Special Weather Statement, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
341 AM EDT Mon May 3 2021

.SYNOPSIS...
An upper level disturbance will move northeast across the region
today accompanied by a humid and unstable air mass. Numerous
showers and thunderstorms...a few of them severe...will occur
today and then again on Tuesday as a cold front approaches from
the west. The front should move across the region early in the
day on Wednesday. We may dry out temporarily on Thursday as high
pressure moves past to our north.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 317 am: Scattered showers continue to develop across the
mtns and along/near the Blue Ridge Escarpment at this early hour,
more or less as suggested by the convection-allowing models and in
response to improving upglide/warm advection ahead of an approaching
short wave. The fcst will maintain a likely precip prob along/near
the Escarpment. Temps will remain seasonably warm as low level
moisture continues to increase.

Today looks convectively active across the fcst area. We've been
looking at the passage of a dampening short wave for a few days now
and the model guidance has been telling a consistent story. Although
the best of the mid/upper forcing lifts northeast across the region
early in the day, and in spite of extensive low cloudiness into
the middle part of the morning, we should quickly destabilize in
mid/late morning. The air mass should be characterized by sfc-based
CAPE on the order of 1500-2000 J/kg by late morning mainly east of
the mtns. As we move into the afternoon, mixed layer CAPE climbs
to around 2000 J/kg east of the mtns and deep layer shear improves
a bit to 30-40 kt. There has been some uncertainty regarding a
trigger, but once again the guidance suggests that will not be
a problem owing to the upslope flow over the Escarpment. In the
expected environment, one would anticipate stronger storms to
rotate, and the convection-allowing models respond in kind. All of
the CAMs have rotating updrafts somewhere in the fcst area east of
the mtns this afternoon/evening, which is compelling. The new Day
1 has raised a Slight Risk along the I-85 corridor and this seems
like a good bet that we will see a few severe thunderstorms. RAP/NAM
fcst soundings over the Upstate suggest that all forms of severe
weather are possible, even with marginal lapse rates and effective
shear that tops out at 40 kt, probably because the wind field looks
favorable as seen on a hodograph at 21Z at Anderson. Will not rule
out a tornado given 0-1km shear around 20kt and 0-1km shear around
150 ms/s2 and a few updraft helicity stripes seen on the 00Z HREF
over the upstate. The storms should bring a period of heavy rain,
but for now the flash flood threat appears secondary to the severe
thunderstorm threat given the dry conditions as of late. Temps may
be kept down afew degrees compared to yesterday because of the
cloud cover and relatively early precip development. The storms
should move off to the east in the early evening at the latest,
and thereafter we should have a relative lull for the overnight
hours. A chance of showers will be retained over mainly the
southwest mtns of NC because of the continued moist low level SW
flow. Low temps will once again be mild.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
As of 330 AM EDT Monday...Another day with a severe tstm threat
looks on tap for Tuesday, as an upper trough approaches from the
west. The forecast area is expected to be in a humid, warm sector
air mass, with sfc dewpoints in the upper 60s to near 70. Enough
of a lull in precip and some breaks in the clouds should allow
temps to warm into upper 70s to lower 80s and result in sbCAPE over
2000-3000 J/kg by early aftn. Shear looks a little more marginal
than today's (Monday), but still 30-40 kts of 0-6 km bulk shear
across the mountains. The CAMs all have plenty of convection
develop to our west by 18z, then westerly steering flow brings
activity into the forecast area thru the aftn hours. There may
not be enough shear for supercells, but rather more of multicell
clusters. Interestingly, the NAMnest and HRW-NMM have another round
of storms develop near the sfc front over western TN/KY and grow
upscale to a squall line. This line pushes into the NC Mountains
Tuesday evening. For now, confidence in the details is modest at
best, but ingredients are in place for organized severe storms. The
new Day 2 SPC convective outlook has the western 2/3rd of the CWFA
in a slight risk, and a marginal risk east for all but Union County,
NC. Will continue to mention in the HWO. The other concern is any
possible excessive rainfall. While we could see some training storms
within the SWLY flow, the current soil conditions are quite dry,
and anything more than very isolated minor flooding looks unlikely.

