Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
244 PM EDT Tue Aug 11 2020

.SYNOPSIS...
Daily showers and thunderstorms will steadily become more numerous
as the week progresses.  High temperatures of a couple degrees above
normal through Wednesday will become near to below normal by the end
of the week with the deeper moisture and considerable cloud cover.

&&

.NEAR TERM /THROUGH WEDNESDAY/...
As of 230 pm EDT Tuesday: Several of the convection allowing models
continue to show two windows of convection this afternoon through
tonight. The first, afternoon triggering along the Blue Ridge
mountains, has already begun - with outflows expected to migrate
east across the foothills and into the lower Piedmont through early
evening. Later evening triggering along the northern Blue Ridge then
looks more likely as a 500 mb vorticity lobe approaches the northern
mountains, with activity moving out over the I-77 corridor through
the late evening and overnight hours. Will continue to feature the
best likely PoPs along and just east of the escarpment through late
day, and then advertise lingering, lower chances mainly across
western NC for the overnight hours. SBCAPE values are maxing out at
about 2000 to 2500 J/kg this afternoon, with dewpoint mixing weak
thus far. Fortunately, DCAPE values remain modest at 800-900 J/kg.
Cannot rule out some gusty winds with any storms coming east off the
mountains, but the severe potential appears limited. Hydro issues
remain possible with any training, but still expect enough eastward
propagation to limit the flooding threat. Min temps tonight should
run 1 to 2 degrees above climo throughout with residual high cloud
cover and mountain valley fog and stratus development.

Mid to upper heights will recover very modestly over the southern
Appalachians Wednesday as heights fall over the MS River Valley
ahead of a MO/IL cutoff low. Still anticipate low-level convergence
along with the heating through the day, and flow toggling back
toward southerly will enhance a weak mountain upslope component. All
told, this should mean a slight uptick in diurnal PoPs for tomorrow
afternoon across the mountains and adjacent foothills. Temperatures
will be comparable to today, with 80s mountains to lower 90s
Piedmont.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
As of 200 PM Tue: Wednesday night our overall pattern will
remain weakly forced, with little to no low-level flow. We will
be in the relative lull between a well defined shortwave over
the mid-Mississippi Valley and a weak cutoff low off the SC
coast. Diurnal instability will wane as usual, but with cyclonic
flow aloft and potentially abundant outflows from the daytime
activity, stray redevelopment could occur much of the night. As
the shortwave begins to move up the Ohio Valley later Thursday,
promoting a clear southerly flow, convective coverage should
increase compared to Wednesday. That flow continues and amplifies
slightly Friday as the low nears, warranting high likely to
categorical PoPs for the area that day. Shear will remain
seasonably low, and sfc-midlevel delta-theta-E will support
an equally seasonable threat of isolated/localized damaging
microbursts. With storm motion likely slow due to the weak shear,
and PWAT values elevated AOA the 90th percentile, a few spots will
be at risk of flash flooding; as the pattern takes on more influence
from southerly flow, our attention may be drawn to the Blue Ridge
Escarpment Friday. Temps will run near or slightly above normal.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
As of 240 pm Tuesday: The start of the medium range will be active
as an upper trough begins to set up shop over the eastern CONUS.
Lower heights should enter the region and an embedded low begins to
build from the Mid Mississippi Valley and into the Ohio Valley over
the weekend. With summertime heating and a persistent southwesterly
flow through much of the atmospheric column, tropical moisture from
the Gulf will provide an uptick in diurnal convective coverage over
the cwa on Saturday. Pieces of shortwave energy will allow the
activity to linger into the late evening hours. PWAT values are
likely going to be =>2.00" everywhere outside of the mountains and
could cause a possible excessive rainfall event, especially over
areas that get hit with a couple of rounds of convection during the
rest of this week. The overall pattern will continue to support
mainly diurnal development as the axis of the upper trough remains
to our west, but will gradually propagate eastward over the weekend
and into the early part of next week. Model guidance is picking up
on a weak frontal boundary late Sunday into Monday of next week. The
GFS is a little more progressive than the ECMWF, but both are in
agreement of the overall setup. The ECMWF continues to show a wetter
solution than the GFS and could be linked to the slower push of the
upper trough. A diurnal pattern will be the likely outcome Sunday
and early next week with no clear signal of any organized setup
expected across the area. Both models do pick up on another frontal
boundary associated with the trough for next Tuesday and into the
middle part of next week, but this could be one that is not typical
for August. Although it is being shown on day seven, the boundary
looks to set up just on the east side of the trough axis and along a
200 mb jet streak that stretches from the Tennessee Valley through
the Northeast. The right entrance of the jet could provide more
influence to the overall weather pattern during this time frame. If
this was the case, another potential excessive rainfall event would
be in store for parts of the cwa as an increase in lift and moisture
will be available. Temperatures for the weekend will be a degree or
more below climo and could downtick a degree or more for the early
part of next week if the cloudy and wetter pattern remains in place.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
At KCLT and Elsewhere: Convective coverage is building this
afternoon along and east of the Blue Ridge mountains, with more
isolated activity forming east of the mountains. Anticipate that
KAVL to KHKY will see the earliest chance of TSRA, unless a rogue
TSRA can reach KCLT or the Upstate sites sooner. Will hold onto
TEMPO TSRA the latest at KHKY to KCLT to account for weak upper
forcing arriving over western NC later in the evening, but this
timing will likely have to be refined as earlier convection forms
and propagates. The SC sites should see a more traditional diurnal
timing through mainly 00Z. Expect light winds through the period,
with directions driven by any outflows moving through the terminal
area. More of NE wind component will set up overnight, except for
persistent NW flow in the French Broad Valley at KAVL. IFR to LIFR
low stratus and fog are likely in the mountain valleys again
overnight, and cannot be ruled out at a few other sites depending on
where heavy rain falls. Anticipate VFR bases from cumulus with
heating Wed morning, and then building TSRA chances again through
the afternoon from the west.

