Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 021957
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
257 PM EST Fri Dec 2 2016

.SYNOPSIS...
Dry and cool conditions will persist through Saturday. Increased
moisture will return from the west on Sunday and linger into early
next week. Another cold front will approach the area later next week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 110 PM EST Friday:  H5 zonal flow atop surface high
pressure highlights the synoptic pattern across the southeast this
afternoon, and through the near term period.  Meanwhile, a closed
upper disturbance will deepen across the Desert Southwest before
diving southward over the Gulf of California into NW Mexico by
periods end.  With that, Gulf moisture looks to advect northward
across the southern states leading to increasing sky coverage
through the period.

Conditions this afternoon remain rather pleasant amidst sunny skies,
moderate wly/swly surface winds, and near/at normal temperatures.
Visible sat imagery does indicate upstream cirrus as stated above,
with e/ne advection of such expected tonight into/through Saturday.
In addition, moist upslope flow along the western slopes near the TN
line will initiate stratus development overnight, possibly spreading
as far south as the French Broad and Little TN River valleys.
Fortunately this mtn/piedmont increased sky cover will limit diurnal
cooling slightly, therefore yielding min temps tonight a degree or
two warmer than last night.  Likewise, this will also 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 /SATURDAY NIGHT 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 /20Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: VFR through the period as high pressure
continues to dominate atop the region while a rather potent upper
disturbance deepens over the southwest.  Skies will remain mostly
clear, however given wly flow aloft, am expecting moisture to
stream into the region as high cirrus tonight.  This moisture is
consequent of the afore mentioned upper low which will promote
increasing Gulf moisture advection through the later half of
the period.  As a result, said cirrus will lower gradually from
high based to mid based and thicken.  Otherwise, moist upsloping
along the TN line tonight is expected to produce low stratus,
likely to advect into the French Broad Valley to affect KAVL.
This stratus will be in the mvfr range, but should remain sct
at worst.  Winds will remain lgt/mod out of the wsw given lee
troffing this afternoon at all sites aside for KAVL where nw flow
will persist.  Expect weaker/veered nwly flow tonight at all sites,
while KAVL could see persistent gusting into early/mid morning per
latest NAM soundings.  Lastly, winds will remain light vrb/nly on
Saturday as the surface ridge sets up overhead.

Outlook: Look for dry high pressure to prevail into Sunday before
moisture spreads into the region from the southwest leading to
increasing chances for precip/restrictions to start the work week.
Chances for precip/restrictions will increase further into midweek
as a potent and moist surface wave moves through the region.
Conditions will dry out behind a passing cold front into midweek,
with temperatures becoming much cooler/drier to end the work week.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     Low   55%     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...CDG
SHORT TERM...RWH
LONG TERM...WJM
AVIATION...CDG


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