Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210353

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1153 PM EDT Thu Oct 20 2016

After a stretch of record warmth, a cold front will cross the area
tonight through Friday morning, increasing rain chances mainly over
the mountains. Drier and much cooler conditions will set up across
the area over the weekend. A dry "back door" front will push through
from the north on Monday with cool high pressure remaining in place
through the middle of next week.


As of 1030 pm Thursday: Surface analysis shows that the front is
right on our doorstep, pretty much along the VA/KY border stretching
southwest through the southern part of the NC/TN border. Have made
another few minor tweaks to temperatures for the late evening
update, but the bigger adjustments are to sky cover (lower) and PoPs
(also lower). HRRR and other CAMs are picking up pretty well on the
lack of precipitation across the mountains, as the initial weak band
that developed is pretty much gone now. Zero CAPE to speak of so in
addition to slowing down the incoming pops pretty drastically, have
removed all mention of thunder.

Otherwise, by late tonight, low pressure is forecast to deepen
across PA, sweeping the front across the southern Appalachians
during the early morning hours, reaching the central Carolinas by
sunrise. South-southwest winds should remain ahead of the front,
then veering from the northwest in the wake of the boundary. The
environment should yield marginal instability across the
Appalachians, supporting showers and thunderstorms. East of the
mtns, it appears that showers will be limited to the NC foothills
and northern Piedmont. Low clouds may remain across the TN border
counties through Friday morning.

By mid Friday morning, shallow mixing should provide enough momentum
transfer to support gusts around 20 kts, gradually strengthening
through early afternoon. By mid day Friday, gusts are expected to
regularly reach into the 30s to mid 40 kts across the high terrain
and east facing mtn slopes. The strongest gusts will likely occur in
the wake of the mtns, with the forecast development of a mtn wave.
At this time, the window of gusts and coverage appears too limited
to highlight with a Wind Advisory, but we will monitor trends in
guidance. Otherwise, Friday will feature much lower temperatures,
highs ranging nearly 20 degrees cooler that today. Using a blend of
guidance, highs should range from the upper 50s across the mtn
valleys to low 70s across the upper Savannah River Valley. NW flow
may support ISO to sct showers across the TN border counties well
into Friday afternoon, I will highlight with SCHC to CHC PoPs.


As of 220 PM EDT Thursday: A weak short wave will move through an
upper trough and across the area Friday night and Saturday. At the
surface, a tight pressure gradient will be over the area between the
departing cold front and high pressure over the Mid-South. The
result will be very windy conditions across the mountains and breezy
to windy conditions elsewhere. With the cold advection Friday night,
expect 850 mb temps to bottom out slightly sub-freezing across the
mountains, but low level moisture still does not look very
impressive. Will still feature a ridge top rain/snow mix Friday
night and cannot rule out some light accumulations on Mount Mitchell
by Saturday morning where temperatures should fall into the upper
20s. A widespread Saturday morning freeze does not look likely in
the NC mountains and winds will be too high for frost formation so
no frost/freeze products will be issued. Elevations above 4000 feet,
however, could well see plenty of lower 30s temperatures. Any
lingering mountain precip will end quickly Saturday morning with
clouds slowly dissipating through the day. Sunny to mostly sunny
skies expected elsewhere. Windy to breezy conditions will continue
across the area as well. Highs will be 5 to 10 degrees below normal.

Heights begin to rise Sunday as the trough axis moves east of the
area. The surface pressure gradient will relax as well the center of
high pressure moves along the Gulf Coast. Winds should diminish
Saturday night, but will pick back up a little on Sunday as a lee
trough develops. A better chance of frost develops Sunday morning
across the mountains with the continued cold conditions and
diminished winds. Will have to keep an eye on the normally colder
locations outside of the mountains for frost development as
radiational cooling conditions improve. Highs will bounce back to
near normal.


As of 200 PM EDT Thursday...the extended period begins Sunday night
with the area under the influence of WNW flow and a weak surface
high centered over the southeast. Plentiful insolation and some
continued weak downslope flow east of the mountains will allow max
temps on Monday to climb to about ten degrees above average. A
clipper system will sweep through well to our north during the day
on Monday, pushing a backdoor cold front through by Monday night.
Though the frontal passage will feature no QPF response in our area,
it will bring min and max temperatures down closer to average levels
on Tuesday. An upper ridge will build in from the west by the middle
of next weak, keeping the region dry. Guidance is currently
suggesting a surface wedge will set up during the middle of the next
work week, maintaining temperatures near average levels.


At KCLT and elsewhere: Only change to the KCLT TAF for the 03z
AMD is to remove the lower VFR Cu for this evening. Otherwise, only
restrictions continue for KHKY and KAVL in closer proximity to the
front. Rain chances still not high enough to include prevailing SHRA
at this time so continued VCSH. S to SSW winds should continue this
evening and early overnight ahead of the front, but veering around
to NW Friday morning (current timing at KCLT is 14z). Post-frontal
mixing still looks like enough momentum transfer for decent wind
gusts tomorrow and for Friday afternoon have increased some of the
gusts through the end of the period.

Outlook: Pressure gradient will remain remain across the western
Carolinas through Saturday with breezy conditions. Dry high pressure
will build across the region on Sunday into early next week.

Confidence Table...

            04-10Z        10-16Z        16-22Z        22-00Z
KCLT       High 100%     High  97%     High 100%     High 100%
KGSP       High 100%     Med   75%     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%     Med   75%     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:



  STATION      HIGH        LOW         HIGH        LOW
  -------  ----------  ----------  ----------  ----------
   KAVL      83 1985     42 1961     62 1950     25 1981
   KCLT      86 1926     49 1989     66 1993     30 1972
   KGSP      86 1899     50 1989     68 1894     29 1972




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