Area Forecast Discussion
Issued by NWS Greer, SC

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964
FXUS62 KGSP 231838
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
238 PM EDT Sun Jul 23 2017

.SYNOPSIS...
A cold front will become stationary from west to east across our
region through the middle of the week then dissipate.  This should
result in a little cooler temperature and better chances for rain
over the next few days.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 235 PM EDT Sunday: Convection will move off the Blue Ridge and
SE across the foothills and piedmont of the Carolinas and NE GA
through early evening. Some of these storms will be severe with
damaging downbursts the main threat but isolated large hail cannot
be ruled out. The NC Southern Piedmont and eastern Upstate look to
have the best chance of seeing severe storms given the movement of
the current broken line of severe storms. That said, there is plenty
of instability and DCAPE over NE GA and the western Upstate, so
those areas are vulnerable as well.

Convection diminishes this evening as the line moves SE of the area.
However, there will be more MCS convection moving into the NC
mountains overnight. The latest CAM guidance shows this convection
diminishing as it moves into the foothills and dissipating before
reaching the piedmont. Have followed these trends in the PoP
forecast. Mountain valley fog, some dense, is expected again and
locations outside of the mountains that receive rainfall may see
some as well. Lows will again be above normal but not quite as warm
as last night.

Convection redevelops Monday as outflow boundaries from the
overnight MCS move across the area. Coverage will be better along
and south of the I-85 corridor where the timing of the boundary
movement will coincide with the higher instability. North of the
corridor instability will be much less leading to only isolated
coverage at best. DCAPE and sfc delta theta-E values while still on
the high side, will not be as high as Sunday. Instability will reach
moderate levels but just barely. Therefore, the chance of severe
storms will be less but not non-existent. Highs will be in the lower
90s outside of the mountain, and while still humid, heat index
values should remain below 100.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 200 PM Sunday: A weak and nearly stationary cold front
will remain over central GA and SC Monday night into Tuesday,
immediately south of the CWFA. Aloft, we will remain somewhat
under the influence of the northeast CONUS trough, though
without significant embedded vorticity in play early in the
period. More abundant moisture will be present over the south,
near the front, which could increase cloud cover. But otherwise it
appears conditions will be close to climatology, with seasonably
modest instability and weak shear producing a small pulse severe
threat. Partial thicknesses change little from Monday, so max
temps should be similar.

Tuesday night, high pressure will shift into New England in the
wake of the upper trough. A weak upper low is shown to cut off
and remain over the Carolina coast, and allows enough vorticity
to persist over the area that isolated t-storms still could fire
overnight across mainly the Piedmont. The eastward push of the high
will bring cooler air south into the area, which virtually all
guidance indicates will knock max temps down to a couple degrees
below normal, in conjunction with increased cloud cover. Nonetheless
consensus SBCAPE values remain fairly large given still-elevated
sfc dewpoints and good lapse rates afforded by the upper low. PoPs
peak above climo and pulse threats return once again.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 145 PM Sunday: The medium range begins Wednesday evening with
a weak closed off 500mb low over the Carolinas and Georgia. As the
next northern stream shortwave crosses the Great Lakes on Thursday,
it kicks this weak low off to the east with the 500mb trough digging
over the East through the weekend. As the Great Lakes shortwave
moves to far eastern Canada, the associated cold front moves slowly
southeast. This front should be nearing the Ohio River early Friday.
As the front approaches from the north, frontal associated
convection will increase first across areas along and north of I-40
Friday afternoon and then spread across much of the rest of the area
Friday evening. The front becomes stationary west to east across our
region through the weekend. The eventual location of the axis of the
trough next weekend varies among the models. This axis location will
have significant influence on our weather. Also, mesoscale
convective systems are expected to develop near the Rockies or
northern Plains states and track southeast in the northwest 500mb
flow toward our region of the nation. Therefore, our weather will
remain unsettled through the weekend. Temperatures will be near
normal.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
At KCLT and elsewhere: Line of convection developing across the NC
mountains and becoming strong to severe. Have gone pessimistic with
the TAFs in response and indicate VRB winds and TSRA in a TEMPO.
Storms could linger a while in the evening outside of the mountains
so have VFR VCTS in place. Guidance not very favorable for
restrictions outside of the mountain valleys overnight, so have
limited them to KAVL. That said, they will be possible as most
locations will receive rainfall. Scattered convection expected
Monday for all but KHKY. SW wind today becomes light overnight, then
W to SW for Monday.

Outlook: Increasing chances of convection expected through the
middle of next week ahead of a series of weakening cold fronts.
Overnight restrictions will continue in the mountain valleys with
chance increasing elsewhere.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  92%     High  96%
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...RWH
SHORT TERM...Wimberley
LONG TERM...DEO
AVIATION...RWH



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