Severe Thunderstorm Warning 235, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1032 PM EDT Fri Jul 20 2018

.SYNOPSIS...
High pressure will weaken over the forecast area before a large
upper low drops south from the Great Lakes to the Southern
Appalachians tonight. This low will remain nearly stationary for
several days and support unsettled weather through the middle of
next week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 1015 PM EDT Friday: Still seeing MCS convection march across E
TN, and as of this update MRX has issued SVRs right up to the state
line. Have increased pops just along the spine of the Appalachians a
little over the next couple of hours, but based on CSTAR research on
MCSs crossing the southern Appalachians, the thermodynamic
environment across the lee of the mountains really does not support
the MCS sustaining itself much farther east. Would expect that we
might have to issue a few SPSs and not totally out of the question
to see a SVR or two, but should not spread too far east. This is
shown pretty well in CAM reflectivity guidance. Of increasing
concern, however, is a secondary wave of convection just starting to
get going across Middle TN, and the HRRR and other CAMs DO have this
round of convection pushing east overnight and into the southern
mountains and north GA, upper Savannah Valley, around sunrise. We've
been watching these CAM trends for a few hours now and have seen a
consistent uptick in this, especially with some of the 00z HiRes
windows in now. Have increased pops accordingly. Other grids look to
be on track for now.

Otherwise, the potential for severe thunderstorms will dominate the
latter part of the near-term period Saturday. For the setup, a 500
mb low center over Lake Michigan this evening will continue to carve
out height falls across the Tennessee Valley and into the southern
Appalachians through the nighttime hours. Meanwhile, a low pressure
system along the coastal plain will move from SC to NC tonight with
just a continued low end threat of isolated showers along our
eastern piedmont fringe in weak deformation. A series of developing
MCSs will make a run south and east of the OH Valley and central TN,
but with most of the high resolution, convection allowing models
showing little impact across our forecast area overnight into
Saturday morning. The most recent runs of the HRRR are the most
aggressive, showing upstream convection filling in across central
Tennessee and eastern Kentucky by early evening and then brushing
the northern mountains and the Smokies vicinity before dissipating
in the weaker instability across the high terrain late evening/early
overnight. Additional activity may develop and make a run overnight,
but with little more than scattered showers and thunderstorm to
advertise along the TN/NC border through daybreak.

Forcing, instability, and shear should then come together for
Saturday. Although profiles look rather capped early in the day
tomorrow, the cap should be explosively overcome during the
afternoon hours, with 2000 to 3000 J/kg of sbCAPE likely realized
east of the Blue Ridge. The best pocket of 7+ deg C/km mid-level
lapse rates will then cross the region from mid-afternoon to early
evening. The mid-level speed maximum rounding the upper trough will
likely yield surface to 6 km bulk shear values of 30 to 35 kt by 21Z
Saturday. However, shear will be quite a bit weaker in the lowest 1
to 2 km of the atmosphere and this may toggle the severe potential
away from tornadoes to more of a damaging wind/hail threat.
Anticipate the better spokes of upper level vorticity under the
dominant trough to cross the region mainly from mid afternoon to
mid evening and this better forcing should promote the greatest
coverage and highest severe thunderstorm chances. Most of our area
remains in a well-placed Slight Risk of severe thunderstorms for
Saturday.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/...
As of 215 PM EDT Friday: Robust,if not severe, deep convection
should be ongoing and numerous at the start of the period, all
within the unseasonable moderate deep cyclonic flow around Ohio
Valley closed low. Overall coverage will in no doubt wane overnight,
but the pattern will remain favorable for some degree of
regeneration at any point overnight, so at least low chance pop will
remain into early Sunday morning.  Through Sunday, upper low will
continue it's southward drift and slowly fill, which will actually
result in weaker shear and a little bit of drier air working in from
the west. But, at this point, it is hard to believe that diurnally
enhanced deep convective coverage won't become numerous again,
especially across western NC.  Sunday maxes still should about a
category below climo under the influence of the upper low. With the
axis of the filling upper low lingering atop the region through
Monday, the potential for a deeper southerly flow and influx of rich
Atlantic moisture increases. This setup may result in excessive
rainfall chances increasing. With Lots of clouds and rainfall
progged, max temps may wind up being at least 5 deg f below normal.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 1230 PM EDT Friday: Not a lot of changes made to the ext fcst
from the previous package. The op models and ens means agree well
with a broad yet weak stacked low sw of the FA to begin the period.
This low will likely become more disorganized beginning Wed as an
ulvl trof picks it up and shears it out. The mean flow within the
sfc-h85 layer will begin e/ly which will provide very good Atl moist
flux. Soundings show pwats on the order of 2.00 inches Tue and the
potential of some hydro concerns...esp across the eastern facing
upslope zones. As the stacked low dissipates the flow becomes less
defined and rather weak thru the column...so the potential for
upslope heavy precip will lessen. There will be some instability and
tstms during this time...but it will remain conditional with llvl
sbCIN persisting each afternoon. Will keep the higher mention PoPs
each day with some reduction on Fri as a weak cold front shud cross
the region during the day limiting convec moreso across the mtns.
However...there is much uncertainty with the timing of this front
attm. With high cloud cover each day...max temps will be held a cat
or so below normal...while mins persist abv normal within weak rad
cooling.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: VFR this evening and into the early overnight
period, but this will be short-lived as a round of MVFR to possibly
IFR cigs develops especially across NC TAF sites during the pre-dawn
hours as moisture increases. Convection to our west is progged to
fall apart before it crosses the mountains, with another round of
convective potential around daybreak, but confidence still to low to
mention more than VCSH at KAVL. Of greater concern is convective
potential for Saturday afternoon and have mixed a combination of
VCSH/PROB30 TSRA around midday, transitioning to prevailing TSRA at
all sites by 18-20z. With SPC slight risk in place, some of these
thunderstorms could become severe with damaging winds, but will hold
off on including enhanced wind wording until we are in a TEMPO time
frame and can narrow down timing with greater confidence. NNE winds
this evening will continue to back around the compass NW tonight and
toggle WSW to SW just ahead of the approaching trough on Saturday.

