Severe Thunderstorm Warning 308, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
242 PM EDT Mon Aug 17 2020

.SYNOPSIS...
Thunderstorm chances remain at typical mid-August levels through
Tuesday, then increase through the rest of the week as an upper
level trough becomes well established. Highs will be around normal
through the first part of the week then fall below normal for the
latter half of the week.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 2:30pm EDT Monday: Radar shows widely scattered convective
showers along a line that is more or less coincident with a weak
frontal boundary that runs along the foothills.  These showers
should move southeastward out of the foothills, exiting the area by
around midnight.  As storms move southeastward, they will
progressively encounter better CAPE, along with shear slightly
better than recent history.  There is consequently some possibility
of storms developing up to severe levels as the afternoon
progresses.  High-res models, in particular, have stronger updrafts
than typical sub-severe situations.

Front is due to passage of weak embedded shortwave in broader
eastern CONUS longwave trough.  As it passes, light surface flow
will become northwesterly to northerly and will work to reduce
moisture and instability, making convection much less likely on
Tuesday. Temperatures will be near normal, though with a slight
cooling trend on the highs and lows due to persistent eastern
longwave trough.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 230 PM EDT Monday: An ill-defined front will settle into
our area overnight Tuesday night as the upper-level trough axis
stalls just to our west from the central Gulf Coast to the lower
Great Lakes.  The attendant weak front is expected to slowly drift
SE into the coastal Carolinas during the short-term period late
Wednesday into Thursday.

Currently, we are expecting convection to fire in the vicinity
of the front Wednesday afternoon, generally along the favorable
ridge tops.  The NAM is suggesting convection will then propagate
east into the Piedmont Wednesday evening, with cold pool dynamics
possibly helping to push the front east (which would reduce any
heavy rain threat); however given our favorable position east of
the trough axis with the front providing a focus of low-level
convergence, all guidance is expecting numerous showers and
thunderstorms area-wide during peak heating Wednesday.  Model PW
values of 1.25" (mountains) to 1.6" (Piedmont) are about normal
for this time of year and guidance disagrees on how much cell
training potential exists; therefore if any excessive rain/flooding
issues develop, they are expecting to be very isolated.  Given the
increased flow aloft adjacent to the trough, guidance continues
to suggest bulk shear values of 20-30 kts are possible Wednesday
with ample instability, therefore the potential for a few strong
to severe storms remains.  Expect high temperatures to be 1-3
degrees cooler than Tuesday or slightly below normal given better
convective coverage and cloud cover.

By Thursday, the upper-level trough axis remains nearly stationary
thanks to a blocking high pattern over the Atlantic.  The front
is expected to push east of our area by this time and NE flow
is forecasted to advect a cooler and less unstable airmass into
the region.  However, at this time model SBCAPE values remain
in the 500-1200 J/kg range with bulk shear still in the 15-25kt
range so we still expect another round of likely PoPs area-wide
with the best chances for thunderstorms across the Piedmont closer
to the better instability.  With persistent SWly flow aloft, PWs
may actually increase to near the 90th percentile especially over
the Piedmont. In addition, NEly low-level flow with SW flow aloft
better supports cell training, therefore the locally excessive
rainfall threat may be better than Wednesday wherever convection
manages to develop.  However, future trends will be worth watching
because the pattern suggests a early-fall wedge-like environment
could develop which would support high PoPs, but low instability.
Nevertheless, the CAA and persistent cloud cover/PoPs suggest
highs will struggle to reach the lower 80s east of the mountains,
which is about 5-7 degrees below normal.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
As of 230 PM EDT Monday: On Friday, the overall pattern changes
little from Thursday and we continue to expect unsettled conditions
to persist across the region with numerous to widespread showers
and thunderstorms expected and a continued threat for isolated
excessive rainfall and a few strong storms.  Once again, the setup
supports a wedge-like pattern suggesting the mountains and Foothills
will be more stable and that the best chances for thunderstorms
may remain over the Piedmont.  High temperatures are expected to
remain several degrees below normal under cool NEly low-level flow.

