Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 121437

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
937 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 930 AM Tuesday: With temperatures in the upper 30`s to around
40 degrees, a strong, dry cold front continues to move west to
east across the Carolinas this morning as latest observations
show the notable wind shift from the mountains just up to
Piedmont attm. Expect this boundary to continue it`s eastward
propagation out of the forecast area within the next hour or two
as winds begin to increase with strong CAA. Snow showers are
slowly making their way into areas along the TN/NC border as
seen from latest MRX and GSP radars along with an earlier report
of light snow in Madison County (NC). For this morning update,
no major changes were needed as latest near term forecast
remains on track.

Otherwise, a vigorous shortwave will continue to cross the southern
Appalachians this morning, helping further dig a deep longwave
trough over the eastern CONUS. As the strong cold front pushes
through the forecast area, northwest winds will increase to around
40-50 kts across the NC mountains, and scattered to numerous snow
showers are expected to 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 cold front has pushed into the NC
mountains as of the 12z TAF issuance, and should rapidly cross the
rest of the area by mid-morning. Winds will shift at all sites to NW
by about 15z. Other than a period snow showers and low clouds along
the TN border, conditions should be VFR thru the period. A brief
period of snow shower activity may reach KAVL, most likely in the
14-17z time frame, where a tempo for some MVFR cigs and a mention of
-SHSN will be carried. Winds at all sites will become gusty, with
strongest gusts at KAVL, as low-level CAA ramps up. Winds should
subside gradually outside of the mountains, but will remain very
gusty in the mountains overnight.

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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