Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
751 PM EDT Thu Apr 8 2021

.SYNOPSIS...
A cold front will approach slowly from the west tonight into Friday,
with increasing shower and thunderstorm chances. The cold front
finally pushes through the forecast area late Saturday into early
Sunday keeping shower and thunderstorm chances around through
midday. High pressure builds in behind the departing cold front
Sunday into Monday, leading to drier conditions. Another cold front
tracks into the region on Tuesday, but models disagree on the
sensible impacts of this system.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 510 PM Thursday...multiple bands of showers continue to move
NE over the Upstate and Piedmont, with very little activity over the
mountains. So far, we have yet to see any lightning over our zones
with any stronger convection still well to our NW.

Otherwise, a closed occluded low pressure system will drift
north/northeast across the Upper Midwest tonight, while an upper
ridge axis shifts off the East Coast. A weakening warm conveyor belt
(WCB) associated with the Midwest low will track across the FA this
evening, then sort of wash out over our southern zones Friday. The
WCB is bringing a fair amount of mid and high clouds, which is
limiting destabilization. There is better instability developing
within a dry slot over the TN Valley and central Gulf states, and
this is where deeper convection is expected to fire and approach NE
GA and the Upstate late aftn thru this evening. The 12z NAM and also
a few of the high-res models still bring in about 1000-1500 J/kg of
sbCAPE in NE GA by 00z, with about 40-50 kts of 0-6 km of bulk
shear. The latest HRRR still has a few discrete cells with some
updraft helicity swaths in NE GA and into the western Upstate. So
cannot rule out a few strong to marginally severe storms in the
southwest corner of the FA this evening. But overall, it looks like
sfc-based instability will wane and most activity should be elevated
and sub-severe. With that said, there will be elevated CAPE thru the
overnight across most of the area, especially the Upstate. So a few
rumbles of thunder cannot be ruled out overnight. Activity should
wane during the the pre-dawn hours. Lows will be about 10-15 deg
above normal under the clouds and elevated dewpts. Friday's fcst
looks a little murky. Earlier thinking was that overnight precip
would produce stratus that takes a while to scatter out, resulting
in cooler temps. But all the latest guidance shows these clouds
scattering by mid-morning, and highs could be back to upper 70s to
lower 80s east of the mountains. So have bumped up the highs by
blending in latest NBM. Despite clouds being less stubborn, mid and
high clouds will linger, and we still take a while to become
unstable. Overall, guidance is not excited about convection across
the FA tomorrow aftn. And sure enough, the new Day 2 convective
outlook has removed our area from the marginal risk. Still, with a
moist SLY/SWLY low-level flow and sbCAPE of 1000-2000 J/kg, we
should see some showers and tstms develop. it just looks like there
will be a lack of upper support and focusing mechanism, as the
expected wedge-like boundary may not materialize.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 200 PM EDT Thursday: Lingering showers and thunderstorms will
be in store across portions of the CFWA Friday night before the the
nocturnal cycle gradually diminishes the convective environment
that is introduced Friday afternoon/evening. The pattern remains
amplified going into Saturday as a deepening upper low over the
central High Plains pushes into the mid-MS Valley during the day
Saturday. At the same time, a shortwave trough on the southern
stream portion of the upper low will rotate within the broad
cyclonic flow from the lower MS Valley through the Southeast by
Saturday afternoon. With a stacked ridge setting up shop just
offshore over the western Atlantic, the incoming shortwave will
derail the dry air that was entrained in the mid-levels and
allow for profiles to become more saturated. When factoring in
the arrival of a 30-40+ kt low level jet and daytime heating,
convective development ahead of the cold front will be likely across
the CFWA. With good moisture transport entering the region, PWAT
values ranging between 1"-1.5" could alter convective development
slightly and thus, heavy rain will be the primary threat with any
embedded thunderstorms that form during the afternoon and evening
hours on Saturday.

