Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190800

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
400 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 350 AM EDT Monday: After a quiet end to the weekend, unsettled
weather begins it`s journey across the Southeast today as a potent
storm system slowly makes it`s way out of the central Plains,
bringing severe weather to the Deep South. 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. With the aid
of isentropic lift/upglide and SW flow infiltrating in plenty of
moisture across the FA, latest radar imagery depicts weak returns
moving across the southwestern NC mountains and higher
reflectivities across northeast GA and the extreme southern portions
of the Upstate attm. Elsewhere, conditions remain dry as opaque high
cloud cover persists with low to mid clouds beginning to move in. Do
anticipate showers to gradually expand across portions of the
western Carolinas over the next few hours as showers are already
moving across eastern TN.

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.

As the front lifts northward as warm front into the evening hours
and the sfc low moves through into the upper TN Valley, increasing
elevated instability and shear (hodographs from fcst soundings are
quite impressive) will come into play across the extreme southern
portions of the FA. 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 stay confined well
south of the I-85 corridor across the southern portions of the FA
(northeast GA eastward through SC), occurring late, roughly between
midnight through just before daybreak. Could not rule out an
isolated (subsevere) thunderstorm or two making it`s way up near the
Charlotte Metro area as well. 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 will prevail over the next few hours, as
conditions are then expected to gradually deteriorate to MVFR with
lowering cigs and increasing chances for -SHRA through the early
morning hours due to increasing low level moisture and isentropic
lift. After daybreak, the continuation of increasing moisture and
moist upglide will aid in the further lowering of cigs, with IFR
anticipated through the rest of the afternoon. However, if the
insitu wedge weakens any this afternoon, a brief improvement to MVFR
is possible, especially at KCLT. Otherwise, expect VCSH/-SHRA
throughout the day, with a potent storm system bringing the
potential for -TSRA at the SC TAF sites, along with KCLT late this
evening into the early overnight hours. Lingering low cigs and
precip are expected into Tuesday, with a mix of MVFR/IFR.

Outlook: Colder air may wrap south into the region behind the system
into Tuesday night and early Wednesday, allowing for the potential
for mixed precipitation. Expect conditions to retreat to VFR on

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  91%     Med   78%     High  82%     Med   69%
KGSP       High  94%     High  95%     High  91%     High  94%
KAVL       High  93%     High  83%     High  98%     High 100%
KHKY       High  83%     High  98%     High  89%     High  98%
KGMU       High  85%     High  95%     High  89%     High  98%
KAND       High  81%     High 100%     High 100%     High  93%

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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