Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 152029

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
329 PM EST Mon Jan 15 2018

Cold and dry high pressure will linger across the region ahead of an
approaching cold front crossing the Plains through tonight. This
front will usher in a quick shot of cold air, with light snow
possible across the forecast area Tuesday night and Wednesday. High
pressure will slide off the East Coast by Friday, allowing a warming
trend for the end of the week.


As upper low over the Corn Belt wobbles across the Great Lakes
through tomorrow, a short wave trough digging west through south of
the low will carve out a deep trough across the Mississippi/ western
Ohio Valley by the end of the period. Associated Arctic cold front
will march across the TN Valley tonight through Tuesday, as surface
high continues to weaken across the Eastern Seaboard. While some
thin cirrus may begin impinging on the area toward sunrise, fairly
ideal radiational cooling conditions should be in place tonight,
allowing min temps to average 5-10 degrees below climo.

Front and associated moisture encroach on the southern Appalachians
tomorrow afternoon and evening, and it appears likely that light
precipitation will spread into far southwest NC at some point during
that time, with exact timing being the primary question. The
forecast has been nudged a bit toward the (typically faster)
high-res windows in terms of timing, with chance pops spreading into
the Smokies and vicinity by early afternoon, increasing to likely by
early evening. This timing should be sufficient to allow temps to
warm above freezing (in fact, around 40) in the valleys before any
precip falls. Thus, precip should begin as rain or rain+snow. Once
light precip begins falling, wet bulb effects should  allow temps to
cool toward freezing, allowing for a transition to all-snow, but
this may be too late to produce any significant accums, and in fact,
forecast amounts in the valleys struggle to reach even a half inch
through the day tomorrow. Meanwhile, only snow is expected in the
high elevations, with forecast accums in the 1-3 inch range forecast
during the afternoon/evening. A Winter Weather Advisory will be
issued for areas above 3500` beginning tomorrow afternoon in the
Smokies, Balsams, and surrounding areas. Max temps tomorrow will be
roughly 5 degrees warmer than today across the mtns, and 5-10
degrees warmer across the Piedmont/foothills.


As of 315 PM EST Monday: The forecast period begins Tuesday evening
as focus remains on the increasing chances for wintry precipitation
across the forecast area through into Wednesday. With the afternoon
forecast package, a Winter Weather Advisory has been raised for the
higher elevations of Graham, Swain, and Haywood (NC) counties,
beginning in the near term forecast period, continuing through
overnight Tuesday attm.

Latest model guidance continues to be in agreement with a positively
tilted upper trough, digging from the Great Lakes into the central
plains Tuesday evening, becoming slightly more neutral as it pushes
across the eastern seaboard Wednesday evening, trailing behind it`s
associated cold front. With anticipated lower QPFs, timing will
essentially be everything with this system in regards to expansion
of colder temperatures and the infiltration of drier air behind the
cold front.

With temperatures at or below freezing across the mountains Tuesday
evening, expect any falling precipitation to fall as light snow.
Beyond the escarpment, with sfc temperatures in the mid to upper
30s, the leading edge of the precip will be a rain/snow mix. Across
the Upstate where temperatures will be warmer around 40 degrees,
expect any falling precipitation to begin as all rain. Towards
midnight, the rain/snow mix line will progress eastward to the I-77
corridor, as temperatures continue to drop west to east down to just
at or slightly below the freezing line for overnight low
temperatures. With temperatures nearing freezing across the entire
forecast area will allow for any falling precipitation to switch
over to all snow a few hours before daybreak on Wednesday.

PoPs will quickly drop off west to east, remaining mentionable just
along/east of the I-77 corridor by mid-morning on Wednesday as the
infiltration of dry air quickly moves in. Temperatures will
gradually warm throughout the day but not my much, only 5 to 10
degrees in most locations, climbing above freezing along and
southeast of the I-85 corridor. Increasing NNW winds (gusty at times
across the mountains) behind the front on Wednesday will help dry
surfaces throughout the day. However, any residual water on roadways
Wednesday night into Thursday, especially in shaded areas, will re-
freeze as temperatures are expected to drop into the single digits
across portions of the mountains and teens elsewhere. Thus, expect
black ice to potentially create hazardous travel conditions
overnight Wednesday into Thursday morning. After daybreak on
Thursday, expect dry conditions to prevail as sfc high pressure
builds in. With clearing skies, temperatures will warm into the low
to mid 40s across the Upstate and Piedmont, cooler in the 30s across
the mountains.

Overall, with lower QPFs and wet, cold roads/surfaces to begin, am
anticipating snow accumulations to be light Tuesday night through
Wednesday evening with this system, especially across the Upstate
and Piedmont. For the Upstate, a dusting up to a half of an inch
will be possible, with up to just short of an inch across the NW
Piedmont, though some locations, especially east of I-77, could see
slightly higher amounts towards an inch. Back across the mountains,
where roads/surfaces will be cold but not wet, anticipate anything
that falls will instantly stick to roadways, with up to an inch
possible, up to 2 inches possible across the higher elevations.


As of 220 PM EST Monday: Good confidence is had in the ext range
fcst. The models continue to support strong sfc hipres dominating
the pattern over the SE CONUS. With little moisture/temp adv to deal
with...the GEFS ens members are displaying rather tight clustering
around the mean as well as the the op run.

Soundings show a very dry column to begin the period with PWATS
arnd 0.2 inches. This airmass will slowly modify moister each day as
sw/ly flow persists along the periphery of the Atl sfc high. Expect
a warming trend with maxes beginning the period a couple cats below
due to a lingering llvl thermal trof...but by Sun maxes should
easily reach abv normal levels. Mins will also follow the same daily
warming trend. On Sun...sfc cyclogenesis will begin across the srn
plains developing a cold front which will interact with existing GOM
moisture flux as it pushes east throughout the day. There are timing
and QPF differences btw the op will hold off on the
onset of precip across the wrn zones until later Sun night. Slight
chance PoPs will rise to high-end chances Mon as the front is
expected to push over the region. No p/type concerns will be had
with maybe some diurnal mixing of non/accum snow across the highest
terrain early Mon.


At KCLT and Elsewhere: VFR through the period. Light/vrbl winds may
briefly become light S/SW this afternoon, before becoming light/vrbl
again this evening, then returning to the S/SW by late Tue morning.
SKC conditions this afternoon/tonight gives way to gradually
increasing/thickening high clouds early tomorrow morning through the
end of the period.

Outlook: A fast-moving system may bring a brief shot of moisture and
possible restrictions Tuesday night into Wednesday. The main concern
is that temps will be cold enough that any precip that falls would
most likely be in the form of light snow. The main impact at KCLT
would be from around midnight Tue night through Wed morning.
Conditions will improve to VFR by late Wednesday, likely continuing
through the week.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     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 from 10 AM Tuesday to 1 AM EST
     Wednesday for NCZ051-052-058.


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