The pre-frontal activity expected on Saturday will gradually wane by
the nighttime period despite the approaching cold front still
lagging behind over the TN Valley early Sunday morning. A secondary
shortwave is shown swinging from the core of the upper low that is
located over the Midwest and Upper Great Lakes at this point will
enter the Mid South Sunday morning and sweep across the CFWA during
the afternoon and early evening hours on Sunday along with the
attendant cold front. The cold front will propel eastward into the
CFWA during the day on Sunday, but the timing of it will be crucial
to what we see, which can make these types of forecasts tricky. With
the cold front expected to be centered just east of the mountains at
the start of daytime heating and dewpoints remaining around 60F,
areas south and east of I-85 and along the I-77 corridor could
experience another day of convection, but model guidance have been
fairly sparse on simulated reflectivity returns. If the cold front
slow downs just a little bit, then it's possible that widespread
convection could develop, but if it becomes more progressive then
the weather will remain relatively quiet on Sunday. Current trends
indicate that SBCAPE will be on the order of ~500-1000 J/kg, with 30-
40 kts of deep layer bulk shear, and PWAT values around 1" will set
the environment up for convective development ahead and along the
cold front. With more dry air entrained in the mid-levels the
overall storm coverage will likely be capped to only isolated to
scattered before the full fropa takes place between 18z-21z Sunday,
but this is subject to change. Temperatures will be slightly above
normal for highs over the weekend, while lows are expected to be way
above normal each night as values should range in the 50s to near 60
for most locations.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 145 PM EDT Thursday: GFS/ECMWF/CMC all in good agreement that
precip should clear out by Sunday evening, with a weak surface high
building in behind the front.  Aloft, a 500mb ridge pushes in from
the west, suppressing vertical motion until Tuesday.  At this point,
all the models depict an upper low pushing through well north of our
area, across the northern Great Lakes and Canada, and dragging a
cold front through the region Wednesday (where before they'd
depicted this feature earlier, on Tuesday night, models are now
trending toward generally slower progression).  They differ in the
amount of moisture present for this frontal passage; while the GFS
and to some extent the CMC now as well are producing showers with at
least a few hundredths of an inch of QPF, the ECMWF keeps us dry
through the period, with little more than some cloud cover
accompanying the fropa.  For now, keeping chance PoPs across the
region from late Tuesday through Wednesday.  Cool high pressure
builds in on Thursday behind the front, and temps swing from well
above normal on Monday and Tuesday, to around normal on Wednesday,
to at or slightly below normal on Thursday as cooler air filters in
from the northwest.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
At KCLT and Elsewhere: Most of the shower activity has moved east
of our fcst area with just a few isolated light showers lingering
in the vicinity of KCLT and KAVL. Once these move out of the area,
we'll likely see a lull in shower activity for the rest of the night
thru early tomorrow. Guidance continues to suggest MVFR to IFR cigs
developing overnight/early tomorrow morning across the Upstate in
the wake of the showers, then gradually mixing out to a VFR cigs
by roughly noon. Another round of showers and tstms is expected to
develop to our south and expand northward tomorrow aftn/evening.
Thus, I kept PROB30s for TSRA at all sites beginning in the 18 to
20z window. Winds should become light over the next few hours but
still favor a SLY direction overall. They will pick back up from
the south by tomorrow aftn and remain SLY thru the evening.

Outlook: Scattered to numerous showers and thunderstorms will be
possible late Friday and into the weekend, leading to associated
flight restrictions, especially under any heavier showers. Drying
returns on Sunday and lingers into early next week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  99%     High  85%     High 100%
KGSP       High  98%     High  85%     High  89%     High 100%
KAVL       High  95%     High  85%     High  96%     High 100%
KHKY       High 100%     High  90%     High  87%     High 100%
KGMU       High  97%     High  87%     High  92%     High 100%
KAND       High  93%     High  80%     Med   78%     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...HG/MPR
NEAR TERM...ARK/JPT
SHORT TERM...CAC
LONG TERM...MPR
AVIATION...JPT

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
511 PM EDT Thu Apr 8 2021

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-090800-
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-
511 PM EDT Thu Apr 8 2021

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

.DAY ONE...Tonight.

Isolated thunderstorms are in the forecast thru tonight. The main
threat with any of these thunderstorms will be cloud-to-ground
lightning strikes. One or two of the storms may become strong enough
to pose a damaging wind risk.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

An isolated severe thunderstorm producing damaging wind and large
hail is possible Friday.

.SPOTTER INFORMATION STATEMENT...

None.

Wimberley

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
859 PM EDT Thu Apr 8 2021

GAZ010-017-090130-
Rabun GA-Habersham GA-
859 PM EDT Thu Apr 8 2021

...A STRONG THUNDERSTORM WILL IMPACT NORTHERN HABERSHAM AND
SOUTHWESTERN RABUN COUNTIES UNTIL 930 PM EDT...

At 859 PM EDT, Doppler radar was tracking a strong thunderstorm 11
miles north of Clarkesville, or near Lake Burton, moving east at 20
mph.

Locations to be impacted include...
Clayton, Tiger, Tallulah Falls, Lake Rabun, Lake Burton, Lakemont,
Tallulah Gorge State Park, Batesville, Seed Lake and Turnerville.

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.

This storm may intensify, so be certain to monitor local radio and tv
stations, as well as local cable tv outlets for additional
information and possible warnings from the National Weather Service.

Lakes Burton and Rabun are in the path of this storm. Cloud to
ground lightning and gusty winds will create dangerous boating
conditions. Move to shore immediately.

&&

LAT...LON 3481 8366 3483 8365 3491 8336 3474 8335
      3470 8335 3469 8334 3467 8337 3470 8364
      3471 8364 3473 8362 3476 8363 3479 8367
      3480 8368
TIME...MOT...LOC 0059Z 262DEG 18KT 3477 8358

8