Heading into the weekend, the quasi-stationary front over the
Carolina coastal plain is expected to yield to broad southerly
flow on the backside of the Atlantic subtropical high and ahead of
the deep-layer trough to the west of our region as we head into
Saturday. During the weekend, the trough should slowly propagate
east as the heat ridge over the western CONUS builds east and the
Atlantic high deamplifies a bit.  The upper-level pattern will
continue to support weakness overhead which keeps above-climo PoPs
in the forecast through the weekend, albeit we do expect coverage
to decrease Sunday and Monday as we lose the more-focused upper
support due to the deamplification of the trough.  As a result
of the pattern, highs will rebound to slightly below normal on
Saturday and near normal for Sunday and the new work week.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
At KCLT and Elsewhere: A weak frontal zone is passing through
the area this afternoon and helping to focus a round of
thunderstorms, with storms currently forming in the foothills. These
storms are expected to move southeastward and intensify as they
move into the piedmont.  It is possible that KAVL will be missed by
storms this evening, but all other TAF sites have a good chance of
being affected.  Storms move east of the area by 00Z with winds up
to 10kts becoming more northerly behind the front.  Front will
decrease moisture levels leaving reduced chances for fog or low CIGs
Tuesday morning, and very little chance for precipitation on
Tuesday.  KAVL is the only location where fog has a high enough
chance of occurring to be worth mentioning.

Outlook: Thunderstorms are again possible Wednesday. Fog and low
stratus restrictions will be possible each morning, especially over
the usual mountain valleys. Otherwise expect VFR.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High 100%     High 100%     Med   66%     High  92%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  99%     High  99%     High 100%     High  99%
KHKY       High 100%     High  92%     Med   62%     High  92%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High  92%     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...WJM
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...WJM

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
450 AM EDT Mon Aug 17 2020

GAZ010-017-018-026-028-029-NCZ035>037-056-057-062>065-068>072-082-
502-504-506>510-SCZ001>014-019-180900-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Alexander-Iredell-
Davie-Catawba-Rowan-Macon-Southern Jackson-Transylvania-Henderson-
Cleveland-Lincoln-Gaston-Mecklenburg-Cabarrus-Union NC-
Greater Caldwell-Greater Burke-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-
450 AM EDT Mon Aug 17 2020

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

.DAY ONE...Today and tonight.

Scattered thunderstorms are expected to develop this afternoon as a
front pushes east off the Blue Ridge escarpment and across the
Piedmont. The main threats will be cloud-to-ground lightning, heavy
downpours, and gusty winds. A severe storm or two producing damaging
wind gusts cannot be ruled out.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

NCZ033-048>053-058-059-501-503-505-180900-
Avery-Madison-Yancey-Mitchell-Swain-Haywood-Buncombe-Graham-
Northern Jackson-Caldwell Mountains-Burke Mountains-
McDowell Mountains-
450 AM EDT Mon Aug 17 2020

This Hazardous Weather Outlook is for western North Carolina.

.DAY ONE...Today and tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

ARK

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
250 PM EDT Mon Aug 17 2020

NCZ035-036-056-171915-
Iredell NC-Alexander NC-Catawba NC-
250 PM EDT Mon Aug 17 2020

...A STRONG THUNDERSTORM WILL IMPACT NORTHEASTERN CATAWBA...SOUTHERN
ALEXANDER AND WEST CENTRAL IREDELL COUNTIES UNTIL 315 PM EDT...

At 249 PM EDT, Doppler radar was tracking a strong thunderstorm near
Taylorsville, moving southeast at 20 mph.

Locations to be impacted include...
Taylorsville, Stony Point, Catawba, Lookout Shoals Lake, Bethlehem,
Lake Hickory, Hiddenite, Millersville, Scotts and Oxford.

Wind gusts up to 40 mph, pea size hail and torrential rainfall rates
up to 3 inches per hour will be possible in these areas.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

The combination of gusty winds and saturated soil conditions will
cause scattered trees and power lines to fall. Seek shelter inside an
interior room.

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 3583 8129 3596 8123 3590 8093 3568 8107
TIME...MOT...LOC 1849Z 294DEG 16KT 3587 8118

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