Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210028

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
828 PM EDT Tue Mar 20 2018

A complex low pressure system will gradually move off the Atlantic
Coast tonight and tomorrow with colder air overspreading the region
in the low`s wake. Drier high pressure will briefly return over the
area on Thursday, with another moist low arriving from the west
during the weekend.


As of 815 PM EDT...Two clusters of tstms tracking thru the area
attm. One just pushing east of I-77, and the other across the NC
escarpment area. Overall, instby is starting to wane, and the
convection is heading into the more stable wedge to the east. So I
expect tstms to weaken into just showers over the next couple hours.
Guidance keeps shower activity around thru most of the overnight
across NC, as an upper low deepens over the area, keeping good mid
lvl forcing. Snow levels should also start falling this evening, and
accums should start above 3500 ft by around midnight. The forecast
was updated with latest obs. Winds and sky were updated with latest
guidance. I bumped up thunder wording on previous update for the NC
and eastern Upstate zones thru the evening.

A meso-low takes over the pattern this evening as the wedge retreats
north. This will keep generally unsettled TROFINESS over the FA as
another sfc low and a strong ULVL s/w approach and cross the region
overnight. There looks to be a continued feed of upstream moisture
associated with an ULVL low which will create -SHSN across the NC
mtns in good mech lift aided by pockets of h5 DPVA overnight. Snow
will continue across the higher terrain thru Wed night with accums
the greatest along the NC/TN spine....where the Smokies and NRN mtn
spine areas could receive arnd 8-10 inches. Across the mtn
valleys...generally 1-3 inches is expect during this time.

With the passing strong s/w thru mid-day Wed...there`s a chance that
snow will break mtn containment and fall across the NC foothills and
piedmont. These areas will see the best temperature drop overnight
cooler cP air mixes in. However...guidance consensus has come in
warmer so the best area for snow looks to be arnd the I-40 corridor.
This snow shud be non/accumulating...but a dusting to a tenth is
possible by 16z or so.

Will keep the warning and advisory as is and continue the mention of
non/mtn snow in the HWO. Winds still look to gust into 40-50 mph
across the higher elevations of the NC the adv levels
winds will be wrapped into the WSW products. A cold day on tap Wed
with highs held abt 15 degrees below normal. Min Wed night will also
drop below normal with freezing temps or just below likely across
all locales.


At the beginning of the period, gradient winds will be the primary,
but gradually diminishing factor across the high terrain with high
pressure building into the Deep South and a <990mb coastal low
moving away and into the northern Atlantic.  In the interim, gusty
NW winds are expected through the late afternoon hours on Thursday,
albeit less signficant than those experienced on Wednesday, with
25-35 mph gusts generally confined to the highest elevations.
With tranquil high pressure building into the region for Thursday
night into Friday, our attention will shift to the next system to
our west...

On Thursday, shortwave energy ejecting to the lee of the Rockies
will trigger low pressure development to the lee of the Front
Range and attendant warm-frontogenesis is expected across Ohio
Valley on Thursday.  The resultant warm front will extend SE into
the southern Appalachains late Friday and resultant upglide will
induce some light precip at least by Friday night.  The primary
guidance (ECMWF and GFS) is in disagreement (surprise) about the
commencement of precip-inducing upglide on Friday, and given the
very light amounts and 00Z to 12Z ECMWF consistency have opted
to only slowly ramp up pops on Friday holding any measurable QPF
off until after 00Z on Saturday.  At this juncture, temperatures
throughout the column are forecast to be cold enough for snow in
the northern mountains and the highest elevations of the central
mountains.  However, with a liquid equivalent of less than 0.05",
any accumulations are expected to be minimal, which makes sense
given the weak return flow/lack of deep moisture flux and lack of
significant energy aloft.  Finally, a rain/snow mix is possible
across the northern NC Piedmont as low temperatures dip into the
33-35 degree range with a subfreezing column just above the surface
(>750-1000ft) so this too will have to be watched.  Winter will
hold on till the bitter end...

Regarding sensible weather, though heights aloft will be building
in advance of the longwave ridge over the eastern Great Plains,
reinforcing shortwave energy embedded within the deep-layer
northwest flow will dominate and force the surface high pressure
ridge to remain subbornly just to our west.  Cloud cover and
developing precip associated with the approaching warm front
on Friday into Saturday morning will influence temps as well.
Therefore, high temperatures on Thursday and Friday are expected to
remain 7-12 degrees below normal regionwide, with low temperatures
near normal.


As of 225 PM Tuesday: Our forecast area will be stuck between a
ridge to our west and a trough over the Northeast CONUS through much
of the period. This means a series of short waves will rotate
through the NW flow over the area until Tuesday when the ridge axis
finally moves east and into the area. At the surface, moisture and
isentropic lift develop along a warm front extending toward the area
from a low pressure center over the mid MS valley. This spreads
precip into the area and over the top of a developing cold air
damming high building in from the north. Precip chances ramp up
Saturday and remain relatively high through Sunday as the surface
low skirts the southern boundary of the damming high. The GFS has
come in colder with temps and thermal profiles due to a colder
initial air mass and the surface low moving south of the area
limiting the strength of any warm advection. This keeps the
potential for accumulating snow in place for the higher elevations
of the northern mountains through Saturday morning and again
Saturday and Sunday nights. Temps could be cold enough for snow to
mix in across the rest of the NC mountains and even the I-40
corridor but no accumulations are expected in these locations. That
said, the snow is a low confidence forecast as it is getting very
late in the season even for the northern mountains. Precip tapers
off Sunday night as the low moves east and a drier air mass briefly
moves in. However, there will be a small chance of precip Monday and
Tuesday with lingering moisture and weak insentropic lift over the
warming damming high. Temperatures will be highly dependent on the
strength of the damming high and timing of precip. They will likely
range from around 5 to 10 degrees below normal through the period.


At CLT and elsewhere...At time of the 00z TAF issuance, there were
two clusters of tstms. One pushing thru the KCLT area, and another
near KAVL and along the NC escarpment both drifting east. These
should overall weaken as we lose heating and they move into more
stable air, but showers will likely linger across the much of
western NC, as an upper low deepens over the area. Gusty west to
southwest winds will continue behind a passing cold front,
especially across the Upstate sites thru the evening. VFR conditions
area expected at these sites, but MVFR to possibly IFR cigs will
persist across the NC foothills and piedmont (KHKY and KCLT) thru
the night. On Wednesday, a low pressure system will deepen over the
Mid-Atlantic and allow winds to turn out of the NW. Cold air will
spill in, and snow is expected in the mountains, with a changeover
to RA/SN mix or briefly all SN across the NC piedmont before
moisture exits to the east by late morning. Temps should hover in
the mid to upper 30s, so little to no snow accums are expected at
any of the TAF sites. The exception being at KAVL, but most of the
snow showers should remain north of the airport. VFR conditions
should return to all sites by around midday with gusty NW winds thru
the aftn.

Outlook: VFR conditions expected through Thursday. Another moist system
will arrive from the west this weekend.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  82%     High  91%     High  96%     High 100%
KGSP       High  99%     High 100%     High 100%     High 100%
KAVL       High  96%     High  95%     High 100%     High 100%
KHKY       High  84%     Med   79%     High  95%     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:


NC...Winter Weather Advisory until 8 AM EDT Thursday for NCZ053-059-
     Winter Storm Warning until 8 AM EDT Thursday for NCZ033-


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