Outlook: An active/wet period is expected to continue from Sunday
through the middle part of next week as a slow-moving storm system
begins impacting much of the East. Numerous showers and
thunderstorms are expected during this period. While there will
likely be an afternoon/evening peak in coverage on most days, higher-
than-normal chances will also exist during the overnight and morning
hours.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High  92%     High  90%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     Med   75%     High 100%     High 100%
KHKY       High 100%     Med   77%     High  86%     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...SBK
NEAR TERM...HG/TDP
SHORT TERM...CSH
LONG TERM...SBK
AVIATION...TDP

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
1035 PM EDT Fri Jul 20 2018

GAZ018-026-028-029-NCZ035>037-056-057-064-065-068>072-082-501>510-
SCZ001>014-019-220245-
Stephens-Franklin-Hart-Elbert-Alexander-Iredell-Davie-Catawba-Rowan-
Transylvania-Henderson-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-
Union-Caldwell Mountains-Greater Caldwell-Burke Mountains-
Greater Burke-McDowell Mountains-Eastern McDowell-
Rutherford Mountains-Greater Rutherford-Polk Mountains-Eastern Polk-
Oconee Mountains-Pickens Mountains-Greenville Mountains-
Greater Oconee-Greater Pickens-Greater Greenville-Spartanburg-
Cherokee-York-Anderson-Abbeville-Laurens-Chester-Greenwood-
1035 PM EDT Fri Jul 20 2018

This Hazardous Weather Outlook is for northeast Georgia, piedmont
North Carolina, western North Carolina and upstate South Carolina.

.DAY ONE...Tonight.

Hazardous weather is not expected at this time.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

..Saturday...Severe weather possible. The environment is expected to
support numerous thunderstorms across the region Saturday afternoon
and evening. Storms will have the potential to produce damaging wind
gusts, large hail, and locally heavy rainfall.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.
..Thursday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Skywarn spotters are requested to forward any reports of large hail,
damaging winds, tornadoes, or flash flooding to our office.

GAZ010-017-NCZ033-048>053-058-059-062-063-220245-
Rabun-Habersham-Avery-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-
Graham-Northern Jackson-Macon-Southern Jackson-
1035 PM EDT Fri Jul 20 2018

This Hazardous Weather Outlook is for northeast Georgia and western
North Carolina.

.DAY ONE...Tonight.

Late tonight, a cluster of storms may cross Tennessee and push into
the western North Carolina and northeast Georgia. There remains a
small possibility of gusty wind and/or hail. Any storms that enter
the mountains late tonight are expected to quickly weaken as they
cross east.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

..Saturday...Severe weather possible. The environment is expected to
support numerous thunderstorms across the region Saturday afternoon
and evening. Storms will have the potential to produce damaging wind
gusts, large hail, and locally heavy rainfall.
..Sunday...No hazardous weather is expected at this time.
..Monday...No hazardous weather is expected at this time.
..Tuesday...No hazardous weather is expected at this time.
..Wednesday...No hazardous weather is expected at this time.
..Thursday...No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Skywarn spotters are requested to forward any reports of large hail,
damaging winds, tornadoes, or flash flooding to our office.