The latest model guidance seems to be a little slower on pushing
the cold front thru the area Wednesday, with the NAM especially
slow. All but the GFS have the eastern half of the forecast area
still ahead of the front with sbCAPE of 1000-2000 J/kg and better
bulk shear than we'll have Tuesday. There is the chance though,
that if the NAMnest-like squall line blasts thru the area Tuesday
night, we may not see as much destabilization Wednesday. But in any
case, the new day 3 convective outlook has a marginal risk for the
eastern half of the forecast area, and that seems reasonable. Temps
will be above normal again Wednesday with highs in the lower 80s
east of the mountains. The front should push thru Wednesday night
and bring low temps close to normal.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
As of 300 AM EDT Monday...The medium range starts 12z Thursday
with a digging upper trough across the Ohio Valley, carving out
a longwave trough across the eastern CONUS that persists into the
weekend. The upper flow gradually flattens out over the weekend,
with flat upper ridging returning by Sunday night. There will be
a slug of moisture with the digging trough, bringing a chance of
showers across mainly the NC mountains. Although, the 00z ECMWF has
come in a lot drier for Thu-Thu night. Will feature mainly slight
chc to low-end chc PoPs. Cool high pressure will builds in under
the trough Friday, with the sfc high center reaching the forecast
area by 12z Saturday. Temps will return to slightly below normal,
and there may be some frost concerns in the NC mountains Friday
night. The high drifts east off the Carolina Coast by Sunday and
return flow allows temps to warm back slightly above normal. The
next front looks to stay to our west thru Sunday, but a chance of
showers creeps into our western zones Sunday night.

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: An active day shaping up across the
region. Showery precip has already developed in the southerly
moist warm advection upslope/upglide flow near the Blue Ridge
Escarpment. Guidance shows the development of an MVFR cloud deck
in the pre-dawn hours and observations support that conclusion. A
ceiling restriction of at least MVFR looks like a good bet by 09Z
or so, with increasing confidence in an IFR restriction developing
by daybreak. Holes should open up in the IFR by mid/late morning
with deep convection developing rapidly in the increasingly
unstable/sheared environment across the region. Model guidance
develops numerous showers and thunderstorms affecting the terminals
from mid-morning through late afternoon. Have tried to shoot for
the most likely 4-hr time range with a TEMPO at the beginning
of the active period, followed up by a lengthy PROB30 group for
the stragglers in the late afternoon and early evening. MVFR
restrictions will continue through the showers/storms, with
an improvement to VFR after storms move off to the east in the
afternoon. Wind should be S to SW through the period, and gusty
before/during thunderstorms.

Outlook: A cold front will likely bring another round of SHRA/TSRA
to the area by Tue afternoon, with chances persisting into Wed.
Another lull in convection is expected for the end of the week.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High  89%     High  84%     High  91%     High  98%
KGSP       High  94%     Med   78%     High  83%     High  94%
KAVL       High  95%     High  81%     High  89%     High  87%
KHKY       High  91%     High  81%     High  84%     Med   77%
KGMU       High  95%     High  89%     High  86%     High  94%
KAND       High  94%     High  84%     High  90%     High  97%

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...PM
NEAR TERM...PM
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...PM

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
500 AM EDT Mon May 3 2021

GAZ010-017-018-026-028-029-NCZ068>072-082-508-510-SCZ001>014-019-
040900-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Cleveland-Lincoln-
Gaston-Mecklenburg-Cabarrus-Union NC-Greater Rutherford-Eastern Polk-
Oconee Mountains-Pickens Mountains-Greenville Mountains-
Greater Oconee-Greater Pickens-Greater Greenville-Spartanburg-
Cherokee-York-Anderson-Abbeville-Laurens-Union SC-Chester-Greenwood-
500 AM EDT Mon May 3 2021

This Hazardous Weather Outlook is for northeast Georgia, upstate
South Carolina, and the southern foothills and southern Piedmont of
North Carolina, including the Charlotte metro area.

.DAY ONE...Today and tonight.

Numerous thunderstorms are expected to develop across the region
later this morning and through the afternoon hours. Enough
instability and wind shear will be present for some of the storms to
become severe this afternoon, perhaps as a squall line that moves
east across the area along and southeast of Interstate 85. The
stronger storms will produce damaging wind gusts and hail the size
of quarters or larger. An isolated tornado will also be possible. The
thunderstorms should move away during the early part of the evening.

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

A cold front approaching from the west will be the trigger for another
round of strong to severe thunderstorms during the afternoon and
evening hours on Tuesday. The storms will increase the threat for
isolated flash flooding across areas that see storms with heavy rain
on Monday. The cold front could trigger isolated severe thunderstorms
east of the mountains again on Wednesday as it crosses the region.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information about
observed severe weather to the NWS while following all local, state,
and CDC guidelines.

