Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 161153

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

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.


As of 645 AM EST: All quiet across the western Carolinas and
northeast Georgia at daybreak. Temps bottoming out a bit lower
than expected. Clouds should start to increase over the wrn zones
by mid morning, but earliest arrival on the TN border should not
be until around noon.

Main concern as of this morning is the potential for a light
snow/advisory-level event across most of the western Carolinas
tonight/Wednesday morning. Models have not really changed a whole
lot over the past few days with respect to the overall scenario,
that being the approach of a sharp, positively-tilted upper trof
today that will drive a cold front eastward across the fcst area
tonight. The 06Z guidance kept on the same track in that precip
amts continue to trend slightly upward. Forcing looks decent 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.  The moisture has always been the big question, and
thus how much precip this system can manufacture, which still looks
relatively light. 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, perhaps
beginning as light rain if precip begins early enough. 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. However, the GFS has shown a slight upward trend, as
has the SREF, so in spite of the meager-looking operational NAM,
still think we are looking at the potential for an inch of snow
or thereabouts, mainly across the area N/E of I-26. As one inch
meets our Advisory criteria, that will be the course we take. Onset
would be late evening or around midnight over the foothills, and
then early morning hours over the wrn Piedmont and Upstate SC.
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.


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.


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.


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...

            12-18Z        18-24Z        00-06Z        06-12Z
KCLT       High 100%     High 100%     High 100%     High  86%
KGSP       High 100%     High 100%     High  94%     Med   72%
KAVL       High 100%     High 100%     Med   65%     High  81%
KHKY       High 100%     High 100%     High  99%     High  87%
KGMU       High 100%     High 100%     High  92%     Med   75%
KAND       High 100%     High 100%     High  95%     High  99%

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:


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 from noon today to 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.


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