Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191049

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
649 AM EDT Mon Mar 19 2018

A stalled surface front will remain draped just south of the area
today as a complex low pressure system arrives from the west
tonight. Abundant moisture ahead of the low will bring rain showers
at times with instability producing some thunderstorms tonight. The
trailing low pressure system will cross the region Tuesday through
Tuesday night, with falling temperatures in the wake of the system
producing mountain snow showers through Wednesday. Snow showers
could spill out along the I-40 corridor as well. Drier high pressure
will return late in the week, but with another moist low arriving
from the west for the weekend.


As of 650 AM EDT Monday: Main update for 12Z TAF issuance along
with minor tweaks made to morning temperatures/PoPs and Wx
(thunderstorm coverage) for tonight, as the rest of the near
term forecast period remains on track.

Showers continue to move eastward across the SC counties this
morning with the aid of isentropic lift/upglide, as isolated
showers from the west will continue to propagate eastward per
latest radar trends. A lull in precipitation is expected over
the next few hours, before activity picks up again and spreads
into the NC counties.

Otherwise, after a quiet end to the weekend, unsettled weather will
make it`s journey across the Southeast today as a potent storm
system makes it`s way out of the central Plains and into the
northern TN Valley tonight, bringing severe weather to the Deep
South this afternoon. With a quasi-stationary front extending from
this system down along the Gulf states and eastward off the eastern
seaboard, closer to home, sfc high pressure continues to push off
the Carolina coast this morning.

Temperatures will be tricky today as WAA bumps up against an insitu
wedge from the sfc high offshore, which may weaken a bit today per
latest guidance. For now, anticipate temperatures across the
northern portions of the FA along with the NC mountains (currently
lower 50s, cooler across the higher elevations) to only increase by
a couple of degrees this afternoon from what they are now, with
temperatures along and south of the I-85 corridor expected to warm
into the upper 50s/lower 60s.

Isolated to scattered thunderstorms are expected to develop tonight
as the front to the south lifts northward as a warm from through the
afternoon and evening hours, and the sfc low moves through into the
upper TN Valley towards the Carolinas. Increasing (elevated)
instability and shear (hodographs from fcst soundings are quite
impressive) will become available across the southern portions of
the FA (into the southwestern NC mountains) as a result, increasing
the potential for severe weather. For now, am thinking the potential
for any severe weather (primary threats: damaging winds and hail,
though an isolated tornado could not be ruled out) will increase
just after midnight south of the I-85 corridor, heading into the
lower piedmont during the early morning hours. Areas elsewhere will
see scattered showers through the overnight hours, though an
embedded isolated (sub-severe) thunderstorm could not be entirely
ruled out. Do expect any thunderstorm activity to be over by
daybreak on Tuesday as the sfc low pushes overhead and it`s
accompanying cold front pushes through, with a secondary cold front
setting up to our west. Cooler temperatures are expected overnight
into Tuesday morning, with lower 50s along and south of the I-85
corridor, into the low to mid 40s elsewhere.


As of 315 AM Monday: The snowfall potential appears to be on the
upswing for at least the North Carolina mountains in the Tuesday
night to Wednesday night period, with snow showers also possible out
over the NC foothills/piedmont at times Wednesday morning.

For the specifics, a pronounced shortwave embedded in the complex
central/eastern CONUS trough will cross the region from the west
Tuesday through Tuesday night. The associated surface low will pass
over the area Tuesday and then deepen off the Outer Banks Tuesday
night. Meanwhile, a mid-level dry slot will wrap across the region
Tuesday morning, before deeper moisture, lift, and steeper lapse
rates aloft return from the west through Tuesday afternoon/evening.
Surface cold wedge conditions should get reinforced on Tuesday
despite the lack of precipitation early, resulting in a very sharp
temperature gradient from NE to SW. Cannot rule out some thunder
Tuesday afternoon given the steeper lapse rates, but wind profiles
will be rather tame and the better instability should keep any
severe thunderstorm threat off to our southeast.

Snow levels will steadily fall from the west Tuesday night behind
the coastal low. Low-level northwest flow moisture will
simultaneously ramp up and a low to mid-level deformation zone could
provide a measure of frontogenesis along the Blue Ridge as well
early Wednesday. Snow showers should thus increase in coverage and
intensity Tuesday night through Wednesday, with any better shower
rates at all providing some potential for light snow accumulations
across the NC foothills and piedmont as well Wed. morning. In
addition to the snow potential, 45 to 50 kt 850 mb winds during the
peak of the cold advection could produce some advisory level wind
gusts across the high terrain as well. Anticipate the northwest flow
moisture to gradually dry up Wednesday night. For now, will hit the
snow and wind potential in the HWO and via Decision Support
briefings, but delay any Watch/Warning/Advisory issuances to later
shifts given the current marginal confidence on any Warning criteria.


As of 325 AM Monday: Heights will rise from the west on Thursday,
with deep layer northwest flow persisting through Friday. Dry
surface high pressure will briefly dominate the forecast area into
Friday morning, but with some activation along a nearby warm front
to the west allowing PoPs for rain showers to creep back into the
picture. Falling heights over the central plains Friday night will
then sweep eastward through the central Appalachians through
Saturday night. Moisture will quickly return in the westerly flow
ahead of the system, with PoPs for all liquid ptypes rising sharply
along the reactivated warm front Friday night, and then persisting
through Saturday. A trailing cold front will cross the region
Saturday night, and northwest flow moisture could wring out
northern/western mountain snow showers on Sunday morning. A strong
ridge will return to the east on Sunday.


At KCLT and Elsewhere: VFR cigs have held on longer than previously
expected, even with -SHRA moving across the SC TAFs which only
created low end VFR cigs, though a brief patch of MVFR cigs moved
over KCLT within the past hour. With this update, have pushed back
the onset of areas of MVFR cigs by a few hours, with the gradual
deterioration to widespread MVFR this aftn and IFR tonight as moist
upglide brings in lower cigs. However will note, with insitu wedge
in place, some guidance continues to suggest minor weakening of this
feature, which could allow brief improvement in flight conditions,
especially at KCLT. Otherwise, expect VCSH/-SHRA with areas of fog
at times throughout the day, with a potent storm system bringing the
potential for -TSRA at the SC TAF sites and potentially KCLT
overnight tonight into the early overnight hours. Lingering low
cigs, fog and precip are expected into Tuesday, with a mix of
MVFR/IFR prevailing.

Outlook: Colder air wrapping around the exiting system will bring
snow to the mountains, and potentially mixed precip to to KAVL, KHKY
and KCLT Tuesday night into Wednesday morning, VCSH/-SHRA is
expected elsewhere. MVFR/IFR anticipated, with VFR returning late

Confidence Table...

            10-16Z        16-22Z        22-04Z        04-06Z
KCLT       High  86%     High  87%     Med   77%     Med   75%
KGSP       Med   77%     High  94%     Med   75%     High 100%
KAVL       High  94%     High  83%     High 100%     High 100%
KHKY       High  80%     High 100%     High  80%     High  93%
KGMU       Med   65%     High 100%     High  80%     High 100%
KAND       Low   59%     High 100%     High 100%     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 and ensemble forecasts
are available at the following link:




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