Tornadoes and Damaging Winds in South Carolina, April 10, 2021

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
248 PM EDT Sat Apr 10 2021

.SYNOPSIS...
Scattered showers and thunderstorms are possible this afternoon into
the evening as a weak cold front crosses the region.  High pressure
builds in behind the departing cold front Sunday into Monday,
leading to drier conditions. Another cold front will likely cross
the area during the middle of next week, but without much moisture
to work with, this front may struggle to produce precipitation, and
inactive weather could prevail through next Friday.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 230 PM: Minor adjustments to the PoPs and Max T's as showers
become more widespread over the western half of the CFWA. With the
convective debris overspreading the area from the large Deep South
MCS this morning. The enhanced cloud cover has caused a drop in the
forecasted highs for today and as well as instability all together,
with 50s and 60s in the mountains and 70s elsewhere. Mesoscale
analysis shows ~500-1000 J/kg of SBCAPE over the Charlotte area and
portions of the Upstate. Once the activity is able to tap into this
environment, thunderstorm development should take place as the pre-
frontal showers and thunderstorms track across the CFWA.

Otherwise, a deep stacked low currently located over the mid MS
Valley will pivot northeast towards the Great Lakes through the
period. In response to this, an impulse shortwave will track from
the Deep South through the Southeast during the afternoon/evening
hours. With marginal instability in place over the I-85 and I-77
Corridor, a ~35-45 kt LLJ will help support convective development
during the afternoon/evening. The shortwave trough should enter the
region as a vigorous vort axis slides in as the last line of storms
begin to work into the CFWA between ~20Z-03Z. This is capable of
producing organized convection in the form of a small MCS/QLCS,
which is indicated in high-res CAMs late this afternoon through this
evening. If this scenario prevails, strong to severe thunderstorms
will be possible. A few discrete cells out ahead of this line could
show vague supercellular structure, especially considering the
moderate shear parameters and relatively destabilized environment.
However, weak mid-level lapse rates and only modest available
instability will likely limit large hail potential, so damaging wind
is expected to be the main threat with these cells. Additionally,
moderate helicity values, veering at low levels, and low cloud bases
supports a very low end risk for a tornado. One other concern in
this set up are the 1"-1.5" PWAT values, upslope enhancement, and
heavy rain rates could cause minor flash flooding issues, mainly
over the Escarpment through the rest of this evening.

Most of the showers and thunderstorms should push out of the CFWA no
later than 06Z as the occluded front works its way into the region.
This is expected to make a full fropa across the CFWA by lunchtime
Sunday as the upper trough axis and associated front will likely
swing through the western Carolinas and northeast GA during the day
as well. Low stratus will likely stick around before daytime heating
and fropa occurs. High-res CAMs do initiate a few showers in the
southern and eastern tiers between 12Z-18Z. The environment is
adequate for weak convective development once again, but will likely
be capped during the window of opportunity, especially when
factoring in the amount of drier air aloft that's expected to filter
into the area during this time. The rest of Sunday looks warm and
dry with partly cloudy skies once the low stratus burns off.
Temperatures will be in the 60s and 70s over the mountains, while
the foothills/piedmont reach into the mid to upper 70s.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
As of 230pm EDT Saturday:  Fairly quiet weather pattern for Monday
and Tuesday, with region in a dry post-frontal regime. Upper flow
flattens as trough moves eastward, with a slight rise in
thicknesses. Westerly to northwesterly surface flow behind front
continues at a brisk 10 to 20 kts on Monday, with a gradual
weakening of winds on Tuesday. Forecast soundings show a little
moistening aloft and a littler recovery of depressed dewpoints on
Tuesday, leading to some scattered clouds, and maybe an isolated
brief shower in the mountains Tuesday afternoon.  Highs and lows
Monday and Tuesday look similar, and will run 5 to 10 degrees above
normal.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
As of 200 PM Saturday: An upper level low north of of the Carolinas
situated over the Ohio Valley will track eastward into the western
Atlantic Tuesday night into Wednesday. The associated cold front
with this system will be east of the CWA at the start of the long
term. This cold front will continue to push eastward Tuesday evening
into Wednesday as Canadian high pressure builds in behind the FROPA
across much of the central and eastern CONUS. Global models are in
good agreement in trending drier for much of the extended forecast.
Therefore maintained a slight chance for PoPs Wednesday into Friday
in collaboration with neighboring WFOs and also due to a few models
showing isolated chances. The cold front looks to stall across the
northern Gulf of Mexico Thursday into Saturday. Meanwhile a low
pressure system tracking out of the northwestern CONUS will fracture
into multiple shortwaves. A few of these shortwaves will eject into
the central CONUS. These shortwaves will interact with the stalled
front to the south near the Gulf Coast potentially increasing rain
chances for the region towards the end of the forecast period. High
temps are expected to return to near normal or just a degree or two
below normal in the extended.

&&

.AVIATION /19Z SATURDAY THROUGH THURSDAY/...
At KCLT and Elsewhere: Cigs have teetered between MVFR and IFR
over the last 12-18 hours, with some spots even at VFR occasionally.
This continues to bring low confidence in the TAFs. A more potent
deck of clouds has moved in from the southwest due to the convective
debris from the morning MCS over the Deep South. This could perhaps
limit instability, but buoyancy has developed over the Upstate and
KCLT area. Looking at the timing of occluded front and associated
peak probs for SHRA/TSRA low cigs/vsbys should be on track, but
could vary by an hour or two. Used tempo/prevailing to pick the most
likely window for TSRA at all terminals. Model guidance continued to
indicate an organized squall line with a wind shift from roughly 160
to 220 for most sites with the front itself. Brief moderate LLWS
will be possible along and just ahead of the front. IFR or lower
cigs/vsbys are likely at least for a brief period at all terminals
east of the mountains, including KCLT once the squall line pushes
off to the east overnight. Low stratus will likely stick around for
parts of the morning hours as the front slides across the CFWA
during that time. Kept IFR/MVFR cigs in through ~14Z-16Z, with
restrictions relaxing a bit and ending the TAF period with VFR
conditions and low end wind gust. Model guidance indicate vsby
restrictions over the French Broad Valley overnight into the early
morning hours, but confidence is low enough for it not to be
mentioned in the KAVL 18Z TAF, but that could change with the next
update.

Outlook: VFR conds return Sunday and prevail into early next week.

Confidence Table...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       Med   75%     Med   75%     Med   78%     High  88%
KGSP       Med   70%     Med   77%     Med   74%     High  89%
KAVL       Med   77%     Med   75%     Med   76%     High  87%
KHKY       Med   78%     High  80%     Med   72%     Med   78%
KGMU       Med   71%     Med   75%     Med   76%     High  92%
KAND       Med   77%     Med   74%     High  89%     High  95%

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...MPR
NEAR TERM...CAC
SHORT TERM...WJM
LONG TERM...AP
AVIATION...CAC

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
605 AM EDT Sat Apr 10 2021

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-111015-
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-
605 AM EDT Sat Apr 10 2021

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

.DAY ONE...Today and tonight.

Numerous to widespread showers and thunderstorms are expected across
the area today, mainly from late afternoon through the evening.
Isolated severe thunderstorms are possible, with damaging straight
line wind gusts and perhaps a brief, weak tornado being the primary
threats. Higher rainfall rates associated with thunderstorms could
also result in localized flash flooding, primarily in urban areas and
other locations that drain poorly. Frequent cloud to ground lightning
can occur with any thunderstorm.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

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.

Lane