Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
227 PM EDT Sat Jul 24 2021

.SYNOPSIS...
Air quality should improve somewhat over the weekend while afternoon
showers and thunderstorms increase in coverage and intensity through
Monday.  A backdoor cold front builds into the forecast area early
next week, bringing more active weather at least through Tuesday.
Late week, additional rainfall is possible, though significant
uncertainty remains at that point.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 140 PM: Low clouds scattering out but convection developing
across the area. Guidance in decent agreement showing better
coverage initially over the mountains which then spread south and
east across the foothills and Piedmont through the afternoon and
early evening. Of course, some convection has developed along the
differential heating boundary south of I-85. This convection will
likely be more isolated with better coverage once convection moves
off the higher terrain. Meso analysis shows better than expected
effective shear across the area, with moderate to very unstable CAPE
values. DCAPE values are generally modest but could increase to 800
J/kg ore higher with heating. Therefore, a few severe storms will be
possible. Storm motion looks to be faster than previous days, but PW
values remain high. Isolated areas of excessive rainfall and
flooding will be possible, especially locations where multiple
storms traverse.

Showers will linger into the early overnight but should diminish as
the low level flow turns more southwesterly. That said, can't rule
out an isolated shower in any location overnight. Expect mountain
valley fog and stratus once again, but confidence lower for
locations outside of the mountains. Lows should be near or slightly
above normal.

Northwest flow remains over the area Sunday with moderate
instability developing. Again, no strong trigger mechanism, but
differential heating and a lee trough should be enough for scattered
diurnal convection. An isolated strong to severe storm or area of
heavy rainfall will also be possible. Highs will be near normal.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 150 PM EDT Saturday: Broad ridging over the western two-thirds
of the country will entrench the eastern CONUS in a subdued weather
pattern at the start of the short term forecast period on Monday.
A deep shortwave digs into the Mid-Atlantic throughout the day,
but guidance still keeps the most potent QG forcing associated
with this trough well north of our forecast area.  Instead,
our active weather will be driven primarily by low-level forcing
associated with a cold front impinging on the region from the north
by Monday evening.  With the Bermuda High enhancing southerly
flow throughout the weekend and Monday, the front will find a
moisture-rich environment in the Carolinas - models agree that
PWATs across the foothills and Piedmont will have risen into the
1.9-2.3" range, and will be pushing at least 1.75" in the mountains.

Prefrontal precipitation should begin by Sunday night or early
Monday morning for the NC mountains, and over the course of the
day, this activity will gradually spread south and east over
the CWA, eventually encompassing the entire forecast area.
Coverage and intensity of showers should increase throughout
the day, before convective potential begins to rise late morning
and early afternoon.  The ECMWF, CMC, and NAM all suggest SBCAPE
values of 1500+ J/kg, with the outlier GFS carrying a more modest
500-1000 J/kg.  Given that we'll be overcast much/all of the day,
and that rain will likely be underway for most areas by midmorning,
the current thinking is that those higher instability values are
overdone.  So, expect this front to feature widespread showers
with some embedded non-severe thunderstorms.  The big concern will
be excessive rainfall: with PWATs at/near 2", a deep warm cloud
layer in excess of 12 kft, and weak MBE vectors less than 5kts
all afternoon, backbuilding should be a concern for some cells.
Furthermore, with the front likely stalling somewhere near our
forecast area, some spots are likely to see long durations of
heavy rainfall Monday night into Tuesday.

Temps are still on track to be above-normal both Monday and Tuesday,
though guidance continues to trend toward lower highs in light of
widespread clouds, continued rainfall persisting through the
forecast period, and weakened southerly flow/WAA as the front stalls.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 200 PM EDT Saturday: The details of the medium-range forecast
remain in question.  Global models continue to suggest that a strong
Bermuda High will limit the cold front's ability to fully exit our
region on by Tuesday night, so even into Wednesday we'll likely
continue to see frontally-forced rainfall as the boundary stalls
over our CWA or just to the south.  The GFS, ECMWF, and CMC all keep
us locked in a wet pattern through Wednesday, before the weakening
boundary finally sags out of our area, carrying low-level forcing
with it and allowing weak high pressure to filter into the area.
With broad high pressure persisting over the Atlantic, however,
the prevailing surface flow will still be primarily southerly,
albeit with an increasing westerly component.  This slight shift
in overall wind direction should limit PWATs - comparatively,
at least, as they still keep near 1" into Thursday and Friday.

By Friday, we may see the arrival of a shortwave as the upper
pattern slightly amplifies - which would result in another wave
of precipitation late in the week.  Once this passes, we may be
in store for some dry weather next weekend.

In any case, temperatures will generally be near- or slightly
above-normal throughout the period.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
At KCLT and elsewhere: Lingering MVFR cigs will scatter out soon.
Convection has already begun and have mix of VCSH/VCTS and TEMPO
TSRA based on current coverage and guidance showing TSRA will be
over or near all TAF sites sometime this afternoon or early evening.
Showers may linger into the early overnight, so have VCSH for those.
Guidance agrees on mountain valley fog/stratus again by morning but
disagree on restrictions outside of the mountains. Have restrictions
at KAVL and KHKY where chances are highest, but left out elsewhere
given the low confidence. Convection will develop again Sunday
afternoon, but only have PROB30 at KCLT for now given the longer TAF
period there. S to SW wind this afternoon becomes light S or light
and variable overnight. Light SW wind for Sunday.

Outlook: Diurnal thunderstorm chances will steadily increase through
early next week, with some drying expected during mid-week. Early
morning fog or low stratus will be possible each day, primarily in
the mountain valleys.

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  91%     High  95%
KAVL       High  99%     High  93%     Med   64%     Med   79%
KHKY       High 100%     High 100%     High  81%     High  83%
KGMU       High 100%     High  95%     Med   74%     High  96%
KAND       High 100%     High 100%     High 100%     High  99%

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...MPR
NEAR TERM...RWH
SHORT TERM...MPR
LONG TERM...MPR
AVIATION...RWH

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
358 AM EDT Sat Jul 24 2021

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-250800-
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-
358 AM EDT Sat Jul 24 2021

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

.DAY ONE...Today and tonight.

Scattered showers and thunderstorms should develop late this
afternoon across the mountains. Activity should track southeasterly
into the Foothills and South Carolina Upstate this evening. A few
isolated showers and thunderstorms could survive and track into the
North Carolina Piedmont tonight. The main potential impacts are
cloud- to- ground lightning and locally heavy rainfall. An isolated
strong to severe storm cannot be ruled out.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
313 PM EDT Sat Jul 24 2021

SCZ001-002-241945-
Pickens Mountains SC-Oconee Mountains SC-
313 PM EDT Sat Jul 24 2021

...A STRONG THUNDERSTORM WILL IMPACT NORTHWESTERN PICKENS AND
NORTHEASTERN OCONEE COUNTIES UNTIL 345 PM EDT...

At 312 PM EDT, Doppler radar was tracking a strong thunderstorm 15
miles northeast of Walhalla, or near Devils Fork State Park, moving
south at 10 mph.

Locations to be impacted include...
Salem, Devils Fork State Park, Keowee Toxaway State Park, Jocassee
Gorges, Lake Jocassee, Lake Keowee, Bad Creek and Nine Times.

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

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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.

Lake Keowee is in the path of this storm. Cloud to ground lightning
and gusty winds will create dangerous boating conditions.
Move to shore immediately.

&&

LAT...LON 3502 8303 3506 8288 3490 8278 3484 8302
TIME...MOT...LOC 1912Z 343DEG 8KT 3497 8296

ARK