Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
239 PM EDT Mon Jul 19 2021

.SYNOPSIS...
An upper-level trough of low pressure will remain just west of the
southern Appalachians through Tuesday, while a weak stationary front
remains stalled over the region. The trough will finally move
southeast of the area on Wednesday. Until then, increased clouds and
precipitation chances will keep high temperatures below normal.
Somewhat drier air arrives from the north on Thursday and
temperatures rebound to near-normal or slightly above through late
week.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 205 pm: The latest surface analysis indicates the leading edge
of the weak frontal zone has pushed into the northern tier of SC
counties. The boundary will continue to ooze slowly south across the
area through the period, and continue to be the primary focus of
convective development...although terrain effects will work their
usual magic, while PWATs above 1.75" and a weakly forced environment
acting on modest instability will allow showers to develop just
about anywhere. PoPs therefore ramp up to likely-to-categorical this
afternoon across all areas excepting locations north of the front
(i.e., the northwest NC Piedmont), where chance PoPs should suffice.
As was the deal yesterday afternoon/evening, the potential will
exist for cell training, esp in the vicinity of the frontal zone,
resulting in a continued threat of localized flash flooding.
Meanwhile, thunderstorms will be difficult to come by in the weakly
forced warm/moist environment, and the severe storm threat is
therefore virtually non-existent.

Overnight, an upper low is forecast to take shape across the TN
Valley by daybreak Tue. Most short term guidance sources are
developing a weak surface reflection along the boundary to our
west...most likely in the vicinity of the AL/TN/GA border. This
would act to focus low level convergence across far western portions
of our forecast area, with the addition of a respectably strong (for
the time of year) E/SE upslope flow. Therefore, if any part of the
forecast area stands to see more of an organized heavy rainfall
and/or flash flood threat, it would be our northeast GA and
northwest SC zones. We are currently forecasting around 2 inches in
this area through the period, but locally higher amounts are
likely...possibly much higher amount depending upon terrain
exposure. This area is too small and confidence still too low to
consider a Flash Flood Watch, but this area will need closer
scrutiny as time goes forward. Otherwise, PoPs ramp up to
categorical across the southwest quarter or so of the area between 6
and 12Z...tapering to low chance at best across the northwest NC
Piedmont.

Leading edge of the frontal zone is expected to sag south of the
area by the end of Tuesday, taking what little instability there is
along with it. Nevertheless, moisture will remain abundant over the
forecast area, and numerous to widespread showers ongoing at the
start of the day should persist through at least the morning hours,
with a continued potential for locally heavy/excessive rainfall. Max
temps will again be below normal, perhaps well-below normal where
dense low clouds are more persistent (i.e., the Blue Ridge and SC
foothills.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 230 PM EDT Monday: Tuesday night into Wednesday the trough
axis will remain in place across the CWA, however, PoPs will
gradually decline during this time frame. Models continue showing a
low pressure center breaking off from the trough Tuesday night into
Wednesday and tracking southwest into Texas. The upper-level trough
will track out of the region Wednesday night. The low remains in
Texas as high pressure builds across the Desert Southwest Thursday.
Meanwhile broad troughing remains across the East Coast Thursday
thanks to a large low pressure system in eastern Canada. Moisture
sticks around Tuesday night into Wednesday as the stationary front
remains draped across southern NC and northern SC. PWATs will
continue to hover around 1.5 inches for the mtns and 1.5 to 2.0
inches outside the mtns. The stalled front will lead to southerly
and westerly flow ahead of the frontal boundary with northerly flow
behind it. As a result, diurnal shower and thunderstorm chances
should decrease for the far northeastern zones due to drier air
while scattered diurnal activity looks best across the SC Upstate
and southwest NC mtns on Wednesday. Moisture will gradually decrease
from north to south Wednesday afternoon into Thursday as the
stationary front dissipates. This will allow flow on Thursday to
become mostly westerly and northerly across the CWA leading to
further drying of the lower and mid-levels. In response, PWAT values
should decrease across the northern tier of the forecast area
Thursday. Diurnal shower and thunderstorm chances return Thursday
with the far SW NC mtns and NE GA mtns having the best chance for
PoPs. Highs will gradually increase throughout the short term
period. However, high temps Wednesday will remain a few degrees
below climo, climbing to near or just above climo Thursday.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 230 PM EDT Monday: The high pressure system persists out
across the Desert Southwest while low pressure lingers across Texas
for much of the extended forecast period. Broad troughing hangs
around the East Coast Friday into the weekend as a result of large
low pressure systems tracking east across Canada. Lower moisture
content can be expected due to northerly and westerly flow across
the forecast area Friday allowing PWAT values to decrease across the
southern tier. Diurnal shower and thunderstorm chances continue on
Friday with the best chances for PoPs across the mtns. PWATs are
expected to climb back up this weekend into early next week as a
long wave trough digs across the eastern CONUS. A cold front
associated with this trough could track out of the Ohio Valley early
next week approaching the forecast area. This will lead to an uptick
in moisture and PoPs this weekend into early next week. High temps
throughout the extended will be a few degrees above climo.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
At KCLT and Elsewhere: Gradual heating/destabilization combined with
very moist conditions and a weak frontal zone sagging south across
the area will result in a continued expansion of convective coverage
through the afternoon into the evening. TS are expected to remain
quite isolated, so TAF mentions will be limited to VCSH and tempos
for SHRA at most sites into early evening. Flt conditions will
generally be MVFR/low VFR this afternoon/evening, but periods of IFR
visby will be possible in heavier downpours. Convective coverage is
expected to become increasingly confined to the upstate SC terminals
later this evening into the overnight. In fact, shower chances are
actually expected to increase in these areas between 6 and 12Z. As
such, categorical SHRA will be carried at these terminals at some
point during the overnight. Meanwhile, VCSH appears to be adequate
at the NC terminals. Cigs will also begin to lower again at all
sites later this evening, likely reaching IFR, or at least low MVFR
by daybreak Tuesday. Conditions are again expected to be slow to
improve during the daylight hours Tue, while shower chances, albeit
diminishing somewhat, will persist.

