Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 030540
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1240 AM EST Sat Dec 3 2016

.SYNOPSIS...
Increased moisture will return from the west on Sunday and linger
into Tuesday. A strong cold front will arrive early Thursday
bringing some of the coldest air of the season so far at the
end of the week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1230 AM EST...Cirrus continues to stream in from the storm
system developing over northern Mexico and Texas. This cloud cover
may be having a slight impact on temps, as they are running a little
warmer than forecast trends. But otherwise, the forecast is on track.

As of 945 PM EST: IR imagery continues to show mainly thin cirrus
moving into the area from the southwest late this evening. The high
clouds should slowly thicken and lower through the near-term period.
Anticipate the sky cover overnight remaining just thin enough to
permit minimum temperatures to fall into the 20s in the high
mountain valleys and 30s east of the mountains. However, some
upslope stratus will be possible along the immediate TN border and
possibly pushing into the French Broad and Little TN river valleys.

Otherwise, zonal 500 mb flow atop surface high pressure will persist
while a closed upper disturbance deepens across the Desert
Southwest. The upper system will move southward over the Gulf of
California into NW Mexico by late Saturday. Gulf moisture ahead of
this system will advect northward across the southern states to
increase sky coverage through the period. Increasing clouds will
work to keep max temps cooler on Saturday, generally 1-2 degrees
below normal. Winds will weaken tonight as the aforementioned
surface ridge moves east, setting up overhead atop the Appalachians,
or perhaps further east over the Carolinas on Saturday leading to
light NLY/VRB winds.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
As of 255 PM EST Friday: An upper ridge axis slides over the area
Sunday as a short wave moves across the Great Lakes. Short wave
ridging returns for Monday as an upper low moves into TX and weak
short waves move through the flow and across our area. At the
surface, the center of high pressure builds into the Carolinas on
Sunday in a cold air damming pattern. Low level moisture an
isentropic lift move into the mountains Saturday night and into the
rest of the area Sunday ahead of a weak frontal feature. Forecast
soundings and surface temps show the potential for some snow to
develop or mix in with the rain across the higher elevations
Saturday night and early Sunday morning. However, the SREF and GEFS
ensemble means show only low chances for snow, even at Boone. Will
keep the mix wording in the forecast, but do not expect anything
more than a dusting at the highest elevations given the very light
QPF. Precip chances increase across NE GA and the Upstate through
the day Sunday as low level moisture and isentropic lift move into
those areas as well. Lows Saturday night will be nearly 5 degrees
above normal, while highs Sunday will be nearly 10 degrees below
normal due to clouds and precip.

Low level moisture and isentropic lift remain over the area and
spread into the Piedmont Sunday night as the weak frontal feature
slides across the area. Temps, especially low level temps with an
increasing low level jet, will be warm enough for all rain. The best
forcing slides east of the area Monday morning bringing a brief
break to the precip chances, especially across NC. Precip begins to
return from SW to NE during the late afternoon ahead of the next
weather system moving toward the area. Lows will be a few degrees
warmer Saturday night and around 5 degrees warmer Sunday with less
precip. QPF for the 2 days will range from around 0.15 inches along
the I-40 corridor to around an inch across the Upper Savannah River
Valley.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 2pm EST Friday:  Main issue for the extended forecast for
Monday night onward is the chances for and quantity of precipitation
Monday through Wednesday, plus the chances for very cold air at the
end of the forecast.  Current expectations are for a break in rain Monday
night prior to the next significant wave with widespread significant
rain Tuesday morning and all day on Tuesday, trailing off Tuesday
evening.  The 2 main forecast models, GFS and EC, generally agree on
the location and timing of significant precipitation, though the EC
model is more aggressive in Precip quantity.  GFS has been more
accurate with recent systems, and tend to favor it for QPF.  Current
expectations are (to within a factor of 2) for 1 to 1.5 inches of
rainfall for the Monday night through Wednesday period, with more
rain over Piedmont areas, which is in addition to the .5 to 1 inches
expected in the short term forecast Sunday and Monday.  Minimum
temperatures Tuesday and Wednesday are still in the upper 30s at
elevation, so we are still in an all rain regime.

Surface trough/front associated with the Tuesday system extends down
into the Gulf of Mexico and enables the flux of decent moisture into
the area with 850mb winds of 30kts and dewpoints rising from the
current upper 20s into the upper 40s by Monday evening.
Surface-based convective instability, though, is marginal at best
with the most unstable air Tuesday afternoon.  Optimistic
assumptions are necessary to get positive CAPE, given expected state
of moisture return.  However, deep layer shear is again very
impressive with 80kts of bulk shear on Tuesday as very strong speed
max in main shortwave rotates through.  Strong 850mb winds on
Tuesday also give the strongest surface winds in the extended
forecast on Tuesday of 15 kts, with higher amounts at higher
elevations.

Following the exit of potent shortwave/front/precip Tuesday night,
the chances for precip. decline as surface pressure rises.  A high
off New England may generate some CAD flow Wednesday, followed by a
major cold frontal passage Wednesday night.  Cold frontal passage
late on Wednesday is potentially impressive with cold northerly flow
behind it and temperatures by Friday 10 degrees below normal. Models
have been consistent in showing this major cold air outbreak, which
is a longwave and relatively predictable feature.  The end of the
forecast period on Friday is expected to be the coldest day of the
season so far with lows down to the low 20s in Piedmont areas and
down into the teens in the mountains.  However, while the EC model also
has this cold air outbreak that will likely affect much of the
eastern CONUS with 850mb temps down to -12C at GSP, it delays the
front by a day and a half relative to the GFS, and the current
forecast Friday morning is a blend of the colder GFS with the more
moderate EC.  The EC also gives some hope for light precipitation
with and behind the major front on Thursday, with some light snow
possible, especially at the highest elevations.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: A moist low pressure system will slowly eject
out northern Mexico and TX today thru tonight, which will result in
a gradual increase in moisture atop the region from the top down.
Cirrus will gradually thicken and lower into mid cloud deck late
this afternoon thru this evening. Otherwise, winds will be fairly
light, as surface high pressure lingers over the area. Light winds
will favor a northwest to northeast direction, then turn to
southeast at the Upstate TAF sites and ENE at KCLT. As CIGS lower
this evening, light rain may begin to develop in the west, so PROB30
will be added to KAVL and KAND.

Outlook: Moisture will gradually spread into the region Saturday
night through Sunday leading to increasing chances for precipitation
and restrictions. A brief lull is possible Monday with the best
chance for heavier rain and widespread restrictions on Tuesday.
Brief dry weather on Wednesday, with another front approaching from
the west.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     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%     High 100%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...LEV
NEAR TERM...ARK/CDG/HG
SHORT TERM...RWH
LONG TERM...WJM
AVIATION...ARK


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