Area Forecast Discussion
Issued by NWS Greer, SC

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386
FXUS62 KGSP 241731
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
131 PM EDT Sat Sep 24 2016

.SYNOPSIS...
A broad upper ridge and surface high pressure will remain
in place over the southeast through tonight then break down Sunday
as an upper trough approaches from the Mississippi Valley.
A slightly cooler and moist airmass will begin Sunday as a frontal
boundary settles into the area from the north. Unsettled weather is
likely to linger through the first half of next week before drier
and cooler air enters behind the passing cold front by the middle of
next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A quiet, but warm afternoon remains on tap for the area, as a
high-amplitude upper ridge shifts slightly east across the Ohio
Valley to the southern Appalachians. Dry air and sunny to mostly
sunny skies will allow temps to climb into the lower 90s across most
of the Piedmont, and mid to upper 80s in the mountain valleys, 10-12
deg F above climo, and only a few degrees from record highs. (See
Climate section below).

Tonight...a back door cold front will approach from the north, as a
trough digs into the northeast states. The front won`t initially
have a lot of moisture or oomph to it, but may be entering the NW NC
Piedmont by 12z Sunday with a wind shift to NELY and some
stratocumulus developing. Temps will be similar to last couple
nights, still several degrees above normal.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
As of 215 PM EDT Saturday: At the start of the period, a back
door/wedge front will have moved into the area and is progged to
stall across the region with the onset of a classical damming
scenario. With a midlevel ridge still in place from the Gulf Coast
to the Great Lakes, should not see much impact to temperatures on
Sunday with highs still several degrees above seasonal normals and
only a few degrees cooler than what we`ll see today. The front
should bring with it enough lift for isolated to scattered
convection, with the best chance of thunder over the Blue Ridge
where mechanical lift from the front is enhanced by topography.

Low level moisture will increase more substantially Sunday night
into Monday with the stalled wedge front. Meanwhile a strong closed
upper low moving across southern Canada will sweep a cold front into
the Great Lakes and down the Ohio Valley. This will push the upper
ridge over and just east of the Appalachians by early Monday which
will help to erode the preexisting front and swing winds around to
the southeast and eventually south through the day. Despite the WAA,
increasing cloud cover and better precipitation chances with the
approach of the cold front will help to bring highs down even
further, just a hair above seasonal normals for Monday afternoon.
Have increased pops slightly for Monday midday, though of the
operational guidance the GFS seems the most aggressive. WPC QPF
brings a swath of 0.25" basin-average along and just to the east of
the Blue Ridge - some much-needed rainfall but not really enough to
put much of a dent into the yearly deficit. With the WAA comes
increasing instability between 500-1000 J/kg so with increasing
convective potential think high chance to low likely pops are
warranted, and would not be surprised to see these increased in
subsequent forecasts.

The GFS is a little faster than the ECMWF with pushing the front
through, but both do show a slight diurnal increase in pops
especially across southeast zones Tuesday morning/midday, but with
pops rapidly decreasing Tuesday evening as the front pushes through.
The mountains should really feel the effects of the post-frontal CAA
for highs on Tuesday, but the real effects will start being felt
Tuesday night as the drier and cooler air advects in. Precip will
exit to the southeast overnight Tuesday night (note this is quite a
bit faster than previous forecast) with lows ranging from the upper
40s (brrrr!) in the mountains to the lower 60s (not quite as brrrr
but still a LOT cooler than we`ve seen lately) across the Piedmont.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
As of 245 AM EDT Saturday: We begin the medium range/extended
with quite the pattern change behind the passage of the short term
cold front. Definitely still some uncertainty in details but the
main shift will be to mean troughiness over the area. Of the
deterministic guidance, the GFS takes the cutoff upper low, digs it
briefly over the Central Appalachians, and then lifts it toward the
end of the week back toward New England. The new ECMWF however
continues to dig the upper low south, eventually parking itself over
the Southern Appalachians, and with the steep lapse rates aloft and
moisture wrapping around the system, wants to bring convection to
the mountains and some of the northern Piedmont counties. Kept the
trend of going with the blend which keeps us dry but with the caveat
that that could change.  The blend also has a slow but steady
cooling trend through the period, with afternoon highs just a tad
below normal on Wednesday and a category below by Thursday and
Friday (same with overnight lows) - some appropriately-fall
temperatures.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...
At KCLT, just a few cu and light ne winds until about sunset leading
to a clear and nearly calm evening.  Toward Sunday morning within
developing NE flow, there is the possibility of stratocu development,
possibly dipping into the mvfr range before scattering out midday.

Elsewhere, light winds with few-sct cu dissipating this evening.
Overnight, the best chance for dense valley fog developing will be
across the southern/sw NC mtn valleys although cig restrictions may
develop by daybreak north of I-40. MVFR/IFR cigs have the potential
to be seen at least briefly all across western NC through about
mid-morning Sunday.

Outlook: A back door cold front will push into the region on Sunday,
bringing increased moisture for early next week. Another cold front
will push in from the west on Tuesday, bringing the best chance for
showers and possibly some TSTMS. Then dry high pressure returns
midweek.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
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%     Med   66%
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...DEO
NEAR TERM...ARK/CSH
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...CSH



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