Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 060004

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
704 PM EST Mon Dec 5 2016

Expect a soaking rain tonight and early Tuesday followed by dry high
pressure returning Wednesday. A strong cold front crosses from the
northwest early Thursday bringing the coldest air of the season for
the end of the week.


As of 655 PM: The initial surge of upglide into the forecast area
resulted in an area of light to moderate rain that has impacted the
majority of the CWA so far this evening, and this prompted a rather
dramatic timing shift in pops with an earlier update. While that
area of precip continues to drift to the ENE, a much more
substantial mod/heavy rain shield is screaming northeast from
central GA, and will overspread the forecast area over the next 0-3
hours, with all areas seeing a 100 pop by the end of the evening.
Healthy rainfall rates of .25-.5 inch have been observed across
central GA, especially with heavy embedded showers and even some
isolated/elevated TS. QPF has been increased across much of the
area, with storm total amounts of 1-1.5 inches appearing likely
across much of the area through Tue morning.

Otherwise, the downstream ridge will push a relatively weak surface
high across PA and the nrn Mid-Atlantic to New England. The surface
high should bridge over the Appalachians through early evening as
precip develops up from the SW. The result will be the development
of a sort of hybrid cold air damming wedge this evening that will
control our weather through Tuesday morning at least. The upper wave
will deamplify as it crosses the region during the middle of the day
on Tuesday. After that happens, we quickly lose the isentropic lift,
and the cold air damming should start to erode. Precip chances will
quickly taper back down from the SW, to where we retain only a
chance on the TN border as NW flow upslope begins in the wake of the
wave`s passing. As it stands right now, we might see some elevated
instability on Tuesday morning, especially over northeast GA and
upstate SC, so thunder cannot be ruled out. However, the threat for
severe storms appears to be low. Precip amts also look as though the
flood threat will be minimal based on the intensity and duration.
Wind will be strong at high elevations, but not nearly to the point
where a wind advisory would be entertained. Expect a low diurnal
range to temps tonight and Tuesday, with high temps again fairly
tricky, depending on when the cold air damming ends.


As of 145 PM Mon: Coastal low pressure will be pulling away from the
area Tuesday evening, with low-level flow quickly veering as the
cold front associated with the Great Lakes low pushes across the
mountains. Almost simultaneously, however, a flat ridge becomes
re-established over the Southeast in the wake of the low, and
upglide is initiated over the front. During the day Wednesday, this
expands and strengthens gradually as a shortwave develops over the
lower Mississippi Valley along a reinforcing cold front. Moisture
over our area remains confined to a relatively shallow layer,
and the upglide is not particularly strong. Therefore a modest
amount of cloud cover is expected to persist, though without a
mentionable chance of precip. Min temps Wed morning will be a few
degrees above normal; maxes will be just slightly above normal.

That shortwave begins to approach from the west late Wed into early
Thu, driving a slow increase in the magnitude of the upglide and
moisture advection over our area. Colder air begins to push into the
mountains around daybreak, taking most of the day to push across the
whole CWFA. Moisture is still not that plentiful, but there will
be a tiny bit of shallow instability ahead of the front. Showery
precip is expected at best as the front crosses. Temps will be
too warm for any wintry precip at the beginning of the event, but
in the wake of the front, the freezing level will drop through the
higher elevations of the WNC mtns Thu morning, reaching the valleys
late in the day after chances have peaked. Wet bulbing might have
an effect on how fast the changeover occurs. The saturated layer
currently looks unlikely to reach high enough to get good crystal
growth; it is possible freezing drizzle or light freezing rain will
occur in non-upslope areas early in the day; precip may just end
before actually changing over to snow. To keep things simple--and
because upsloping may make up for the relatively warm soundings
in producing ice crystals--I have reflected either rain or snow
in the forecast, but without any accumulation.


As of 200 PM EST Monday: Lingering and weak NWF snow showers across
the nrn mtns will begin the ext period. The models are showing a
shallow moisture layer btw 1-3 Kft through 06z Fri...and with little
to no Glakes fetch and minimal dynamic support...any accum -shra
will drop to nil through daybreak. Perhaps an inch or so at most
across the favored higher terrain spots. After that...a strong 1040
mb Canadian high will build across the plains and ridge into the
mtns. This will create a decent p/grad as well as instigate good
llvl caa. Mtn top temps should drop into the lower teens and with
winds gusting to arnd 30 kts...wind chill values will reach adv
levels at elevations above 3.5 Kft. Thus the HWO mention will

A cold day Fri and another cold night is in store Fri night as the
sfc high settles over the FA...however reduced winds will keep
apparent temps above adv levels. A dry cold day on Sat with highs
continuing 10-15 degrees below normal into Sun. There will be an
increased chance of precip Sun afternoon thru Mon as the next system
develops to the west. The models have varying ideas as to the
strength and placement with this feature with the GFS much further
north and less moist than the slower ECMWF. Thus...will continue an
even model blend and raise pops a little across the wrn zones. The
p/type shud be either rain/snow across the mtns with no sigfnt
accums expected....perhaps an inch or so across the higher elevs of
the nrn mtns.


At KCLT and elsewhere: A brief -RA lull of an hour or so will be
seen at most terminals this evening. However, a large area of
widespread moderate rain, with embedded heavy showers will
overspread the area quickly from now through 03Z. Once the rain
begins, the potential for IFR conditions, first with visby, followed
by a cig response will increase significantly, such that all
terminals should see at least tempo IFR by 04Z, with categorical
conditions expected by 06Z. The rain will help to establish cold air
damming across the forecast area, and NE winds are expected to
intensify to around 10 kts at Piedmont terminals after midnight.
While rain is expected to begin tapering off from the west by around
daybreak, cold air damming is expected to keep IFR/LIFR conditions
locked in at most locations through the end of the period.

Outlook: A cold front will sweep through the area late Tue through
Tue evening, which should scour out the cold air damming air
mass/low clouds. Another front will approach on Thursday with some
restrictions possible. Dry and much colder through the weekend.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High  88%     High  82%     High  83%     Med   69%
KGSP       High  84%     High  88%     High  85%     Med   71%
KAVL       Med   75%     High  84%     High  87%     Low   50%
KHKY       High  85%     Med   68%     Med   69%     Med   69%
KGMU       High  83%     High  80%     High  87%     Med   68%
KAND       High  85%     High  87%     High  90%     Med   78%

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:




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