Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 151949
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
249 PM EST Fri Dec 15 2017

.SYNOPSIS...
Dry and cool high pressure will dominate our weather through Sunday
morning. Moisture and a gradual warm-up will return to the area
starting Sunday. A series of disturbances will pass across the area
next week, with a prolonged period of unsettled weather.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 215 PM Friday: With high clouds, mainly opaque in nature,
streaming across the Southeast today, temperatures have peaked just
below normal across the Upstate and Piedmont this afternoon in the
upper 40`s/around 50 degrees, while temperatures remained cold
across the mountains in the 30`s.

As a weak sfc boundary across the southeastern Carolinas continues
to progress eastward offshore, flow will become more zonal and sfc
high pressure will gradually build northeastward into the Southeast
beginning tonight, where it`ll become the dominating weather pattern
on Saturday. As a result, expect high clouds to decrease in both
coverage and opacity tonight, allowing for ideal radiational
cooling. This along with CAA will allow for temperatures to plummet
overnight, with temperatures below freezing expected across the
entire forecast area. However, if high clouds linger for longer than
anticipated, going temperature forecast could be a bit too low. Will
also note, as high pressure builds in tonight, a short-lived
tightened pressure gradient across the NC mountains could bring
gusty winds to the higher elevations, subsiding before midnight.

As surface high pressure continues to build into the Southeast on
Saturday, conditions will remain dry and cool. With nearly clear
skies throughout the day, anticipate a gradual increase in high
clouds late in the afternoon hours as available moisture increases
ahead of the next approaching system. High temperatures on Saturday
will climb into the low to mid 50`s across the Upstate and Piedmont,
while remaining a bit cooler into the 40`s across the mountains.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 215 PM EST Friday: A shallow ridge will build over the
southeast as a phasing shortwave lifts through the southern plains.
Meanwhile, surface high pressure over the region will gradually
migrate offshore the latter half of the weekend. The deamplifying
upstream wave will generally lift north of the OH valley Sunday and
then off the mid-Atlantic coast Sunday night - leaving the southeast
upper ridge intact. The deeper moisture plume ahead of the phasing
shortwave should impact mainly TN to the west Saturday night through
Sunday morning before the moisture lays over across our region later
Sunday into Sunday night, with brief mid/upper drying likely early
Monday Morning. This first round should yield mostly light QPF
across our region and thermal profiles should remain warm enough
through the period for any precipitation to fall as liquid.

The upper flow will then become more southwesterly through Monday as
heights fall again over west Texas. Deeper moisture will return to
the area from the southwest and some better rates and marginal
instability could return to the lower piedmont. Anticipate the
heaviest rainfall with any passing surface waves on Monday to slip
by mainly just southeast of our forecast area. Monday mins/maxes
will run several categories above climo.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 245 PM EST Friday: Forecast confidence has taken a tumble as
the 12Z ECMWF and GFS have come in with significantly different
scenarios, the ECMWF has the larger change from its earlier run, for
the middle of next week. For what it`s worth, the Canadian is in
between the two while the GEFS mean is closer to the operational GFS
but does have a hint of a slower version of the Canadian.

The guidance agrees that the period starts with a split flow across
the CONUS. The GFS keeps the systems more progressive moving the
northern and southern stream troughs into the area by Tuesday night
and off shore Wednesday. The ECMWF keeps the streams separate moving
the northern stream trough east of the area by Tuesday night. The
southern stream wave is much stronger and slower as a closed low
which moves into the lower MS valley Tuesday night, into the TN
valley Wednesday, then across the Appalachians Thursday. Of course,
this creates quite a difference in the surface pattern as well. A
wave forms along the stalled frontal system to the south and spreads
moisture and precip into the area as it moves east. As expected, the
ECMWF is quite a bit slower than the GFS. The GFS then moves a
northern stream cold front through the area Tuesday night with dry
high pressure for Wednesday and Thursday. The ECMWF is slower with a
stronger southern stream low pressure system. This northern stream
cold front moves into the area Wednesday as moisture increases ahead
of the low. The low then slowly crosses the area Wednesday night
with decreasing precip chances on Thursday. QPF is quite a bit
higher on the ECMWF than the GFS. Temps are warm, so all of the
precip would be liquid in both scenarios. Given the wide difference
in solutions, have gone with a guidance blend which has chance PoP
for Monday night and Tuesday with diminishing PoP for Tuesday night
and a dry Wednesday. Temps ramp up from above normal Monday to well
above normal for Tuesday then slowly down to around 5 degrees above
normal for Thursday.

Guidance does agree on a frontal system moving into the area by late
Friday with some timing differences. Have PoP increasing back to the
chance range during the day. Temps will be above normal.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: Expect VFR through the valid TAF period. A
weak, dry surface boundary just south of the forecast area will
continue to progress eastward as high pressure builds in from the
southwest. Expect FEW/SCT/BKN cirrus to persist through this
afternoon, decreasing in opacity tonight. With the exception of the
potential for gusty northwesterly winds at KAVL through early this
evening, expect northwest winds 5 to 10 kts at the TAF sites through
tonight, becoming light and variable/nearly calm overnight as the
pressure gradient slackens. On Saturday, VFR will continue with
nearly clear skies and west-northwest winds becoming west-southwest,
around 5 kts.

Outlook: Potential for short-lived restrictions on Sunday with rain
showers. Otherwise, expect VFR as a series of dry cold fronts move
through the area.

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...SW
NEAR TERM...SGL
SHORT TERM...HG
LONG TERM...RWH
AVIATION...SGL



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