Severe Thunderstorm Warning 325, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
915 AM EDT Mon Aug 31 2020

.SYNOPSIS...
The Bermuda High building into the area will largely limit afternoon
to evening thunderstorms by mid-week and produce seasonably hot
conditions that will last through the end of the work week.
Unsettled and cooler weather may return by this weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 855 AM: Following a remarkable spate of early morning
convection across the I-77 corridor, activity continues to blossom
in the eastern third of the CWA. These cells are firing ahead of
a shortwave axis seen on water vapor imagery and with the modest
LLJ preceding it. Though the HRRR has not initialized it well, it
looks likely that decent coverage of heavy rain producing cells
will continue into the NC side of the Charlotte metro until the
shortwave axis clears the area later this morning. A weak warm
front will effectively remain over the area thru today, bringing
a plume of 1.75-2.0" precipitable water. Minimal heating by the
middle of the day looks likely to lead to at least one more round
of storms, and a weak convergence zone remaining across the NC
zones should encourage new rounds of showers and storms into
evening. Forecast soundings in this area show substantially
deep warm cloud depth. The HREF probability of exceeding 3"
in 3 hrs has a 50pct bullseye encompassing the nrn foothills/nw
Piedmont at 00Z Tuesday, which is right around what our 3 hr flash
flood guidance is currently. The passage of the convection this
morning will probably lay down enough rain to lower the FFG even
more. Some places up in the nrn part have had excessive rain in
the past week. The overnight Day 1 Excessive Rain Outlook brought
the Slight Risk farther southwest in NC. Many ingredients for an
elevated risk of flash flooding appear to have come together, and
with the early morning activity having primed portions of the CLT
metro, we chose to expand the Flash Flood Watch issued on the mid
shift (for the nrn mtns, foothills, Piedmont zones) into Lincoln,
Gaston, Mecklenburg, and Cabarrus counties. Union NC was left out
mostly because they missed out on the early morning activity. Not
going to rule out a few severe storms in the I-77 corridor either,
within a plume of 1000-2000 J/kg sfc-based CAPE may develop. The
isolated tornado risk mentioned by SPC in the D1 outlook will be
mainly associated with the morning activity due to the enhanced
shear associated with the LLJ, but sufficient deeper shear will
persist thru the day to support a Marginal Risk for damaging
wind and perhaps hail later on. Activity will carry on into the
evening as a short wave passes to the north. Most of the precip
will diminish or lift off to the northeast in the late evening
hours. High temps might stay a few degrees below normal owing to
extensive cloud cover. Low temps will remain mild.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/...
As of 1215 AM Monday: The main story Tuesday through Thursday night
will be the building heat. A rather stagnant pattern will be in
place Tuesday through Thursday night as a nearly stationary Bermuda
High pinwheels off the Southeast coast. Weak shortwave energy
passing to the north will largely not affect apparent weather in our
area as long as we are under the protection of the Bermuda high.
Showers and thunderstorms and associated cloud cover will decrease
in coverage and become more diurnally focused as any lingering wedge
or boundaries and associated upglide dissipates Tuesday with the
ridge dominating the weather Wednesday and Thursday. High
temperatures will respond in kind with widespread highs in the low to
mid 90s outside of the mountains and peak heat index values near 100
for portions of the I-85 corridor and points south.

&&

.LONG TERM /FRIDAY THROUGH SUNDAY/...
As of 1245 AM Monday...A progressive polar jet with embedded energy
associated with a trough over the Great Lakes will finally begin to
break down the Bermuda High this weekend. A 1025 continental high
pressure system on the heals of a weak cold front will press from
the mid-Mississippi Valley into the Mid-Atlantic by this weekend,
likely providing enough oomph to push the boundary at least into the
forecast area. A separate frontal boundary near the Gulf Coast may
active towards southern areas, providing additional lift and
moisture over the feature as well as a short-lived wedge that may
take shape as the surface high presses offshore. The net result is
increases showers and thunderstorms with the initial front late
Friday, continued into the weekend with less in the way of thunder
owing to the extensive cloud cover and retreating wedge. One more
hot day is likely Friday, especially east of the mountains, before
temperatures tumble to near to below normal over the weekend.

&&

.AVIATION /13Z MONDAY THROUGH FRIDAY/...
At KCLT and elsewhere: The first group of thunderstorms should exit
to the east early this morning. In behind this activity, a ceiling
restriction was indicated by the guidance, but it has been slow
to materialize. MVFR will probably hold on at KCLT for an hour or
two in the wake of the recent storms, but even if it breaks up,
it should be temporary and should fill back in with only minimal
heating. Meanwhile, a low cloud ceiling upstream over north Georgia
will most likely be what advects in and brings the ceiling down
across the remainder of the TAF sites. Convectively speaking,
after the morning activity moves out, we could be in something
of a lull for the better part of the morning outside of the nrn
foothills/NW Piedmont. Prefer to hold onto my PROB and TEMPO for
the better chances of storms in the late afternoon and evening
hours even though showers cannot be ruled out at any time during
the day. The ceiling restriction should gradually lift and scatter
during the middle part of the day, with VFR after that outside of
storms. Wind direction will be all over the place, but mainly E/SE
for most of the day. This evening, another round of showers and
thunderstorms is expected, particularly over the NC part of the fcst
area. This will be handled with PROB30s for the time being. Once
that activity moves out in the early morning hours Tuesday, more
low cloud restrictions should be expected, perhaps widespread IFR.

