Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 172336
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
636 PM EST Sun Dec 17 2017

.SYNOPSIS...
Weak high pressure will linger over the southeast tonight
through Monday. Low pressure and abundant moisture will return to
the region from the west Tuesday night through Wednesday, with dry
high pressure following Thursday through Friday. A more vigorous
cold front will arrive from the west next weekend and usher in a
period of unsettled weather.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 635 PM EST Sunday: Latest radar imagery shows showers moving
east of the area. Cannot rule out some additional sprinkles or even
light drizzle this evening before an upper level disturbance moves
east of the area. Should see patchy areas of fog overnight, mainly
over the mountains and foothills with widespread cloud cover most
locations. continues. Given cloud cover, expect overnight low
temperatures to be slightly warmer than we`ve seen over the past
couple of nights, with lower 40`s across the Upstate down into the
mid to upper 30`s across the mountains and foothills.

On Monday, expect lingering clouds throughout the day as residual
moisture spreads across the area, though some guidance suggests the
mid levels levels may dry out much sooner. With a subtropical ridge
building into the area, expect overall quiet conditions with a
warmer day in store as temperatures climb into the upper 50`s/lower
60`s.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
As of 2pm EST Sunday:

Tuesday morning begins mild with generally zonal flow aloft with dry
northwesterly 850mb flow.  However, clouds aloft will be increasing
as a well-defined shortwave exits from northern Mexico and crosses
south Texas on Tuesday, picking-up Gulf moisture as it does so.
This wave will bring potentially significant rainfall to the area
beginning Wednesday morning, and lasting into Wednesday evening.
Timing is fairly consistent in the EC and GFS models with the
heaviest rain Wednesday afternoon impacting all parts of the CWA,
and I`m inclined to go with fairly high POPs on Wednesday.  The
mountains will receive the most rainfall with 1 to 3 inches with
amounts tapering southwest with an inch or so in the piedmont.  Late
in the event Thursday morning, when precipitation amounts have
become light and scattered behind the cold front, temperatures will
cool enough at higher elevations to produce some scattered light
snow.

System produces a well-defined closed low/triple point over western
NC Wednesday morning with a cold front trailing southwest, and a
warm front extending eastward.  Areas along and north of the warm
front will be associated with the most rain, and this area will
include much of the CWA.

Warm advection pattern south of the warm front on Tuesday will lead
to warming in addition to moistening with highs perhaps 5 or fewer
degrees shy of seasonal records.

Dewpoints increase to the mid-50s by Wednesday, and GFS actually
predicts a smidgen of CAPE Wednesday afternoon.  Have decided to
keep any mention of thunder out of the forecast at this time as
BUFKIT soundings show any possible convection to be very shallow,
and area with potentially useful CAPE only impacts a handful of
counties in northeast GA and the Upstate.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 2:45 PM EST Sunday: At the start of this period the models are
similar in that the lead wave of energy will be east of our forecast
area (FA) Thursday morning. This will allow a flat ridge aloft to
shift across the area, while surface high pressure takes up a
damming configuration from Quebec Canada south into the Carolina`s
and northeast Georgia. The model soundings show some shallow
moisture in parts of our south FA, as a result spotty light rain or
drizzle will be possible.

The damming configuration should last Thursday night into part of
Friday, and then break down. This is a result of the next strong
spoke of energy lifting from the southwest CONUS to the western
Great Lakes Friday. The flow at most levels will be in the process
of backing to the southwest (slowest at the surface) during Friday.
Moisture will become deeper through the column later Friday and
Friday night, in the southwest flow. Therefore we should note a
subtle expansion of shower activity, but still well in the chance
range.

At the start of the weekend, and beyond, is where we have noted the
greatest divergence (or changes) in how the models are handling the
next front and associated precipitation. Today is no exception as
the new European Model has shifted slower, even slower than the GFS
and Canadian. However, to some degree they appear to be following
sensible weather. A broad high pressure system in the western
Atlantic to the Gulf of Mexico, combined with a southwest flow
aloft, should signal a slower moving frontal pattern. The first wind
shift from the lead energy source will allow some type of wind shift
to approach the FA Saturday or Sunday. The European is now super
slow with its front tucked well west right through the weekend.

We will allow a plume of moisture, to overspread our area, in the
deepening southwest flow aloft and ahead of the initial wind shift
change. A blended approach yields some high chance and low likely
POPS Saturday into Sunday. It would seem the colder air would only
ooze into our area (if it even makes it this far southeast) by the
end of this forecast cycle. Considerable implications where
everything lines up, since we are going slower, PW`s are very high
for this time of year, thus how much rainfall.

We suspect there will be more adjustments needed this far out, as
the models line up with consistency from run to run.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
At KCLT and Elsewhere: Could be some sprinkles overnight but any
precip will be very light. MVFR cigs spreading in from the west this
evening will develop at all sites but KHKY. May be some early, but
fog restrictions will be more likely there overnight. MVFR fog
likely all areas with IFR possible at KAVL and KHKY. KCLT and KHKY
will see the earliest improvement Monday morning as downslope flow
and drier air move in. KAND will see the longest restrictions where
dry air is latest to move in. Light SW winds this evening become
light overnight then pick back up on Monday. KAVL the exception
where NNW winds expected through the period. LLWS is possible there
early this evening until low level winds turn more NW.

Outlook: Could be a good setup for fog Monday night and Tuesday
morning. Potential for restrictions Tuesday into Wednesday as a
moist frontal system moves thru the area. More restrictions possible
over the weekend as another moist cold crosses from the west.

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:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...HG
NEAR TERM...RWH/SGL
SHORT TERM...WJM
LONG TERM...TS
AVIATION...RWH


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