NCZ033-035>037-048>053-056>059-062>065-501>507-509-040900-
Avery-Alexander-Iredell-Davie-Madison-Yancey-Mitchell-Swain-Haywood-
Buncombe-Catawba-Rowan-Graham-Northern Jackson-Macon-
Southern Jackson-Transylvania-Henderson-Caldwell Mountains-
Greater Caldwell-Burke Mountains-Greater Burke-McDowell Mountains-
Eastern McDowell-Rutherford Mountains-Polk Mountains-
500 AM EDT Mon May 3 2021

This Hazardous Weather Outlook is for the mountains, northern
foothills, and northwest Piedmont of North Carolina.

.DAY ONE...Today and tonight.

Numerous thunderstorms are expected to develop across the region
later this morning and through the afternoon hours. Enough
instability and wind shear will be present for some of the storms to
become severe this afternoon. A few of the storms will become strong
enough to produce damaging wind gusts and hail the size of quarters
or larger. Any of the storms will have frequent cloud-to-ground
lightning and heavy downpours of rain. The thunderstorms should move
away during the late afternoon.

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

A cold front approaching from the west will be the trigger for
another round of strong to severe thunderstorms during the afternoon
and evening hours on Tuesday. The storms will increase the threat for
isolated flash flooding across areas that see storms with heavy rain
on Monday. The cold front should move across the mountains early on
Wednesday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed. Please relay any information about
observed severe weather to the NWS while following all local, state,
and CDC guidelines.

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
552 AM EDT Mon May 3 2021

NCZ056-068-504-508-031015-
Greater Rutherford NC-Cleveland NC-Greater Burke NC-Catawba NC-
552 AM EDT Mon May 3 2021

...A STRONG THUNDERSTORM WILL IMPACT SOUTHWESTERN CATAWBA...
NORTHEASTERN RUTHERFORD...NORTHWESTERN CLEVELAND AND SOUTHEASTERN
BURKE COUNTIES UNTIL 615 AM EDT...

At 552 AM EDT, Doppler radar was tracking a strong thunderstorm 9
miles south of Morganton, or near South Mountains State Park, moving
northeast at 25 mph.

Locations to be impacted include...
Morganton, Longview, Valdese, Icard, Drexel, Hildebran, Connelly
Springs, Rutherford College, Lake Rhodhiss and Hickory Regional
Airport.

Wind gusts up to 45 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.

Doppler radar indicates weak rotation with this storm. Spin-up of a
very brief, weak tornado cannot be ruled out. Be prepared to take
immediate action if a tornado is sighted or a Tornado Warning is
issued.

&&

LAT...LON 3578 8155 3579 8145 3577 8144 3577 8141
      3563 8131 3557 8150 3557 8153 3556 8152
      3549 8172 3564 8183 3580 8158
TIME...MOT...LOC 0952Z 239DEG 23KT 3561 8170

ARK

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
614 AM EDT Mon May 3 2021

NCZ056-068-069-504-508-031045-
Greater Rutherford NC-Cleveland NC-Lincoln NC-Greater Burke NC-
Catawba NC-
614 AM EDT Mon May 3 2021

...A STRONG THUNDERSTORM WILL IMPACT SOUTHWESTERN CATAWBA...
NORTHEASTERN RUTHERFORD...NORTHWESTERN LINCOLN...NORTHWESTERN
CLEVELAND AND SOUTHEASTERN BURKE COUNTIES UNTIL 645 AM EDT...

At 614 AM EDT, Doppler radar was tracking a strong thunderstorm 14
miles northeast of Forest City, or 7 miles west of Casar, moving
northeast at 30 mph.

Locations to be impacted include...
Casar, South Mountains State Park, Belwood, Valdese, Pleasant Grove,
Cooksville, Sunshine and Vale.

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.

Doppler radar indicates weak rotation with this storm. Spin-up of a
very brief, weak tornado cannot be ruled out. Be prepared to take
immediate action if a tornado is sighted or a Tornado Warning is
issued.

&&

LAT...LON 3575 8154 3552 8137 3541 8181 3554 8189
      3554 8185 3558 8182 3559 8183
TIME...MOT...LOC 1014Z 242DEG 24KT 3552 8175

ARK