Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 051929
AFDGSP

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

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TUESDAY/...
As of 230 PM EST Monday...our lull in-between systems continues, but
already changes are underway with the next system taking shape to
our SW. Cloud cover remains thin and high over the wrn Piedmont and
nrn Foothills early this afternoon, but that will change as moisture
returns ahead of the upstream system. This has had an affect on
temps, which fortunately was taken into account in the previous
fcst. The brief end to precip chances will give way to an increase
in precip from the SW from now through the early part of the
evening. Already the precip approaches from the SW, nearly at the
edge of the Savannah River at 18Z.

Over the next 12 hours or so, as an old closed upper low opens up
along the TX Gulf Coast, we should see a return of deepening
moisture and the development of isentropic upglide from the S/SW to
the north of a warm frontal boundary lifting NWD from the Gulf.
Meanwhile, 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. Precip chances
ramp up accordingly as moisture and forcing increase, to the point
where the whole fcst area should get wet. 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.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
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.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
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
continue.

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.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
At KCLT and elsewhere: Have seen improvement early this afternoon
and we appear to be in the lull between weather systems at the
outset. Already we can see the next round of restrictions coming
over the SW horizon, so the VFR period will not last much beyond
sunset in many locations. Expect widespread RA to develop with IFR
cigs sometime after 00z, with some sites possibly dropping to LIFR
in the early morning hours. For all but KAVL, NNE winds will shift
more NE and ENE through the period, increasing to over 10kt with
low-end gusts. KAVL will see N up-valley winds shifting SE
down-valley this afternoon. Once in place, the IFR restriction will
probably not lift until midday Tuesday, but these details were
omitted for brevity.

Outlook: Rain will be on the decrease Tuesday afternoon with drying
expected through Wednesday. Another front will approach on Thursday
with some restrictions possible. Dry and much colder through the
weekend.

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       High  96%     Med   64%     Med   72%     High  85%
KGSP       High  88%     Med   72%     Med   76%     High  95%
KAVL       High  89%     High  80%     Med   76%     High  95%
KHKY       High  83%     High  88%     Med   70%     High  93%
KGMU       Med   78%     Med   69%     Med   77%     High  92%
KAND       Med   66%     Med   74%     Med   68%     High  92%

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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...SBK
NEAR TERM...PM
SHORT TERM...Wimberley
LONG TERM...SBK
AVIATION...PM


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