Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 072156

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
456 PM EST Thu Dec 7 2017

A moist frontal zone will linger just to our southeast through the
end of the work week allowing temperatures to drop below normal with
light wintry precipitation possible for Friday and Saturday. 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.
Cold temperatures will persist into the first half of next week.


As of 500 PM EST Thursday: New guidance trickling in continues to
trend cooler and wetter, and while the NAM remains a cold/wet
outlier, other 18z data shifting that direction lends confidence to
a slight uptick in snowfall amounts. Will continue to be a battle
between the cold air vs. moisture as is typically the case, but with
the moisture expected to now make it farther into the mountains, the
prudent course of action was to go ahead and hoist a Winter Weather
Advisory for the upslope areas of the SW mountains, including the
GA/SC mountains. Because NC`s advisory criterion is 2" in 12h and we
have that over the South Mountains in Cleveland/Burke/Rutherford
Counties, included those areas, but most locations <1500ft will be
hard pressed to see anything more than a trace/dusting at this time
(uh, with the current forecast that is - if the trends continue that
may be pushed up as well). Concern is that in the prime overlap area
of the SC mountains (QPF and temps), current forecast is getting
close to warning criteria. Will continue to watch trends through the
evening and especially with the overnight forecast.

Otherwise, guidance is coming into better agreement regarding the
precip potential for tonight and Friday. Deep moisture moves in this
evening with decent upper divergence from the right entrance region
of the upper jet. Short waves move over as the upper trough deepens
to the west. Low level isentropic lift develops as well, but remains
relatively weak with abundant moisture. Despite an overall cooling
trend in forecast temps and surface wet bulb values, they remain
above freezing through the night outside of the mountains, but do
fall below freezing across the mountains. Of course, precip chances
are lower where temps are colder since those locations are farther
away from the better forcing. Therefore, have snow developing in the
colder locations with mainly rain elsewhere. Forecast soundings show
the freezing level dropping low enough for snow to mix in north of I-
85 with a change over possible across the Northern Foothills. QPF is
light but could be enough for around half an inch of snow along and
near the NC/GA to NC/SC border mountain locations. Snow would be
less across the rest of the mountains.

Categorical precip chances continue generally along and south of I-
85 Friday with likely chances north of there across the foothills to
along the Blue Ridge. Good chance PoP west of there. Temps and
surface wet bulbs remain cold enough for the precip to fall as snow
across the mountains with a transition zone across the NC Foothills.
Outside of these areas, freezing levels may fall low enough for snow
to mix or possibly even briefly change over early in the morning
mainly along and north of the I-85 corridor. QPF has increased on
some of the guidance runs which increases the potential for advisory
level snow across the NE GA and Upstate mountains, along with the
Southern Mountains and possibly portions of the NC foothills. After
final national guidance is in, we may need to update to post an
advisory for those locations. While these accums would develop in
grassy and elevated areas, it is still questionable how much
accumulates on roads given the recent warmth and sunshine. Locations
where the precipitation will be mixed or only a brief change over,
no significant accums are expected with any small amounts limited to
grassy or elevated areas.


As of 230 PM Thu: We will remain slightly on the cold northwest
side of a surface frontal boundary through early Saturday. Models
are now more supportive of a resurgence in precipitation over the
CWFA Friday night, as the sharp shortwave moves into the central
Gulf states, and the jet streak strengthens over the southern
Appalachians and adjacent Piedmont. GFS and EC hold the thermal
gradient relatively steady over the area thru the night; wet-bulb
temps do gradually drop across the southeast half of the CWFA,
which should permit snow to mix in with rain if not totally change
over. However, the area with wet-bulb values at or below freezing
looks likely to expand little. This minimizes the accumulating
snowfall in the greater I-85 corridor.

