Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
752 PM EDT THU AUG 4 2016

.SYNOPSIS...
A cool and moist wedge of high pressure will linger into Friday
across our region. Another frontal boundary is expected to enter the
region over the weekend, keeping higher than normal rainfall chances
in the forecast through early next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 740 PM EDT Thursday...we appear to be in the lull the previous
shift alluded to. Shower coverage is less than the previous update,
so some additional adjustments will be made to the precip probs.
That being said...some showers continue to percolate across the
region in our weakly unstable environment. That... combined with a
light easterly upslope flow...may continue to force new shower
development that could occur at any place at any time. Still expect
the smallest chance over the nrn part of the fcst area.

Main players in our weather during the near term will be the
continued moist easterly upslope flow around the southern periphery
of the high off the Mid-Atlantic/New England coast, as well as an
upper disturbance currently over the Ohio Valley. The surface high
is creating a CAD-like environment over our area with widespread low
ceilings, drizzle/light rain, and below-normal temperatures. The
Ohio Valley disturbance can be seen spinning on both water vapor and
IR imagery and is kicking off convection west of the Appalachians.
As this disturbance pushes east, the upper support will combine with
the low-level upslope flow to create an environment favorable for
heavy rain production.

Convection should really blossom across the area late this evening
into the overnight hours as southeasterly 850mb flow and low level
convergence maximize just east of the Blue Ridge. This is reflected
pretty well in updated WPC QPF values. Basin-average QPF through the
near-term has a large swath of 1" and greater across our NC
counties, and of course these are large river basins so we can
expect to see many areas between 1-2", with locally upwards of 5"
storm total. Instability today with the low-level inversion in place
is definitely lacking but we should see some clearing tomorrow which
may increase convective chances. sbCAPE progs for tomorrow range
between 1500-2500 J/kg but with slightly lower PW values and not
quite a saturated-adiabatic profile. Add to this the low-level winds
will swing around to to the S and SW during the day on Friday as the
upper disturbance pushes on through, so we lose our upslope flow
(for now).

So with all this, considering the Flash Flood Watch, it looks like
we might not really need it for the eastern and southern zones where
QPFs won't be quite as high. The eastern most tier of counties in NC
actually have very little QPF, without the terrain influence of
counties farther west. The areas in D2 drought across SW NC and
Upstate SC, despite some locations receiving several inches of rain
last night and early this morning, seem to have handled the
precipitation just fine. If anything, might consider Davie to Char-
Meck and on around to Elbert GA. However, am strongly disinclined to
cancel since it is already out, at least until we see how convective
trends evolve through the evening. The bigger question might be
whether to leave the expiration at 12z Friday or take it through
00z, but since we lose the upslope easterly flow and the disturbance
pushes through, for now will leave the ending time alone. The
evening shift tonight may have a better handle on trends by the late
evening update and if it looks like we'll get more rain than
currently anticipated, it can be adjusted from there.

Temperatures today have been held well below climatological normals
(8 to 10 degrees) and while tomorrow should warm up just a tad,
widespread cloud cover will still keep temperatures below normal,
maybe more like 4-7 degrees.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 200 PM EDT Thursday...The short term forecast encompasses the
upcoming weekend. An upper trough will dig across the Great Lakes
Friday night thru Sunday. This will allow a cold front to push into
the area from the northwest Saturday evening. As the front
approaches, the flow should continue to veer from southwesterly to
northwesterly. Any lingering wedge should be gone and easterly
upslope flow should be over. With that said, the models are in
decent agreement on convection on lingering convection Friday
evening. So PoPs will remain elevated. But by daybreak Saturday,
precip should wane. Temps will remain above normal for overnight
lows. With a downslope flow and more sunshine, Saturday afternoon
should see a rebound in temps back into the lower to mid 90s east of
the mountains. It looks like convection will get a later start, due
to the timing of the front. Will feature PoPs ramping up from NW to
SE, but only remaining slight chance across the Upstate and NE GA.

