Area Forecast Discussion
Issued by NWS Greer, SC

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000
FXUS62 KGSP 251054
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
654 AM EDT Sun Sep 25 2016

.SYNOPSIS...
High pressure that has dominated the Southeast will begin to break
down today as an upper trough approaches from the Mississippi Valley
and a backdoor front moves into the Carolinas from the north.
Unsettled weather with a slight cooling trend will dominate the
first half of the week, before drier and cooler air enters behind
the passing cold front by Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 645 AM EDT...The back door front has pushed thru most of the
NC foothills and piedmont, with stratus seen filling in across the
NW piedmont. Sky cover was tweaked to reflect the satellite trends.
The rest of the forecast is on track.

As of 300 AM EDT...A vigorous trough will advance across the Upper
Midwest, helping to break down the upper ridge over the eastern
states today. At the surface, a back door cold front was analyzed
just entering Davie County, NC, and should push thru most of our NC
zones by midday today. From there, the front is expected to stall,
with most of the guidance indicating it stopping before reaching
I-26, then becoming rather diffuse on the wind field. A large
stratus deck behind the front should at least push into the NW NC
Piedmont, but whether it gets any further southwest is still in
question. The edge of the cloud deck may help sharpen the temp
gradient this afternoon, and be a focus for convection. However,
forecast soundings do not show a lot of CAPE. The best instability
and trigger still looks to be in the high terrain, where scattered
showers and perhaps an isolated TSTM or two can be expected. Across
the Piedmont, mainly isolated showers expected, mainly in NC and the
eastern Upstate. Highs were bumped up per latest MOS, ranging from
the upper 70s north of I-40 to lower 90s in the Upper Savannah
Valley.

Tonight, the stalled front may continue to be a trigger for
convection, with southerly upglide flow developing over the
boundary. Also, some easterly upslope component is seen in the
lowest 2000 ft or so of the forecast soundings. Both the NAM and GFS
show expanding low cloud cover with deepening moist layer overnight.
So PoPs will actually expand across the northern 2/3RDS of the
forecast area. Lows will remain well above normal under the cloud
cover.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
As of 300 AM EDT Sunday: As we begin the short term period, the
back door/wedge front will remain stalled across the area - though
not for long - with surface high pressure ridging down from the
northeast. Strong upper trough/closed low pushing through southern
Canada toward the Great Lakes will sweep a cold front into the Ohio
Valley at the beginning of the period, with a fairly significant
pattern change in its wake for the extended.

For Monday, as the cold front approaches, surface winds will veer
around to the SE and S during the day, with low-level WAA competing
with the increased cloud cover from the incoming moisture.
Instability will be on the increase as well, with the GFS between
1000-1500 J/kg in pockets across the area, especially just to the
east of the Blue Ridge, and the NAM going gangbusters with over 3000
J/kg. Luckily we have no deep-layer shear to speak of. Some warming
at 500mb contributes to poor mid-level lapse rates even with decent
low-level ones. SPC Day2 outlook just has us in general thunder and
overall think that works well, though naturally cannot rule out an
isolated strong to possibly severe storm or two. QPF continues to
inch upwards, with now almost 3/4" basin-average for Day2 across the
mountains and foothills, and well over 1" combined Days 1-3. Not
much to write home about overall but we`ll take what we can get at
this point. As for pops, with those QPFs, see no reason to not paint
a large swath of likely across the mountains Monday afternoon and
evening, spreading into the northern Piedmont by early Tuesday. A
shortwave rounding the base of the trough early Tuesday should serve
to pull the front back to the west ever so slightly Tuesday
afternoon and evening, with a corresponding brief increase in pops
across the Upstate and toward the Charlotte area. The longwave
trough rapidly kicks this out of the way however, with all pops out
of the forecast area by the end of the period.

Should see a slow but steady cooling trend through the period, with
highs back to near normal levels for Tuesday. Big changes ushered in
Tuesday night with cooler and drier post-frontal airmass, with
overnight lows progged a good 4-8 degrees colder Tuesday night than
Monday night.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
As of 315 AM EDT Sunday: Cooler and drier is the rule for the
extended with temperatures dropping below normal after the front
pushes through and remaining on the cool side through the end of the
period, with highs actually progged a good 5-6 degrees below normal
Thursday and Friday. Continued discrepancies however could result in
a variety of impacts to the sensible weather. The ECMWF continues to
want to cut off the upper low over the Southern Appalachians and
spin it around and around, dancing across the mountains and back up
toward the Ohio Valley. With the steepening lapse rates convective
showers would likely develop over the mountains. The GFS, while
kicking the upper low out, does bring it far enough south to warrant
a slight mention of showers across the northern tier Thursday.
However, as WPC noted in the extended discussion, only about 10% of
ensemble members show an ECMWF-like solution, so for now will
continue to discount that.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
At KCLT...IFR stratus can been seen in the obs and on the IR
satellite product slowly advancing south and west across the NC
piedmont early this morning. it looks rather patchy, and progress
has slowed. Guidance is nearly unanimous in keeping the low CIGS
just north of the terminal this morning. So will start the 12z TAF
VFR. However far the clouds get, they should start retreating and
mixing out today, as a back door front will stall across the area.
This complicates the winds, which may favor ENE or ESE today, but
either way should be generally 5-7 kts. Late afternoon thru tonight,
guidance shows deepening moisture along the front and more scattered
SHRA developing across the area. So will keep a PROB30 for SHRA from
00-05z. By that time, guidance generally agrees on MVFR to IFR
stratus returning across the area.

Elsewhere...A back door cold front has pushed to the NC foothills
and extends SE to the eastern Upstate. This is about as far as
guidance takes it, except for the GFS, which pushes it a little
farther southwest, then stalls it around the KAVL-KGSP-KLUX line.
Wherever the front sets up, it should focus a few SHRA and possibly
an isolated TSRA this afternoon into the evening. The highest
coverage will be in the mountains thru the day, then favor the
foothills and piedmont tonight, as flow turns out of the SE and
moisture deepens. So PROB30 for SHRA will be timed with this
thinking in mind. In the meantime, IFR stratus has reached KHKY
behind the front, and should scatter out late this morning to VFR.
Tonight, expect these clouds to expand back to the south and west,
possibly reaching the KGMU/KGSP sites before daybreak Monday.

Outlook: Another cold front will approach the area from the west on
Monday, then slowly cross the region Monday night through Tuesday.
This should result in better chances for convection. Then dry high
pressure builds in behind the front on Wednesday.

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       High 100%     High 100%     High  95%     Med   62%
KGSP       High 100%     High 100%     High 100%     High  87%
KAVL       High 100%     High 100%     High 100%     Med   71%
KHKY       High 100%     High 100%     High  90%     Low   58%
KGMU       High 100%     High 100%     High 100%     High  87%
KAND       High 100%     High 100%     High 100%     High  94%

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...TDP
NEAR TERM...ARK
SHORT TERM...TDP
LONG TERM...TDP
AVIATION...ARK



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