Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 081511

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1011 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 1010 AM EST Friday: Snow is following along closely with the
observed surface wet bulb temps. This is keeping accumulating snow
limited to the mountains and Blue Ridge portion of the foothills
with lighter accums across the lower elevations of the foothills.
Some sleet is mixing in with the rain in portions of the area
outside of the snow areas. This will occur from time to time today
as heavier precip rates can bring freezing levels down closer to the
surface. No significant sleet accums are expected and it should be
brief. Based on latest reports, will add Graham and Swain to the
warning as 4 inches of snow have already fallen in Robbinsville.
Otherwise, the forecast is generally on track with only minor

A 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, ultimately melting in the relatively
warm boundary layer 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 snow continuing
through today and 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, with some of the elevations
above 5k feet getting around 8 inches. 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 sucker`s
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 this morning. Once
precip moves in from the south, we should see a steady progression
of the ceiling down through the MVFR category. The guidance
suggests that the ceiling will fall down to IFR around  15Z to
17Z and stay there, if it doesn`t fall at least temporarily as a
precip band moves overhead, and right now I see no reason yet to
argue. Wind should come around to NE shortly and stay there for the
rest of the period. IFR ceiling will move/develop across most of
the area by late morning or early afternoon as the steadier precip
develops, and then will continue overnight probably falling down
into the low IFR category. The big concern later on in the fcst
will be the possibility of a mix/changeover to snow. The guidance
suggests that a mix could happen some time in the middle part of
the evening with a changeover late in the period, perhaps around
09Z. Elsewhere...suffice to say, as elevation increases, snow is
more likely, so KAVL should expect SN to be prevailing through
the entire period with a significant accum possible.

Outlook: 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 NCZ051>053-
     Winter Weather Advisory until 7 AM EST Saturday for NCZ035>037-
SC...Winter Weather Advisory until 7 AM EST Saturday for SCZ001>003.


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