Outlook: Above normal coverage of diurnal convection is expected
through the upcoming weekend. Morning fog/low stratus restrictions
will be possible each morning, especially over the usual mountain
valleys.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     High 100%     High  81%
KGSP       High 100%     High 100%     High 100%     Med   78%
KAVL       High 100%     High 100%     Low   44%     Low   58%
KHKY       High 100%     High 100%     High  81%     Med   64%
KGMU       High 100%     High 100%     High 100%     Med   78%
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 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...HG
SHORT TERM...08
LONG TERM...CAC
AVIATION...HG

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
1107 AM EDT Tue Aug 11 2020

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-121515-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Avery-Alexander-
Iredell-Davie-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Catawba-
Rowan-Graham-Northern Jackson-Macon-Southern Jackson-Transylvania-
Henderson-Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union NC-
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-Union SC-Chester-Greenwood-
1107 AM EDT Tue Aug 11 2020

This Hazardous Weather Outlook is for northeast Georgia, Piedmont
North Carolina, western North Carolina, and Upstate South Carolina.

.DAY ONE...This afternoon and tonight.

Scattered to numerous thunderstorms are expected across the area,
mainly this afternoon and evening. The thunderstorms will likely
start this afternoon mainly in and near the mountains before
spreading east across the foothills and Piedmont through late day.
The main threats from these storms will be frequent cloud to ground
lightning, locally heavy rainfall, and gusty winds.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

HG

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
245 PM EDT Tue Aug 11 2020

SCZ001-111915-
Oconee Mountains SC-
245 PM EDT Tue Aug 11 2020

...A STRONG THUNDERSTORM WILL IMPACT NORTHERN OCONEE COUNTY UNTIL 315
PM EDT...

At 245 PM EDT, Doppler radar was tracking a strong thunderstorm 9
miles north of Walhalla, or near Salem, moving southwest at 10 mph.

Locations to be impacted include...
Walhalla, Salem, Oconee State Park, Lake Keowee and Mountain Rest.

