Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
300 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/...
Robust showers and embedded thunderstorms will continue to develop
northeast for the rest of the afternoon...especially within the
greater buoyancy from the Blue Ridge to the Piedmont.  Within the
high pwat airmass, the greatest threat from storms this afternoon
will be heavy or excessive rainfall where storms train. Looking
further west, upper level impulse will continue driving stout storms
east toward the mountains, and cam guidance drives a swath of this
convective rainfall into the Southern Appalachians this evening. Very
heavy rainfall remains the greatest concern, and isolated excessive
amounts are possible as well. Behind this s/wv, a distinct precip
probability minima is likely overnight, with the pre-dawn sensible
weather featuring warm and muggy conditions, along with areas of low
clouds and fog.

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 /19Z MONDAY THROUGH SATURDAY/...
At KCLT: Progressive destabilization will continue, allowing deep
convection to develop further ne across the piedmont this afternoon.
Current thinking is that storms could affect the terminal after 22z
today. Until then, periodic MVFR cigs may still occur.  Diurnally
enhanced thunderstorm activity will slowly diminish later this
evening but may not totally dissipate.  Within the very moist blyr,
flight restrictions in the form of low clouds and fog are likely to
develop during the pre-dawn hours.

Elsewhere: Anticipate mainly lower VFR cigs with showers and
thunderstorms redeveloping from the west this afternoon ahead of an
approaching shortwave. KAVL should see TSRA the earliest, by 19Z,
with activity spreading to the foothills from 20Z through 00Z. Winds
will be NE early and become southeast to southerly later in the day.
Another night of low stratus is expected tonight as the
precipitation winds down.

Outlook: Chances for restrictions will remain elevated this 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...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High 100%     High 100%     Med   61%     High  85%
KGSP       High  98%     High  99%     High  86%     High  90%
KAVL       High  99%     High  83%     Med   62%     High  81%
KHKY       High  85%     Med   63%     Med   69%     Med   72%
KGMU       High  97%     High  97%     Med   66%     High  86%
KAND       High 100%     High 100%     High 100%     High  96%

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
SHORT TERM...TS
LONG TERM...CDG/SW
AVIATION...CSH

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