Severe Thunderstorm Warning 9, NWS Greenville-Spartanburg, SC

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
301 PM EDT Mon Apr 8 2019

.SYNOPSIS...
A southern-stream low pressure system will continue to approach
the forecast area from the west this evening and early tomorrow
with the potential to produce organized thunderstorms. Drier high
pressure will return on Wednesday before another cold front
approaches from the west later in the week.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 245 pm Monday: Clusters of heavy rain and thunderstorms have
developed across the western 2/3 of the forecast area, with SBCAPE
values ranging from 3000 J/kg in the southern Upstate to 2500 J/kg
in the NC Piedmont. Severe threat east I-77 looks the most
substantial, with a line of storms likely organizing as a result of
the influence of a well-defined MCV over east-central TN. Areas to
the west of I-85 have generally been worked over, though can't rule
out the potential for some strong storms anywhere in the forecast
area as increasing upper flow creates more favorable shear profiles
through the afternoon. With 850 winds on the order of 40kt (a few
mtn obs sites are seeing SWly gusts to 30kt+) and decently steep low-
level lapse rates, strong winds and possible large hail are the
primary threats with convection with afternoon and evening, though a
brief tornado cannot be ruled out in the NC Piedmont where
hodographs have a bit more curvature in the low levels. The
increased severe threat follows the enhanced risk area well.

Convection will likely linger (per CAMs) through the evening and
early overnight period, though this will likely be weaker overall
(just isolated thunder) but more widespread. With heavier convective
rain rates being observed and PWs decent, some isolated hydro issues
can't be ruled out late tonight, though antecedent conditions are
finally better than "soggy" and most areas may be able to handle the
short periods of heavy rainfall just fine. Temperatures this
afternoon will be near to just above average.

The fun won't end this evening, however. The upper low supporting
much of today's convection will still be meandering through the
southeast tomorrow afternoon, and 500mb winds will be strong enough
to create decent shear profiles (near 40kt 0-6km), at least across
the southern Upstate. Plentiful low cloud will remain over the area
through tomorrow morning, but if some areas are able to destabilize
during the late morning and early afternoon hours, some stronger
storms (with wind being the main threat) will again be possible.
Much of the severe threat depends on the convection that occurs
overnight and how much convective debris remains during the morning
hours. The day 2slight risk from SPC looks well-placed for the areas
that are most likely to destabilize tomorrow afternoon.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 215 PM EDT Monday: the short-term fcst picks up at 00z
on Wednesday with the southern stream upper trof passing to our
south as an upper ridge spreads over the Central CONUS. By late
Wed/early Thurs the trof should be moving offshore as heights
recover and the ridge axis moves over the SE. At the same time,
a broad upper trof will amplify as it moves over the Western
CONUS. By the end of the period late Thurs, the ridge will have
amplified over the SE as the trof moves closer to our region.
At the sfc, a fairly complex synoptic pattern will exist early
in the period as what's left of a sfc low drifts eastward across
the extreme SE while Canadian high pressure pushes its way over
the Great Lakes and farther south into the Carolinas. The low
should dissipate by early Thursday as it drifts offshore. As this
occurs, the high will shift farther SE and try to establish a
very brief wedge pattern over the fcst area on Thurs. Simultaneously,
an impressive-looking low will deepen over the Great Plains and
bring a robust cold front to our doorstep early Fri and push the
high further off the Atlantic Coast. The front is expected to
move thru the fcst area fairly quickly and be east of the CWFA
by early Sat.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 210 PM EDT Monday: the extended fcst picks up at 00z on Friday
with steep upper ridging over the Eastern CONUS and amplified upper
trofing to our west. By late Friday/early Saturday the ridge axis
will get pushed offshore as the upper trof lifts to the northeast
and towards New England. As the weekend progresses, heights will
begin to recover over the fcst area as the upper trof moves offshore.
At the same time, a southern stream upper shortwave will amplify to
our west and approach the CWFA. This feature is expected to move
over the CWFA, or lift just north of it, by the end of the period
next Monday. At the sfc, a large and deep low pressure system will
be making its way towards the Great Lakes by early Friday as the
cold front associated with the system moves towards the Western
Carolinas. The long-range guidance has the front moving thru the
fcst area on Fri and off the coast by early Sat. The front looks
fairly dry with minimal deep-lyr moisture associated with it. In
is wake, drier air will briefly overspread the area on Saturday.
By early Sunday, another low is expected to rapidly spin-up over
the ArkLaTex region and quickly lift to the NE. This will cause
low-lvl flow to become SLY as deep-lyr moisture surges back over
the region from the gulf. The cold front associated with this low
will likely move thru the area by the end of the period on Monday,
however there are some timing differences between the models.
Temperatures will continue to run 5 to 10 degrees above normal,
keeping frost out of the picture through the week.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Convection is ongoing across the area, with
some form of TS/SHRA/TSRA featured at all TAF sites this afternoon
and evening. Multiple clusters of convection are possible at all
sites through at least 00Z at eastern sites and at least 02Z at
KCLT/KHKY. Scattered showers will linger through the early overnight
period at most sites as well, though and end to rain is expected at
all sites by 0906Z. At that point, likely looking at another period
of widespread low cigs and at least IFR restrictions tomorrow
morning. Winds are generally light SWly and will remain so through
the night, except for the higher gusts possible in thunderstorms and
related outflows.

Outlook: Periods of low cigs, patchy fog, and convection will remain
possible across the area into Tuesday. Drying is expected late
Tuesday through Wed, before another front approaches late in the
week.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  92%     High  86%     High  86%     High  89%
KGSP       High  80%     Med   75%     High  81%     High  86%
KAVL       High  80%     Med   65%     Med   63%     High  85%
KHKY       Med   76%     Low   57%     Med   68%     High  86%
KGMU       Med   65%     Med   78%     High  90%     High  90%
KAND       High  87%     High  86%     High  82%     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 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...JPT
NEAR TERM...Carroll
SHORT TERM...JPT
LONG TERM...JPT
AVIATION...Carroll

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