Severe Thunderstorm Warning 239, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1105 AM EDT Thu Aug 22 2019

.SYNOPSIS...
A warm and humid airmass will persist across the region today as a
cold front sets up from the Ohio Valley to the Mid-Atlantic. The
front will approach the western Carolinas on Friday and then settle
slowly south through the forecast area on Saturday. A cool and moist
wedge of high pressure will set up behind the front on Sunday and
linger through Monday. Another cold front will approach from the
west during the middle of next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 1045 AM EDT Thursday: With temp/dew point trends looking good,
the only appreciable changes made to the near term forecast were
made to increase pops for this afternoon/evening, especially along
and north of the I-40 corridor where CAMs are beginning to focus
more PM convection.

A relatively flat upper level flow will exist atop the forecast area
today, as a trough digs across the Great Lakes and suppresses the
upper ridge to our south. While a few small vort maxes will ripple
thru the quasi-zonal flow, the bulk of the height falls and upper
forcing will be across the Ohio Valley and Central Appalachians. The
low to mid lvl flow will increase slightly across the region ahead
of this digging trough, resulting in a slight uptick in deep-layer
shear (in the 15-20 kt) range. But overall, convection should remain
fairly disorganized. The latest SPC Day 1 Outlook does have a
marginal risk that includes the northern half of the forecast area.
Forecast soundings show potential for 2000-2500 J/kg of sbCAPE with
some mid-lvl dry air that will increase the microburst potential.
Convective coverage looks pretty scattered today, with the usual
mountain-triggered storms starting first during early aftn hours,
then expanding/drifting east across the NC foothills and piedmont
mid-aftn thru early evening. Coverage over NE GA and the Upstate
looks more isolated. Temps will continue to be slightly above
normal, similar to yesterday's highs, perhaps a deg or two cooler.

Tonight, the right entrance region of an upper jet will pivot south
to the TN Valley and western NC overnight. This combined with linger
instability and increasing SWLY WAA 850 mb flow will support a chc
PoP well into the night across the western third or so of the
forecast area. Lows will be a couple categories above normal under
partly to mostly cloudy skies.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SATURDAY NIGHT/...
As of 245 am EDT Thursday: A sharp, positively tilted upper trough
axis will stretch from a closed low center over eastern Quebec to
the eastern Great Lakes by Friday. South of this trough, a surface
cold front will settle southward toward the western Carolinas, but
keeping the forecast area in the warm and moist sector through the
day. Abundant moisture, with profiles exhibiting PWAT values around
2 inches, will pool across the region ahead of the front. The
moisture will combine with forcing from both the approaching frontal
zone at low levels and right entrance region jet divergence aloft to
generate fairly widespread showers and thunderstorms - especially
across the northern tier of the forecast area. Surface based
instability and 850 to 500 mb lapse rates will not be terribly
impressive Friday afternoon given the warm and moist profiles, but a
modest uptick in surface to 3 km bulk shear to around 25 to 30 kt
along and north of I-40 could support a few severe thunderstorms.
Heavy rainfall in the high precipitable water airmass may be the
primary hazard, with flash flooding becoming possible with any
training convection. However, confidence remains too low for any
Flash Flood Watches at this point.

The boundary should steadily sag southward through the forecast area
through the day on Saturday. Likely PoPs for showers and
thunderstorms will continue, but with instability getting shunted to
the southern half of the forecast area as cold air damming slowly
sets up across the northwest NC piedmont late in the day. Lapse
rates continue to appear fairly unimpressive through Saturday
afternoon, but still cannot rule out a few severe storm along and
south of the immediate wedge front in the pooled instability. Hydro
concerns will once again arise with any training convection along or
near the front.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 300 am EDT Thursday: Moisture could become just a bit
shallower on Sunday as the surface wedge front sags just south of
the forecast area, and instability will certainly be less with cold
air damming getting fully established. However, strengthening
upglide and upslope flow in and atop the cold air damming layer
should present continued solid PoPs for mainly light to moderate
rain showers. Will relegate any thunder mention to mainly the
southwest NC mountains by Sunday afternoon. Slightly drier air
arriving in the nose of the damming high pressure centered over NY
state should keep precipitation amounts lighter than observed
Fri/Sat, but there should be enough QPF to maintain the wedge, with
most locations east of the mountains struggling in the 70s through
the day on Sunday.

Weak upper ridging may briefly return from the west through Sunday
night, but this will be quickly followed by modest height falls west
of the Appalachians early next week. This could support a resurgence
of upglide flow over the lingering damming layer, and little more
than weak shallowing of the wedge is now expected through late
Monday. A broad upper trough will then set up over the eastern CONUS
through mid week. Deeper moisture may return to the southeast ahead
of the upper trough axis and the associated approaching cold front.
Significant drying now looks unlikely until mid to late next week
with a cold fropa from the west.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
At KCLT and elsewhere: All sites currently VFR, though another round
of scattered shra and tsra is expected this aftn, starting in the
mountains and NC foothills, where VCTS will be carried in the 18-21z
time frame. Then activity will work east, with more isolated
coverage developing across the Upstate. Have kept in prob30 at most
sites in the 21-03z time frame based on model consensus of timing,
though these will likely be transitioned to tempos or possibly even
prevailing convection at Foothills sites (KHKY, KCLT). A light, moist
SWLY flow will continue tonight, which may support lingering shra
and tsra across the western parts of the forecast area. Coverage
should be more isolated and confidence is too low to keep any precip
mention overnight. Mid clouds are expected overnight, but mountain
valleys will likely fog up again, so will mention some IFR vsby at
KAVL late. Sfc winds will prevail out of the SW and should stay
under 10 kts today.

Outlook: SHRA/TSRA chances will remain across the area as a front
lays over the area from the north and stalls through the weekend.
Some morning fog and low stratus will be possible each day,
especially in the mountain valleys. Cold air damming will set up
over the weekend, keeping lower cigs possible.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  85%     Low   36%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     High 100%     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 and ensemble forecasts
are available at the following link:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

SYNOPSIS...HG
NEAR TERM...Carroll
SHORT TERM...HG
LONG TERM...HG
AVIATION...ARK/Carroll

Hazardous Weather Outlook
National Weather Service Greenville-Spartanburg SC
423 AM EDT Thu Aug 22 2019

GAZ010-017-018-026-028-029-NCZ033-035>037-048>053-056>059-062>065-
068>072-082-501>510-SCZ001>014-019-230830-
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-
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-Chester-Greenwood-
423 AM EDT Thu Aug 22 2019

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

.DAY ONE...Today and tonight.

Scattered to numerous afternoon and evening thunderstorms will
develop today. Any of these thunderstorms will have the potential to
produce cloud-to-ground lightning strikes, locally heavy rainfall,
and gusty winds. A few thunderstorms could become severe, with
damaging wind gusts or quarter size hail, especially along or near
the Interstate 40 corridor. Localized flooding will be possible in
any areas affected by multiple thunderstorms. Isolated to scattered
thunderstorms may continue into the overnight hours.

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

Abundant moisture will set up over the region on Friday ahead of a
cold front approaching from the north. Instability in the warm and
moist air ahead of the front may contribute to locally excessive
rainfall, mainly during the afternoon and evening hours on Friday.
Flooding of low-lying and poor drainage areas will be possible at any
locations repeatedly affected by showers and thunderstorms. Small
streams could approach bankfull where heavy rain falls.

These conditions will linger into Saturday as the front slowly moves
south across the area.

.SPOTTER INFORMATION STATEMENT...

Spotters are requested to forward any reports of large hail,
damaging winds, tornadoes, or flash flooding to our office.

HG

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