Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 120823

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
323 AM EST Tue Dec 12 2017

Dry high pressure will build across our region through mid week as a
deep trof crosses the area. Temperatures will be several degrees
below normal on Wednesday then recover to near normal Thursday.
Expect another series of fronts to cross our region going into next


As of 300 AM EST Tuesday: A vigorous shortwave will cross the
southern Appalachians this morning, helping further dig a deep
longwave trough over the eastern CONUS. Meanwhile, an associated
clipper low will deepen over the Great Lakes and force a strong cold
front thru the CWFA by late morning. The front was analyzed just
entering the Great Tennessee Valley a couple hours ago. The front
has no low-level RH to speak of along the boundary; however, an area
of stratocu can be seen on IR and in the METARS behind the front
covering a bit chunk of the Ohio Valley and Mid-MS Valley. The
estimated time of arrival of this moisture to the TN/NC border looks
to be around 10-11z (5 or 6 AM). From there, winds will shift out of
the NW and increase to around 40-50 kts across the NC mountains, and
scattered to numerous snow showers should develop with the upslope
flow. Strong 850 mb CAA will ramp up across the entire area and
winds will approach advisory criteria not too long after daybreak.
Temps will likely peak late morning or midday in the mountains and
early afternoon in the Piedmont, then fall fairly quickly thru the
rest of the day, resulting in a cold, breezy evening. Highs will be
mainly in the 30s in the mountains and upper 40s to lower 50s east.
Snow amounts seem to be barely 1-2" in the winter weather advisory,
with locally higher amounts possible.

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


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: The leading edge of a clipper system can be
seen about to enter the NC mountains early this morning, bringing a
band of mainly high clouds and a wind shift from SW to NW. The
associated cold front should rapidly cross the region by
mid-morning, shifting all sites to NW winds by about 15z. Other than
a period snow showers and low clouds along the TN border, conditions
should be VFR. A brief period of snow shower activity may reach
KAVL, most likely in the 14-17z time frame. I have a tempo for some
MVFR cigs and a mention of -SHSN. Winds will get quite gusty,
especially at KAVL, as low-level CAA ramps up. Winds should subside
gradually outside the mountains, but will remain very gusty in the
mountains tonight.

Outlook: Restrictions may be possible Thursday night into Friday,
especially at KAVL, with a system approaching from the west.
Otherwise expect dry and cool, VFR conditions.

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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