Outlook: The moist, unsettled pattern will continue through Tuesday,
with a steady decrease in shower and thunderstorm activity then
expected during the middle to later part of the week.
Overnight/early morning fog/low stratus will be possible each day.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  94%     High  83%     Med   70%     High  82%
KGSP       High  87%     High  83%     Med   67%     High  90%
KAVL       High  83%     High  84%     Med   78%     High  84%
KHKY       High  80%     High  93%     Med   62%     Med   69%
KGMU       High  83%     Med   66%     Med   71%     High  92%
KAND       High  92%     Med   76%     High  86%     High  90%

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...JDL
SHORT TERM...AP
LONG TERM...AP
AVIATION...JDL

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
350 PM EDT Mon Jul 19 2021

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-202000-
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-
350 PM EDT Mon Jul 19 2021

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

.DAY ONE...This afternoon and tonight.

Showers and thunderstorms will continue to develop across the region
today, especially over the higher terrain. The primary threat with
these storms will be heavy and/or long-lived rainfall, which could
result in localized flash flooding over locations affected by
multiple storms or where drainage is poor. Any of these storms will
be capable of producing gusty winds and the occasional cloud- to-
ground lightning strike.

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

Numerous showers will persist through Tuesday, producing heavy
rainfall throughout the day. The flash flooding threat will gradually
increase throughout the day as rain falls on saturated soils and
raised stream levels already primed by tonight's activity. This will
set the stage for additional flood potential on Tuesday.

.SPOTTER INFORMATION STATEMENT...

None.

MPR

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
557 PM EDT Mon Jul 19 2021

GAZ017-018-026-028-SCZ001-004>014-019-192230-
Hart GA-Franklin GA-Stephens GA-Habersham GA-York SC-
Greater Pickens SC-Cherokee SC-Greater Oconee SC-Laurens SC-
Abbeville SC-Chester SC-Anderson SC-Greenwood SC-Union SC-
Oconee Mountains SC-Spartanburg SC-Greater Greenville SC-
557 PM EDT Mon Jul 19 2021

...THUNDERSTORMS WITH TORRENTIAL RAINFALL WILL IMPACT NORTHERN
HART...NORTHERN FRANKLIN...STEPHENS...HABERSHAM...LAURENS...
SOUTHWESTERN PICKENS...UNION...SOUTHERN OCONEE...SOUTHEASTERN
GREENVILLE...NORTHWESTERN CHESTER...SOUTHWESTERN YORK...CENTRAL
SPARTANBURG...ANDERSON...SOUTHERN CHEROKEE...NORTHEASTERN ABBEVILLE
AND NORTHWESTERN GREENWOOD COUNTIES UNTIL 630 PM EDT...

At 557 PM EDT, Doppler radar indicated thunderstorms with torrential
rainfall along a line extending from 15 miles southwest of York to 14
miles west of Union to 3 miles north of Laurens to 8 miles northeast
of Hartwell to 5 miles southwest of Clarkesville, and moving
southeast at 20 mph.

Locations to be impacted include...
Spartanburg, Anderson, Clemson, Laurens, Toccoa, Seneca, Union,
Walhalla, Clarkesville and Hartwell.

Very heavy rainfall rates up to 2 inches per hour 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.

&&

LAT...LON 3470 8363 3473 8339 3472 8335 3474 8334
      3478 8306 3468 8211 3498 8205 3505 8172
      3483 8103 3453 8164 3441 8175 3431 8256
      3442 8338 3446 8340 3448 8347 3449 8354
      3445 8360 3451 8366 3454 8362 3465 8366
TIME...MOT...LOC 2157Z 299DEG 16KT 3490 8148 3471 8188 3455 8203 3445
8285 3456 8360

RWH