Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220024

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
724 PM EST Tue Nov 21 2017

Moist southerly flow in the lower atmosphere will bring some small
chances for rain or drizzle until a weak cold front crosses our area
tonight.  A passing area of low pressure to our south may bring some
precipitation to the Midlands and Southern Upstate on Thanksgiving
Day. However, from Friday through early next week, dry high pressure
will dominate our weather.


As of 720 PM Tuesday: Upper trough across the Great Lakes
continues to gradually propagate eastward this evening as it`s
associated cold front is just making it`s way into the TN
Valley, per latest obs. High pressure across the Atlantic,
however, continues to dominate the current weather pattern. This
feature along with a developing low pressure off the FL coast,
serving as our moisture source, is creating light rain showers
across portions of the forecast area, primarily along and just
west of the I-77 corridor attm. Triggered by isentropic lift,
anticipate this activity to continue over the next few hours, as
noted in changes made to the latest PoPs. Otherwise, given
lingering cloud cover, have slowed down the initial trend of
decreasing temperatures this evening, after high temperatures
climbed into the 60`s in areas this afternoon. Have kept with
overnight lows staying above normal as cloud cover continues
ahead of the approaching cold front.

Moisture moves out by around daybreak Wednesday, with high pressure
over the Plains beginning to work this way. Gradient will increase
slightly during the day, with N winds picking up around 10mph, with
some low-end gusts (especially upstream of the valleys with the gap
winds) not out of the question. The downsloping component will allow
temperatures to increase a few more degrees tomorrow than today
across the Piedmont, though the CAA behind the upper trough will
cool the mountains down a few degrees. Overall though a very
pleasant day tomorrow.


As of 230 PM Tuesday: A split 500 mb trough will phase into a deeper
longwave over the Appalachians on Thanksgiving, allowing a cool and
dry high pressure to settle in from the NW. As the trough phases,
weak cyclogenesis is expected over the eastern Gulf. The low will
slowly cross the FL Peninsula Thursday thru Friday. The 12z GFS is
still the only model that throws a shallow layer of moisture up into
the southern edge of the CWFA. It keeps a small area of light QPF
just south of Greenwood/Abbeville counties, and even then, the
forecast soundings look too dry for measurable rain. So I will go
with a dry fcst for Thanksgiving. There may be some stratocu across
NE GA and the Upstate, at least thru the morning, but otherwise, it
should be mostly sunny. Temps will be about 10 degrees below normal
on Thanksgiving, then rebound to about 3-5 degrees below normal on


As of 100 PM Tuesday: The medium range fcst period kicks off on
Friday night amidst broad eastern CONUS troffing with a
northern stream wave digging across the upper Midwest, while
ridging prevails out west. At the surface, the wave that we have
been watching evolve in the guidance for a few days now
continues to track offshore, and therefore yielding no sig
impacts across northeast GA and the western Carolinas. Weakening
high pressure across the region will allow for a brief period
of wly/swly backed surface flow into/through Saturday before the
aforementioned northern stream wave and surface cold front move
into the region in the evening/overnight hours. There remains
some discontinuity with regard to any frontal precip as the GFS
favors enough upper support tied to the wave to produce a quick
burst of rain, possibly snow at the highest elevations, while
the ECMWF remains dry. As such, both of these guidance sources
favor no residual nwfs behind the front among the cold
advection. Therefore, will leave the fcst dry through this time
frame. Moving along, caa will prevail on Sunday as deep layer
high pressure settles across much of the southern Appalachians
leading to dry conditions and below normal temperatures into
Tuesday. At that point, the upper ridge begins to shift east
taking the surface component along with it, leading to sly
veered flow and thus waa. This is all ahead of a late period
front moving across the Mississippi River valley into Wednesday,
which shouldn`t yield any affects on the cwfa through periods


At KCLT and Elsewhere: Latest radar imagery continues to depict
light showers moving across eastern portions of the forecast area,
thus have kept with VCSH/-SHRA for KCLT over the next couple of
hours. Overall, expect VFR cigs/vsbys to prevail through this
evening as conditions will gradually deteriorate to MVFR/IFR
(varying in severity) at all TAF sites as a cold front begins to
push through the area from the west. Cigs will continue to lower and
areas of fog develop late overnight into the early morning hours,
with conditions improving to VFR after daybreak on Wednesday. Winds
will become northwesterly, increasing to 4 to 8 kts towards the end
of the period, with the exception of KHKY AOB 10 kts, with gusts up
to 18 kts possible with gap winds.

Outlook: Expect VFR through the forecast period as dry/cool
conditions prevail.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  85%     High  83%     High 100%     High 100%
KGSP       Med   78%     High  82%     High 100%     High 100%
KAVL       High  86%     High  87%     High 100%     High 100%
KHKY       Med   77%     High  85%     High 100%     High 100%
KGMU       Med   78%     High  85%     High 100%     High 100%
KAND       Med   79%     Med   73%     High  95%     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:




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