QPF trends overall have been upward this cycle for the Friday night
period, particularly over the Blue Ridge and adjacent mountain
ranges (namely the Balsams); compare this to earlier suggestions
that all precip may have ended there by 00z Sat. In collaboration
with our neighboring offices, we will wait another cycle to see if
this trend continues before reflecting the higher numbers in the
forecast for these mountain areas. It is possible an additional
couple of inches could result and headlines still could be needed.

Another minor yet potentially significant trend in models is for
Saturday. As before, consensus suggests strong DPVA pushing thru
the area during the day along the trough axis. The difference is
that several sources depict the moisture departing more slowly,
and hence the vort max generates some QPF.  By the time this
precip develops it could have warmed up enough to fall as rain,
particularly in the east. However this is something for us to
watch closely.  A small amount of SBCAPE is depicted in parts of
the eastern CWFA to boot; however it slightly lags the forcing and
colder temps. For now PoPs will be allowed to linger thru Saturday
morning over our eastern zones; temp trends suggest this will be
a rain-snow mix changing back over to rain as it ends.

Winds veer Saturday afternoon as stronger cold advection sets
in across the area. The final phase of precip in this period
thus will be northwest-flow driven snow showers along the Tenn
border, developing by late afternoon, and continuing through early
Sunday. Thermo profiles look reasonably good in terms of saturation
at snow-growth temperatures, which given the very cold air spilling
into the area will be in the upslope layer. Current expectation
is for about another inch to fall Saturday night along the spine
of the Appalachians. Brisk and gusty winds in this same timeframe
may bring wind chills below zero on the higher mtn ridges.

Settled weather is expected Sunday as dry high pressure builds
across the South. Despite abundant sunshine, the very cold nature
of the airmass will be reflected in max temps 11-13 degrees
below normal.


As of 210 PM EST Thursday: the extended fcst picks up at 00Z on
Monday with the broad upper trof lifting NE and over New England
while steep upper ridging persists over the Western CONUS. By early
Tuesday, the long range models re-amplify the upper trof again over
the eastern half of the CONUS bringing more widespread cold air to
the region and keeping temperatures well below normal for most of
the period. By early Wed, the trof axis is expected to be moving
over the fcst area or lifting just NE of it. Beyond this point,
the trof broadens and flattens, but still remains in place over
the entire Eastern CONUS. At the sfc, another progressive low will
surge SE and across the Great Lakes on Monday and bring another cold
front to our doorstep by late Monday/early Tuesday. The front is
expected to move thru the CWFA quickly and be SE of the fcst area
by late Tues/early Wed with high pressure moving back over the area
to end the period.


At KCLT and elsewhere: No changes for the KCLT TAF for the 22z
AMD. VFR through the afternoon before deteriorating conditions
develop this evening and overnight. Precipitation moves in and vsby
and cigs begin to lower through low VFR into MVFR. Expect IFR to
develop near daybreak and continue through the morning and possibly
afternoon, although it could improve to low MVFR by then. Precip
chances have increased but temps remain above freezing across most
locations outside of the mountains. However, freezing levels will
fall low enough that snow may mix in overnight and continue through
the day. A complete change over could develop at KAVL. S to SW wind
outside of the mountains this afternoon becomes light and variable
this evening, then NE Friday. KAVL will see NNW wind throughout.

Outlook: The forecast remains unsettled into the weekend as multiple
upper level disturbances pass over the area and interact with a
stalled front and moisture lingering just to the east. Restrictions
could result, with wintry mixed ptypes not out of the question
throughout the area. The cold air will continue this weekend to keep
the risk of winter weather elevated across the mountains and
northern tier. Some drying is likely on Monday.

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 Weather Advisory from 5 AM Friday to 7 AM EST Saturday
     for GAZ010-017.
NC...Winter Weather Advisory from 5 AM Friday to 7 AM EST Saturday
     for NCZ062>065-068-504-506>510.
SC...Winter Weather Advisory from 5 AM Friday to 7 AM EST Saturday
     for SCZ001>003.


SHORT TERM...Wimberley
AVIATION...RWH/TDP is the U.S. government's official web portal to all federal, state and local government web resources and services.