Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191414

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1014 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 1000 AM EDT Monday: With some peaks of sunshine across the
mountains and an area of clearing heading east, have made
adjustments to afternoon high temperatures to bump them up just a
tad. The clouds should fill back in, but there will be a bit of bust
potential depending how long the sunshine lasts. Have lowered pops
this morning, trending into the previous forecast for this afternoon.

Otherwise, after a quiet end to the weekend, unsettled weather will
make its 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: Trends still look good for KCLT for the
14z AMD, so no changes at this time. Still expect 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...

            14-20Z        20-02Z        02-08Z        08-12Z
KCLT       High  87%     High  94%     High  82%     High  98%
KGSP       Low   58%     High  85%     High  92%     High  95%
KAVL       Med   68%     High  88%     High  88%     High  82%
KHKY       High 100%     High  93%     High  83%     High  88%
KGMU       Low   57%     High  90%     High  92%     High  81%
KAND       High  93%     Med   79%     High  84%     High  90%

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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