudiness, with any lower clouds arriving later Saturday near the
area of potential precipitation. We will continue to edge
temperatures up at the start of the weekend, following the overall
thermal profile and a fair clustering of model temperatures.

&&

.AVIATION /23Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: No restrictions across all TAF sites thru the
period. A very dry atmos will continue to dominate the region as
broad trof axis slowly swings overhead. This trof will bring FEW/SCT
ulvl clouds periodically thru Sun. Winds will remain gusty over most
terminals thru 02z or so as a llvl CAA continues to mix in. Expect
up-valley flow over the NC mtn valleys and n/ly winds veering ne/ly
across the non/mtns as stg hipres centers north and ridges south
into the Carolinas.

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...

            23-05Z        05-11Z        11-17Z        17-18Z
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...DEO
NEAR TERM...SBK/Wimberley
SHORT TERM...RWH
LONG TERM...TS
AVIATION...Wimberley


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