Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 080917

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
417 AM EST Fri Dec 8 2017

A moist frontal zone will linger just to our southeast through
tonight with light wintry precipitation for many areas north of
Interstate 85.  A cold upper level trof will then re-amplify over
the area later on Saturday and bring a shot of snow showers to the
North Carolina mountains. Cool temperatures will persist through
much of next week.


As of 330 AM EST Friday: Tricky forecast for the next 24 hours as
we deal with the passage of several weak short waves today bringing
a large area of light precip up from the SW, then the development
of upper divergence and mid-level frontogenesis tonight. As
usual, there is plenty of uncertainty with how the situation
might unfold. The key will be the boundary layer temperatures,
or more specifically, the wet bulb temp profile. Fcst soundings
show the development of a nearly isothermal layer near zero which
defies thickness schemes and should allow wet snow to fall toward
the ground. However, the boundary layer is starting out warm in
most places outside the mtns. Needless to say, precip probability
ramps up to the categorical range from the S/SW today and stays
there well into the tonight period as all manner of forcing acts
on deep moisture. We are most confident in the precip type over
the mtns/adjacent foothills. Wet bulb temps should be low enough
to allow most (if not all) the precip to fall as snow. This makes
it more of a QPF problem, and the trend in the guidance has been
toward the wet. After considering the guidance from WPC, an area on
the Srn upslope zone and across the upper Fr. Broad Valley should
see a sustained period of mostly snow beginning this morning and
continuing into tonight. We could see 4-6 inches of snow in the
area from Franklin and Clayton across the Balsams and Black Mtns
to the Escarpment above Old Fort. That area was bumped up to a
Winter Storm Warning. The Advisory was extended across the northern
mountains where 2-4 inches could fall. Meanwhile...the cooler trend
in the guidance suggests the potential for precip to mix with or
change to snow extending out across the NW Piedmont later today,
with perhaps 2-3 inches possible mainly tonight.  An Advisory will
be issued beginning at Noon. The situation gets a little more tricky
over metro CLT. Think the SREF plume diagrams are overly optimistic
and do not take into account how warm the boundary layer is. Our
main problem is the lack of a source of cold and/or dry air. The
northern part of the area could see an inch or two, but probably
not until after nightfall. Uncertainty is greatest there, so the
consensus was to wait and see how the situation unfolds. The 06Z
sounding at FFC was a concern as it showed a stronger than expected
warm nose closer to 700 mb, meaning CLT would be more likely to
stay rain longer into the evening. We will let the day shift have
another look at trends in that area. As for the rest of northeast
GA and Upstate SC, they say rain changing to snow is a suckers
bet. It looks like it will just stay too warm in the boundary
layer for the better part of the event for anything but rain,
though some small accum is possible in the I-85 corridor closer
to daybreak Saturday if the precip holds on long enough. The only
caveat for the I-85 corridor will be precip rate. If we can get
a high enough precip rate, the melting level could be depressed
lower to the ground and we could see some periods of wet snow. But,
even if that happened, it would NOT be likely to accumulate.


As of 330 AM Friday: Models have come in much slower in the exiting
of moisture on Saturday. Despite losing low-level forcing and CAA
ramping up, models spit out some light QPF across the eastern half
of the CWFA after 12z Saturday, due to the coastal low deepening. In
the upper levels, forcing will still be quite strong, as a northern
stream shortwave phases with a southern stream wave as it enters the
base of the longwave trough. So with some moisture to work with, a
comma head precip shield will likely linger over at least the I-77
corridor thru late morning. Forecast soundings support precip
staying mainly all snow. Additional snow accums should be light,
generally 0.5" or less. It will remain cloudy to mostly cloudy into
early aftn, which with the combination of decent CAA, result in
chilly max temps. I have lowered them using the latest consensus to
the 30s to mid 40s, about 10-15 deg below normal.

With the deep trough axis swinging thru the mountains Saturday
night, a quick shot of moisture will accompany modest low-level
northwest flow. So scattered to numerous snow showers are expected
along the TN border, mainly Saturday evening, tapering off by
daybreak or not long after. Saturday night looks like the coldest
night so far this season, as arctic air filters in from the
northwest. It will feel especially brisk in the mountains with gusty
NW winds. Lows will be about 10 degrees below normal.

Sunday looks cold, but quiet with temps struggling to get into the
40s east of the mountains, despite mostly sunny skies. Melting snow
in the mountains and I-40 corridor may also limit max temps. Sunday
night will cold, but not quite as cold as Saturday night. Lows 6-8
degrees below normal.


As of 135 AM EST Thursday: the extended forecast begins 12Z Monday
with a dry northwest flow regime for our region under a broad upper
trof extending from the Rockies to the western Atlantic. At that
time, the next shortwave will be moving SE across the upper
Mississippi Valley.  The associated cold front arrives late Monday
night and is moving to the coast by late Tuesday morning. Northwest
flow snow begins along the NC and TN border areas early Tuesday with
trof axis crossing KY and height falls continuing for our area as
colder air drops down from Canada.  Forecast models in fairly good
agreement with axis of trof north to south over our area around 00Z
Wed.  GFS model ends NW FLow snow rather quickly after trof axis
passes while ECMWF continues snow into early Wed with lobes of
energy passing until about 12Z Wed.  Another cold front associated
with a quick moving shortwave will arrive on Thursday. Wednesday
should be the coldest day of the week with the deep trof passage
then a warm up of ten degrees on Thursday which will be just a few
degrees below normal. Monday would be the warmest day of the week
and even that day should have temps near or just below normal.


At KCLT and elsewhere: Grim-looking aviation weather as flight
conditions are expected to deteriorate gradually early this
morning. Once precip moves in from the south, we should see a
steady progression down through MVFR with the ceiling. The guidance
suggests that IFR ceiling will prevail after about 13Z and see no
reason yet to argue. Wind should come around to NE by that time
and stay there for the rest of the period. All sites should be
IFR after 00Z Saturday. From that point onward, we will have to
look at the possibility of a mix/changeover to snow. There was
not the time to address that situation with this fcst, but it will
be handled over the next few hours as we have more time to digest
the new model guidance. Expect it to be fine tuned on the 08z or
10Z amendment. Elsewhere...suffice to say, as elevation increases,
snow is more likely, so KAVL should expect some accum.

Outlook: Restrictions will be likely as well as mixed/wintry precip
through Saturday morning. Precip and associated restrictions will
decrease during the day Saturday, with dry conditions expected at
all sites into early next week. The possible exceptions would be
KAVL/KHKY with potential for northwest flow snow Saturday night.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High  81%     High  82%     Med   68%     Low   56%
KGSP       Med   70%     Med   75%     High  83%     Med   64%
KAVL       High  94%     High  85%     Med   70%     High  87%
KHKY       High 100%     High  83%     Med   66%     High 100%
KGMU       Med   76%     High  86%     High  85%     Med   63%
KAND       Med   76%     Med   65%     High  90%     Med   61%

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 Storm Warning until 7 AM EST Saturday for GAZ010-017.
NC...Winter Weather Advisory until 7 AM EST Saturday for NCZ033-
     Winter Storm Warning until 7 AM EST Saturday for NCZ052-053-
     Winter Weather Advisory from noon today to 7 AM EST Saturday
     for NCZ035>037-056-057.
SC...Winter Weather Advisory until 7 AM EST Saturday for SCZ001>003.


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