Saturday night thru Sunday...the front is expected to stall out
across the Gulf states to central SC, while a 1022 mb sfc high
settles in across the Great Lakes. The guidance is in good agreement
on a hybrid CAD event setting up across the area starting Sunday
evening. So Sunday's forecast is a little more uncertain. Will trim
back max temps slightly, with increased clouds. PoPs will be in the
high-end chc to likely range, highest in the southern tier.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 215 PM EDT Thursday...The medium range forecast starts at 00z
Monday. The first few days of next week look unsettled, with a
return of heavy rain/flooding threat. A summer-time cold air damming
event will persist from Sunday night thru at least Tuesday. The
parent high will slowly drift east from the Great Lakes to southeast
of Cape Cod. Meanwhile, a persistent weak wave will develop along a
stalled front near the FL panhandle. The 12z GFS and ECMWF agree on
this feature, enhancing a low-level SELY flow atop the wedge and
southern Appalachians. The GFS shows PWATS in the 2 to 2.25" range
within this forcing. In addition, a broad subtropical low currently
over the Bahamas will eventually merge with the stalled front,
possibly enhancing heavy rain potential further. Altho, the ECMWF
keeps the low east of FL, the GFS brings it into the NE Gulf and
into the Southeast states Wednesday. So with all that said, this
forecast will feature above climo PoPs each day and below normal max
temps with above normal mins.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
At KCLT: Through the late evening hours...expect scattered clouds
with bases in the 035-045 range. Radar shows widely scattered
showers developing across the Piedmont, and one of them could form
or move over the airfield at any time, and if that happens an MVFR
restriction would accompany the shower. Think lightning is unlikely
with the shallow depth of the convection. Will handle this with a
TEMPO group thru 04Z. A light NE wind should come around to light
ESE in the early morning hours. Expect ceiling to drop down into IFR
category after midnight...similar to last night. Went with 09Z for
this to start, but will not be surprised to see it form as early as
06Z. Avoided including any shower activity through Friday morning
altho there could be some around. Once again, the ceiling will
persist into the mid-morning, and will lift around 13Z as the
boundary layer warms. Expect ceiling to lift to VFR in early
afternoon. Thunderstorms are actually not a bad bet for the
afternoon hours as an upper level disturbance enhances the
convective environment. For now, though, the PROB30 covers more
ground, so it was preferred for this issuance.

Elsewhere: Overall similar conditions to KCLT with plentiful
moisture and easterly upslope flow. The best chance for precip in
the next few hours will be at upstate SC TAF sites. Ceilings should
crash again in the early morning hours widespread IFR/LIFR in most
places. The low clouds will be less likely to hold on in late
morning as the arrival of the upper level disturbance helps to wipe
out the residual cool pool E of the Blue Ridge.

Outlook: Restrictions will be possible with any evening convection
on Friday. Restrictions can also be expected each morning in the
mountain valleys where fog and low stratus are likely, but will be
possible outside of the mountains as well. Some drying develops
Saturday as flow becomes southwesterly. A frontal boundary drops
into the area Sunday and lingers into Monday.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  97%     Med   76%     High  85%     High  98%
KGSP       High  85%     Med   70%     Med   78%     High 100%
KAVL       High  82%     High  88%     High  82%     High  98%
KHKY       High  86%     Med   73%     High  93%     High  96%
KGMU       High  93%     Med   73%     High  84%     High 100%
KAND       High 100%     Med   78%     High  92%     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...Flash Flood Watch through Friday morning for GAZ010-017-018-
     026-028-029.
NC...Flash Flood Watch through Friday morning for NCZ033-035>037-
     049-050-053-056-057-062>065-068>072-082-501>510.
SC...Flash Flood Watch through Friday morning for SCZ001>014-019.

&&

SYNOPSIS...DEO
NEAR TERM...PM/TDP
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...PM

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SPECIAL WEATHER STATEMENT
NATIONAL WEATHER SERVICE GREENVILLE-SPARTANBURG SC
752 PM EDT THU AUG 4 2016

NCZ051-050030-
SWAIN NC-
752 PM EDT THU AUG 4 2016

...A THUNDERSTORM WITH TORRENTIAL RAINFALL WILL IMPACT CENTRAL SWAIN
COUNTY UNTIL 830 PM EDT...

AT 752 PM EDT...DOPPLER RADAR WAS TRACKING A THUNDERSTORM WITH
TORRENTIAL RAINFALL 8 MILES WEST OF BRYSON CITY...OR 5 MILES
SOUTHEAST OF SMOKY MOUNTAINS-FONTANA AREA...MOVING EAST AT 5 MPH.

LOCATIONS TO BE IMPACTED INCLUDE... BRYSON CITY...SMOKY MOUNTAINS-
CLINGMANS DOME...SMOKY MOUNTAINS-FONTANA AREA...SMOKY MOUNTAINS-DEEP
CREEK AND FONTANA LAKE.

PEA SIZE HAIL AND WIND GUSTS UP TO 40 MPH WILL BE POSSIBLE IN THESE
AREAS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

THE COMBINATION OF GUSTY WINDS AND SATURATED SOIL CONDITIONS WILL
CAUSE SCATTERED TREES AND POWER LINES TO FALL. SEEK SHELTER INSIDE AN
INTERIOR ROOM.

TORRENTIAL RAINFALL MAY FLOOD AREAS WITH POOR DRAINAGE...SUCH AS
DITCHES AND UNDERPASSES. AVOID THESE AREAS AND DO NOT CROSS FLOODED
ROADS. WATER LEVELS OF SMALL STREAMS MAY ALSO RISE RAPIDLY. SEEK
HIGHER GROUND IF THREATENED BY FLOOD WATERS. DO NOT DRIVE ACROSS
FLOODED ROADS.

&&

LAT...LON 3557 8354 3556 8350 3558 8347 3538 8343
      3539 8359 3541 8358 3543 8359 3543 8361
      3544 8364 3543 8365 3544 8365 3554 8368
TIME...MOT...LOC 2352Z 265DEG 5KT 3546 8359

08