Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 150247
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1047 PM EDT Mon Aug 14 2017

.SYNOPSIS...
A stationary front is expected to remain across the region through
Tuesday while a second front approaches from the west.  This next
front will move into the region Tuesday night into Wednesday. Drier
high pressure is expected for the end of the week leading to hotter
conditions.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 1030 pm: Two main regions of convection persist across the
forecast area this evening, one along the upper Savannah River
Valley, and the other across the western NC Piedmont (with plenty of
scattered/numerous lighter showers noted across the NC mtns and
foothills). Despite the start of the diurnal cooling cycle, due to
oppressive mid-70s dewpoints, CAPE has been slower to decrease as
usual, and it is this lingering moderate CAPE, along with PWATs
around 2 inches (about as high as they get in this area), along with
some weak upper support that is allowing this activity to continue
holding its own. We haven`t really seen anything approaching severe
storm criteria thus far, and with the slowly waning instability and
moist profiles, this should continue to be the case. However, cell
clusters will have to be watched closely for backbuilding and
training and attendant flash flood threat, especially in urban areas,
but also in locations characterized by poor antecedent conditions
/low FFGs. While the Piedmont convection will be out of the forecast
area by midnight, the remaining activity should gradually trend down
through the early morning hours. Plenty of low stratus, along with
areas of fog should develop during the pre-dawn hours, with low
temps averaging above climo in the juicy air mass.

Upper heights rise a tad atop the SE conus on Tuesday but the flow
remains quasi-zonal if not a little weaker. Subtle impulses within
the mean flow are likely to translate eastward atop the region
enhancing diurnally driven thunderstorms. Just like today, pwat
values will remain above normal, giving rise to another day of a
locally heavy possibility. Max temperatures are slated to be a few
degrees warming than today, and with sfc dwpts remaining in the
lower 70s, heat index values will push the mid 90s.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 230 PM EDT Monday: The upper level flow remains rather zonal
through this part of the forecast cycle, with embedded pockets of
vorticity riding along the mean flow. Meanwhile, the evolution of
the Bermuda High will try to nudge H5 heights slightly northward.

Complicating matters is an old surface frontal boundary which the
models continue show meandered, or waffling, north and south in
parts of our forecast area (FA).

It is nearly impossible to try and time each little wave rippling
through the flow, which would aid any diurnal convection. PWATS
will continue to remain elevated 1.5 to 2.0 inches, or more, with
SBCAPES respectable.

Therefore our plan is to let one wave of energy pass by Tuesday
evening, and bring POPS down through the night. We will then restart
the diurnal cycle Wednesday in the mountains, and let cold pool
interactions drive activity south and east during the afternoon and
evening. Note, we gave a slight nod to the NAM along our I77
corridor where convergence was noted at the surface, and
therefore we started POPS a little earlier.

Once again we will then draw back POPS Wednesday night, and then
reintroduce Thursday, mostly in the chance category.

Of course in both aforementioned scenarios, a subtle movement of the
surface boundary would locally enhance convective action

Note our temperature scheme has been adjusted for a diurnal cycle,
and guide continues to nudge temperatures up. In fact, if little
mixing occurs Wednesday and Thursday afternoon ( prior to storms )
some places in NE Georgia, upstate South Carolina and western
Piedmont of North Carolina will have heat indices near or just above
100 degrees.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 215 PM EDT Monday: Mid-level ridging will be in place across
the southeastern states Thursday and Friday. This will allow highs
to be in the lower 90s on Thursday and Friday across the Piedmont.
With dewpoints in the lower 70s, heat index values will climb to
around 100 both days outside of the mountains. Scattered showers and
thunderstorms are expected with the highest pops over the Blue Ridge
mountains.

A trof in the mid levels will move across the Ohio Valley on
Saturday and northeastern states on Sunday. While pops will not rise
much, the lower heights will lead to temperatures closer to normal
and a few degrees cooler than Thursday-Friday.

Another mid level trof will move into the Ohio Valley next Monday
which will keep temperatures across the area less hot than late this
week. There will continue to be scattered shower and storm activity.

Concerning conditions surrounding the eclipse on Monday, August 21:
Expect partly cloudy conditions during this period. There will
likely be some scattered showers and storms around during the
afternoon. Temperatures look to be in the mid to upper 80s outside
the higher elevations with a few degrees cooldown as the eclipse
occurs.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
At KCLT: Blob of convection moving across Upstate SC will continue
to progress steadily northeast toward the airfield, and tempos for
TSRA are included from 00-03Z. There is a gradual overall weakening
trend, so do not know in quite what state the convection will be in
when it reached the terminal, but restrictions could dip below the
MVFR currently advertised in the tempo. Otherwise, convective
coverage will diminish into the early part of the overnight. After
that, the attention shifts to the potential for redevelopment of fog
and/or low stratus toward daybreak. Timing and magnitude of
restrictions may depend largely upon how much rain ends up falling
at the terminal, but have IFR cigs/MVFR visby developing at 09Z for
now. More convection is expected Tue afternoon, warranting
PROB30s.

Elsewhere: The most likely terminal to see convection this evening
is at KAVL, where a tempo for SHRA is included from 00-02Z. Cannot
rule out activity elsewhere, especially toward late evening at KAND,
but confidence is quite low, so only VCSH appears warranted at the
other terminals this evening. High dewpoints/low dewpoint
depressions is expected to result in another round of low cigs and
perhaps visby restrictions later tonight. In fact, we could see IFR
conditions develop prior to 06Z, although confidence is a bit low
attm. Therefore, most terminals receive a tempo for SCT IFR clouds
and MVFR restrictions after 04Z. Conditions should settle into
IFR/LIFR at most terminals by around 08Z, with KAND being a possible
exception. More convection is expected Tue afternoon, warranting
PROB30s.

Outlook: Chances for restrictions will remain elevated through  late
week as a moist pattern persists. Therefore rounds of afternoon/
evening shra and tsra are favored each day, with morning visb/cig
restrictions possible as well for locales that experience recent
rainfall, as well as in the mtn valleys to affect KAVL.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High  98%     Med   62%     High  86%     High 100%
KGSP       High  85%     Low   46%     High  86%     High 100%
KAVL       Med   78%     Med   78%     High  87%     High 100%
KHKY       Med   79%     Med   71%     Med   73%     High 100%
KGMU       High  83%     Low   58%     High  97%     High 100%
KAND       High  90%     Med   64%     High  93%     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...SW
NEAR TERM...CSH/JDL
SHORT TERM...TS
LONG TERM...CDG/SW
AVIATION...JDL



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