Outlook: More typical coverage of diurnal showers/storms is expected
Tuesday into late week. Early morning low cloud/fog restrictions
will also be possible over the mountain valleys most days.

Confidence Table...

            13-19Z        19-01Z        01-07Z        07-12Z
KCLT       High  91%     High 100%     High  94%     High  80%
KGSP       High  80%     High  98%     High  99%     Med   79%
KAVL       High  83%     High  95%     High  84%     Med   65%
KHKY       Med   79%     High  80%     High  80%     Med   77%
KGMU       Med   79%     High  98%     High  98%     Med   73%
KAND       High  83%     High 100%     High 100%     Med   75%

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...Flash Flood Watch through late tonight for NCZ033-035>037-049-
     050-056-057-069>072-501>506.
SC...None.

&&

SYNOPSIS...65
NEAR TERM...PM/08
SHORT TERM...65
LONG TERM...65
AVIATION...PM

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
931 AM EDT Mon Aug 31 2020

NCZ048-053-065-068-082-507>510-SCZ003-006>009-012>014-011345-
Madison-Buncombe-Henderson-Cleveland-Union NC-Rutherford Mountains-
Greater Rutherford-Polk Mountains-Eastern Polk-Greenville Mountains-
Greater Greenville-Spartanburg-Cherokee-York-Laurens-Union SC-
Chester-
931 AM EDT Mon Aug 31 2020

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

.DAY ONE...Today and tonight.

Scattered thunderstorms developing today, and possibly continuing
into tonight, will pose a risk of cloud-to-ground lightning and
torrential downpours. A handful of storms could produce damaging
wind gusts. Isolated flash flooding is also possible particularly
where heavy rain has fallen recently.

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

No hazardous weather is expected at this time.

.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>037-049-050-056-057-069>072-501>506-011345-
Avery-Alexander-Iredell-Davie-Yancey-Mitchell-Catawba-Rowan-Lincoln-
Gaston-Mecklenburg-Cabarrus-Caldwell Mountains-Greater Caldwell-
Burke Mountains-Greater Burke-McDowell Mountains-Eastern McDowell-
931 AM EDT Mon Aug 31 2020

...FLASH FLOOD WATCH IN EFFECT THROUGH LATE TONIGHT...

This Hazardous Weather Outlook is for piedmont North Carolina and
western North Carolina.

.DAY ONE...Today and tonight.

Please listen to NOAA Weather Radio or go to weather.gov on the
Internet for more information about the following hazards.

   Flash Flood Watch.

Also, scattered thunderstorms producing cloud-to-ground lightning are
expected today. A few storms could produce damaging wind gusts.

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

No hazardous weather is expected at this time.

.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.

GAZ010-017-018-026-028-029-NCZ051-052-058-059-062>064-SCZ001-002-004-
005-010-011-019-011345-
Rabun-Habersham-Stephens-Franklin-Hart-Elbert-Swain-Haywood-Graham-
Northern Jackson-Macon-Southern Jackson-Transylvania-
Oconee Mountains-Pickens Mountains-Greater Oconee-Greater Pickens-
Anderson-Abbeville-Greenwood-
931 AM EDT Mon Aug 31 2020

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

.DAY ONE...Today and tonight.

Scattered thunderstorms developing today, and possibly continuing
into tonight, will pose a risk of cloud-to-ground lightning and
torrential downpours. Isolated flash flooding is also possible
particularly where heavy rain has fallen recently.

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

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

None.

8

Special Weather Statement
National Weather Service Greenville-Spartanburg SC
1001 AM EDT Mon Aug 31 2020

NCZ082-311445-
Union NC-
1001 AM EDT Mon Aug 31 2020

...A THUNDERSTORM WITH TORRENTIAL RAINFALL WILL IMPACT NORTHEASTERN
UNION COUNTY UNTIL 1045 AM EDT...

At 1000 AM EDT, Doppler radar was tracking a thunderstorm with
torrential rainfall 6 miles northeast of Monroe, or near Unionville,
moving east at 15 mph.

Wind gusts up to 40 mph, half inch hail and very heavy rainfall rates
up to 2 inches per hour will be possible in these areas.

LAT...LON 3496 8030 3495 8054 3513 8058 3517 8046
      3516 8044 3517 8042 3516 8041 3518 8037
      3517 8036 3518 8033 3517 8032 3519 8030
      3520 8028
TIME...MOT...LOC 1400Z 258DEG 12KT 3506 8048

CSH

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