Wind gusts up to 40 mph and pea size hail will be possible in these
areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Gusty winds can blow down trees that are dead, or that have weakened
root systems. Outdoor furniture and similar items can be tossed
around if not anchored. Seek shelter indoors.

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.

&&

LAT...LON 3487 8324 3488 8324 3488 8321 3489 8320
      3492 8316 3493 8316 3493 8314 3494 8313
      3493 8311 3495 8313 3497 8312 3499 8310
      3502 8306 3489 8291 3488 8291 3470 8306
      3486 8325
TIME...MOT...LOC 1845Z 044DEG 7KT 3491 8304

ARK

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
313 PM EDT Tue Aug 11 2020

NCZ068-070-SCZ008-009-111945-
Gaston NC-Cleveland NC-York SC-Cherokee SC-
313 PM EDT Tue Aug 11 2020

...A STRONG THUNDERSTORM WILL IMPACT SOUTHEASTERN CLEVELAND...SOUTH
CENTRAL GASTON...NORTHWESTERN YORK AND NORTHEASTERN CHEROKEE COUNTIES
UNTIL 345 PM EDT...

At 312 PM EDT, Doppler radar was tracking a strong thunderstorm 11
miles southwest of Gastonia, or near Kings Mountain State Park,
moving east at 10 mph.

Locations to be impacted include...
Kings Mountain, South Gastonia, Crowders Mountain State Park, Kings
Mountain State Park, Grover, Clover, Bethany and Crowders.

Wind gusts up to 40 mph will be possible in these areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Gusty winds can blow down trees that are dead, or that have weakened
root systems. Outdoor furniture and similar items can be tossed
around if not anchored. Seek shelter indoors.

&&

LAT...LON 3503 8127 3514 8147 3527 8139 3523 8119
TIME...MOT...LOC 1912Z 289DEG 7KT 3518 8136

ARK

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
314 PM EDT Tue Aug 11 2020

SCZ001-004-005-111945-
Greater Pickens SC-Greater Oconee SC-Oconee Mountains SC-
314 PM EDT Tue Aug 11 2020

...A STRONG THUNDERSTORM WILL IMPACT SOUTHWESTERN PICKENS AND EASTERN
OCONEE COUNTIES UNTIL 345 PM EDT...

At 314 PM EDT, Doppler radar was tracking a strong thunderstorm 5
miles northeast of Walhalla, moving southeast at 15 mph.

Locations to be impacted include...
Clemson, Seneca, Walhalla, Central, Norris, Six Mile, Salem and Lake
Keowee.

Wind gusts up to 45 mph and pea size hail will be possible in these
areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Gusty winds can blow down trees that are dead, or that have weakened
root systems. Outdoor furniture and similar items can be tossed
around if not anchored. Seek shelter indoors.

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.

&&

LAT...LON 3481 8313 3493 8297 3489 8291 3481 8290
      3484 8283 3475 8270 3457 8295
TIME...MOT...LOC 1914Z 318DEG 12KT 3482 8299

ARK

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
321 PM EDT Tue Aug 11 2020

NCZ504-508-111945-
Greater Rutherford NC-Greater Burke NC-
321 PM EDT Tue Aug 11 2020

...A STRONG THUNDERSTORM WILL IMPACT NORTHEASTERN RUTHERFORD AND
SOUTHWESTERN BURKE COUNTIES UNTIL 345 PM EDT...

At 321 PM EDT, Doppler radar was tracking a strong thunderstorm 5
miles southwest of Morganton, or near Glen Alpine, moving southeast
at 15 mph.

Locations to be impacted include...
Morganton, Glen Alpine, South Mountains State Park, Pleasant Grove
and Oak Hill.

Wind gusts up to 40 mph will be possible in these areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Gusty winds can blow down trees that are dead, or that have weakened
root systems. Outdoor furniture and similar items can be tossed
around if not anchored. Seek shelter indoors.

&&

LAT...LON 3564 8154 3557 8167 3558 8169 3556 8169
      3551 8178 3560 8183 3570 8186 3571 8186
      3578 8174
TIME...MOT...LOC 1921Z 318DEG 12KT 3568 8177

ARK