Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 121756

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1256 PM EST Tue Dec 12 2017

Northwest flow moisture will dissipate over the mountains tonight,
with dry high pressure building over the region through mid-week. A
cold front will cross the area from the northwest Wednesday night
through Thursday before high pressure sets up through much of the
weekend. Low pressure and moisture are expected to return from the
west by early next week.


As of 1255 PM Tuesday: Main update for 18Z TAFs along with minor
tweaks to hourly temperatures. Otherwise, near term forecast remains
on track attm.

Latest radar imagery from MRX depicts light snow showers moving
across the TN/NC border as quiet conditions continue across the rest
of the forecast area. The strong, dry cold front that pushed through
this morning has just about exited the forecast area as latest obs
show northwest winds across the entire area, which will
continue to increase and become gusty through tonight. Wind
gusts up to 40 to 50 kts are possible across the higher
elevations of the NC mountains. Temperatures are a few degrees
warmer than previously anticipated, but expected they have
peaked for today as they will begin to drop fairly quickly later
today into tonight.

Otherwise, expect snow amounts to range between 1-2" in the winter
weather advisory, with locally higher amounts possible across the
higher elevations. Snow showers should taper to flurries by early
evening. The NAM is the one operational model that seems to hold
onto some snow shower activity well into tonight. But looking at the
NAM sounding at TNB reveals very shallow moisture depth. So I still
like the diminishing PoPs this evening. As temps continue to plummet
and winds remain quite gusty, wind chill values will dip to 5 below
zero or colder across much of the areas above 3500 ft near the TN
border. With all that said, the current advisories look well placed
and no changes will be made with this forecast package. Lows tonight
will be quite cold, in the teens to single digits in the mountains
and 20s in the piedmont.


As of 245 AM EDT Tuesday: The rest of the middle part of the week
should be relatively quiet. A short-wave ridge moves overhead on
Wednesday with surface high pressure located to our south. The
850 mb thermal trof will already be past by midday Wednesday with
some warm advection returning in the afternoon. Temps are still
expected to be below normal, but the fcst reflects a warming
trend. On Wednesday night, a clipper-type low is forecast to zip
from the Midwest to the Mid-Atlantic region, dragging another
cold front down from the NW toward daybreak on Thursday. The
combination of some weak dpva and NW flow moisture could generate
some snow shower activity mainly over the nrn mountains near the
TN border beginning before daybreak but only continuing through
early afternoon Thursday when the moisture pulls out. Think a
20/30 pct chance will suffice for the time being, with minimal
accumulation expected. In spite of the passage of this next front,
temps return to near normal for Thursday as cold advection is weak
and short-lived, being overwhelmed by downslope warming E of the
mtns in the afternoon. The next system will not arrive before the
end of the Thursday night period.


As of 135 AM Tuesday: the extended fcst picks up at 12z Friday with
an approaching 500mb trof which has an axis roughly from the Great
Lakes to Memphis at 12Z. Models show height falls through the day on
Friday. The GFS has a slug of moisture arriving late Friday for the
NC and TN border areas with max impact in NW Flow early Friday
night. The old ECMWF has a quick shot of NW Flow Friday morning and
gone by early PM. The 500mb flow nearly levels out over the weekend
from west to east with dry surface high pressure. By late Sunday,
however, there are significant differences between the GFS and EC.
The GFS starts good Gulf influx of moisture along an approaching
cold front bringing rain to the mountains late Sunday and to the
rest of our forecast area Sunday night and Monday. The EC has a
strong low crossing the Great Lakes with low POP chances for us
Sunday night with the frontal passage. We hope for better agreement
as we get closer in time. The current forecast ends Monday and
Monday night with a dry period beginning. The new EC just came out
with even drier period Friday. However, the new EC has trended much
more in agreement with the GFS and even wetter than the new GFS. I
will incorporate this trend into this forecast, but not go
completely into that trend as it is nearly a week away.  Things can


At KCLT and Elsewhere: Expect VFR through the valid TAF period as
latest satellite imagery depicts SCT/BKN high clouds moving overhead
across much of the forecast area, along with intermittent areas of
FEW/SCT/BKN MVFR stratus. Winds have shifted to the northwest behind
a recently passed cold front and are expected to persist out of the
northwest through the forecast period. Expect winds to increase to
10 to 15 kts with gusts up to 25 kts through this evening, and
higher to 30 kts possible at KAVL continuing through Wednesday
morning. A brief period of snow could reach KAVL over the next few
hours, but overall anticipate the majority of snowfall to stay
pinned to the TN/NC border, not affecting any other terminals.  On
Wednesday, VFR continues with passing high clouds and northwest
winds around 10 kts.

Outlook: Potential for flight restrictions  Thursday night into
Friday, especially at KAVL, with a system approaching from the west.
Otherwise expect VFR.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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:


NC...Winter Weather Advisory until 7 AM EST Wednesday for NCZ033-
     Wind Advisory until 7 AM EST Wednesday for NCZ033-048>052-058-
     Wind Chill Advisory from 10 PM this evening to 7 AM EST
     Wednesday for NCZ033-048>052-059.


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