Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 210604

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
204 AM EDT Wed Mar 21 2018

A complex low pressure system will gradually move off the Atlantic
Coast today with colder air overspreading the region as the low
moves over the Atlantic. Drier high pressure will briefly return
over the area on Thursday, with another moist low arriving from the
west during the weekend.


As of 200 AM EDT: Main update for the 06Z TAF issuance, along with
minor tweaks to temperatures and Pops/WX to coincide with latest
trends, as the rest of the near term forecast remains on track.

Latest radar depicts a a few batches of rain moving eastward along
and north of the NC/SC border and east of the escarpment this
morning with the eastern fringe just now moving into CLT/I-77
corridor. Expect this precip to continue to expand in coverage over
the next few hours with the anticipated change over to a rain/snow
mix as temperatures decrease towards daybreak, with weak returns
continuing across the NC mountains as well, where a light rain/snow
mix or all snow has been falling across the higher elevations for
the past few hours.

Otherwise, the most recent change with the forecast in the past few
hours was the expansion of the Winter Weather Advisory across our
northern tier of zones from Caldwell County to Davie, and including
Rowan from midnight tonight thru noon Wednesday, as temps continue
to cooler faster than guidance. Thicknesses will definitely fall to
allow any precip that develops to change over to a mix of rain and
snow or even all snow in these areas late tonight. Accums should be
very light, generally less than an inch. But there may be enough
impact such that an advisory seems prudent.

A meso-low takes has taken over with the retreat of the wedge to the
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.

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 significant 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 Appalachians 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 stubbornly 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: A mix of low stratus, SCT/BKN mid level clouds
to nearly clear skies encompasses the forecast area at the time of
the 06Z TAF issuance, creating VFR at KGSP, KGMU, KAVL, KAND, and
MVFR at KHKY and IFR at KCLT (both TAF sites accompanied with -RA as
well). Overall expect mainly VFR/MVFR mix through the morning hours,
but would not entirely rule out a brief return to IFR before
daybreak, but confidence is low attm. Do expect -SHRA to linger over
the next few hours, with a gradual transition to RA/SN mix to
patches of all SN across the NW Piedmont into the morning hours,
over to all SN across the mountains as cold air wraps in behind an
exiting system. For now, would not entirely rule out a few
snowflakes mixed in with light showers at KCLT towards the morning
hours, however did choose not to introduce into a prevailing/tempo
group given lower confidence attm. Any precip across the NW Piedmont
(including KHKY/KCLT) will return to all rain by 15Z as temperatures
warm, with precipitation tapering off at all sites by 18Z (KAVL
~22Z). For KGSP, KGMU and KAND, other than -DZ-SHRA within the next
few hours, expect conditions to remain dry throughout the valid TAF
period. Expect VFR to prevail this afternoon and persist through the
period, with the winds becoming the bigger story. Gusty NW winds
will continue to pick up through the morning hours, lasting well
into the afternoon hours, gradually subsiding tonight (except for
KAVL where they will continue into Thursday  morning.

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

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Med   79%     High  95%     High 100%     High 100%
KGSP       High  99%     High 100%     High 100%     High 100%
KAVL       High  97%     High 100%     High  99%     High 100%
KHKY       Med   76%     High  98%     High 100%     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 Weather Advisory until noon EDT today for NCZ035>037-
     Winter Storm Warning until 8 AM EDT Thursday for NCZ033-


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