Area Forecast Discussion
Issued by NWS Greer, SC

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878
FXUS62 KGSP 161519
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1019 AM EST Tue Jan 16 2018

.SYNOPSIS...
A cold front will approach from the west today and cross the area
tonight. This front will usher in a quick shot of cold air, with
light snow possible across the forecast area tonight into early
Wednesday. High pressure will slide off the East Coast by Friday,
allowing a warming trend for the end of the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
As of 950 AM EST: Frontal band of light snow currently extends from
northern AL/southern Middle TN into eastern Kentucky. The leading
edge of this band is forecast to enter far western NC during the
early afternoon. Clear skies in advance of the band should allow
temps to warm above freezing across the mtn valleys, thus precip
will likely begin as liquid there, with wet bulb effects allowing
for a gradual transition to snow by late afternoon. Meanwhile,
all-snow is expected above 3500 ft or so. Light precip rates and
initially dry air suggests at least a couple of hours of
flurries/sprinkles before anything begins to accumulate, but it
appears that anywhere from 0.5-1" is likely across the Smokies and
vicinity by 00Z, while snow will likely be in progress across the
remainder of the mtns by that time.

Otherwise, not much change in the forecast scenario at this point...
forcing looks decent late today/tonight with weak upper divergence
developing east across the region in response to a strengthening jet
streak, along with increasing dpva and a upper level pv contributing
to upward vertical motion later today and tonight.  Fortunately, the
models are in excellent agreement with falling thickness and temp
profiles that lack a warm nose, thus we are most confident that
precip type will be mostly snow, beginning as -RA or -RA/SN where
temperatures are well-above freezing, before wet bulb effects force
a transition to snow. We are also most confident in precip amts near
the TN border, resulting in 1-2 inches of snow mainly late this
afternoon and this evening. East of the mtns is where confidence
starts to trail off. Several things could still go wrong with this
fcst, mainly related to the amt of precip that falls because of the
quick movement of this system. The 12Z NAM has finally come on board
with the upward trend in qpf across the Piedmont tonight, suggesting
at least an inch of snow is possible, mainly across the area N/E of
I-26, while as much as 2 inches could be seen along the I-77
corridor.

Precip Onset would be mid/late evening over the foothills, and then
within a couple hours of midnight over the Piedmont. Confidence in
the one inch total is greatest across the NW Piedmont and then falls
off across the GSP metro area. Ordinarily, an inch of snow might not
be a big deal, especially across the NC mtns, but as the snow would
be falling the temp will be falling off into the mid/upper 20s,
which could result in numerous travel problems on untreated roads
during the morning rush. Even though the snow will end in the early
morning hours over the NC mtns, expect some problems with black ice,
thus the ending time extended to daybreak.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/...
As of 330 AM EST Tuesday: Overall, guidance is in good agreement on
the timing of exiting snow to the east Wednesday morning. A little
additional accums may be possible, mainly east of I-77 past 12z Wed.
Otherwise, skies should clear out from west to east during the aftn
with a cold NWLY wind preventing temps from warming much at all from
the morning lows. Add expected new snow cover, and I think going on
the colder side of guidance makes sense. So temps were adjusted down
with CONSRAW, resulting in highs in the teens to lower 20s in the
mountains and upper 20s to mid 30s piedmont. The NWLY winds will
gradually diminish overnight Wednesday night, as temps drop into the
teens (with a few single digits on the highest peaks). This
combination will result in wind chills in the -5 to -15 F range in
the elevations above 3500 ft. Wind chills will be generally in the
single digits above zero in the mountain valleys.

Thursday and Thursday night, sfc high pressure quickly settles in
across the Deep South, allowing winds to weaken further and turn
more out of the west-southwest by the aftn. Skies should be mostly
clear and temps should rebound to 40s in most places, except the
highest elevations, helping melt the snow. Depending on how much
snow falls and subsequently melts the following day, there may be
black ice concerns Thursday night. Temps should bottom out in the
upper teens to mid 20s.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 300 AM EST Tuesday: The models are in good agreement for the
medium range, starting with a relatively flat upper flow at 12z
Friday, amplifying into a deep western trough and eastern ridge by
Sunday. At the sfc, high pressure will linger over the Southeast on
Friday, then slide east over the weekend, allowing return flow to
set up atop the region. So a warming trend will ensue for the medium
range, leading up to a cold frontal passage on Monday. Guidance
shows basically no sbCAPE with the fropa, so severe threat looks
very low. The front should push thru quickly enough that excessive
rainfall also does not look likely. Temps will start out near normal
Friday, then warm to about 5-10 deg above normal for Saturday and
Sunday. Temps will return to around normal by Tuesday. PoPs will
ramp up to high-end CHC to low-end likely on Monday. Before that, it
looks dry. The way the sfc low tracks into the Great Lakes and into
Quebec, NW flow snow behind the system looks limited.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
At KCLT and elsewhere: VFR through the daylight hours, and most
likely through the end of operations this evening at KCLT. We
begin with a few high clouds and a calm or light/variable wind,
eventually becoming light S after sunrise. Clouds will increase
from the top during the afternoon as moisture arrives ahead of the
approaching trof. Guidance shows the wind coming around to SSW at
16Z-17Z and that seems reasonable. It will take some time for clouds
to thicken and lower to where it would impact operations, and that
could happen after sunset, but more toward 05Z Wednesday as low
level moisture increases at KCLT. The fcst includes a PROB30 group
in the 05Z to 08Z time frame as a hedge for early precip arrival,
and if that happens, it would be a rain/snow mix. Confidence
increasing that we will have some light snow for the start of
operations on Wednesday morning, so went ahead with prevailing
light snow with IFR visibility and MVFR ceiling as a starting
point. More detail will be added in future issuances. Elsewhere,
the main concern will be the arrival of frontal precip over the mtns
late this afternoon and this evening, with light snow restricting
vis/ceiling at KAVL after 22Z.

Outlook: Expect increasing possibility of restrictions Wednesday
morning with some light accumulation possible at all terminals,
but least likely at KAND. Conditions will improve to VFR by late
Wednesday, likely continuing VFR through the week.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     Med   77%
KGSP       High 100%     High 100%     High  85%     High  82%
KAVL       High 100%     High 100%     Med   75%     High  84%
KHKY       High 100%     High 100%     High  93%     High  84%
KGMU       High 100%     High 100%     High  85%     High  86%
KAND       High 100%     High 100%     High  91%     High  94%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Winter Weather Advisory from 6 PM this evening to 7 AM EST
     Wednesday for GAZ010-017.
NC...Winter Weather Advisory from 10 PM this evening to noon EST
     Wednesday for NCZ035>037-056-057-068>072-082-501>510.
     Winter Weather Advisory until 7 AM EST Wednesday for NCZ051-
     052-058.
     Winter Weather Advisory from 6 PM this evening to 7 AM EST
     Wednesday for NCZ033-048>050-053-059-062>065.
SC...Winter Weather Advisory from 10 PM this evening to noon EST
     Wednesday for SCZ001>003-005>009-012>014.

&&

$$
SYNOPSIS...ARK
NEAR TERM...JDL/PM
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...PM



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