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000
FXUS62 KGSP 042017
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
317 PM EST Fri Dec 4 2020

.SYNOPSIS...
Low pressure will rapidly cross our area this evening, bringing
rain to most areas and light snow to the mountains. Another
fast-moving system will bring light precipitation to the region
Monday. Seasonably cool and dry weather is otherwise expected for
the remainder of next week.

&&

.NEAR TERM /THROUGH SATURDAY/...
As of 237 PM Friday...Scattered showers will move across the
Upstate and wrn Piedmont/foothills of NC for the next few hours,
out ahead of a developing band of precip moving in over the wrn
mountains. We expect that as the forcing increases rapidly late
this afternoon, the band of precip will blossom further as it
spreads east over the fcst area.

The forcing will come courtesy of a compact and dynamic upper low
moving steadily from the mid-MS valley this afternoon to middle
TN/KY early this evening. A strengthening sub-tropical jet streak
across the southeast will bring a left exit region overhead this
evening as the upper low approaches and acquires something of a
negative tilt. Strong mid-level dpva will arrive concurrent with
the upper divergence to result in strong synoptic-scale lift. A low
level jet out ahead of the system will strengthen dramatically as
it translates northeastward this evening as well. All this forcing
will contribute to a rapid deepening of a surface low to the lee
of the Appalachians tonight. There are a number of concerns to
address. The system should move through rapidly, which will cut
down on precip amounts. At the present time, it does not appear that
we will be able to realize even the most meager of sfc-based CAPE,
which in this situation, is a very good thing given the amount of
shear that will develop over the wrn Piedmont this evening. Will
not rule out some elevated storms, but without any of them rooted
to the boundary layer, our chances for severe weather are very
limited. Model guidance keeps a shallow stable boundary layer across
metro Charlotte, that will effectively provide some insulation from
the winds which are expected to stream overhead at around 3k-5k
feet. We will not rule out some heavier showers bringing some gusty
winds to the sfc briefly. This will have to be watched carefully
for the development of a gravity wave east of the mtns, which
sometimes occurs in the wake of a strongly-forced and deepening sfc
low. Winds will likely become strong at the highest elevations this
evening as the low passes, but not to the extent that an advisory
is warranted at this point. Colder air moving in behind the low
will change precip over to snow at the high elevations along the
TN border this evening, but the moisture pulls out rapidly on
Saturday, which should keep snowfall below advisory criteria. We
should clear out rapidly later tonight east of the mtns. Saturday
looks like a sunny and breezy winter day as high pressure builds
in from the west. Temps should be right around normal.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
As of 230 PM Friday: Overall, there have not been significant
changes made to the previous forecast in the current package with
respect to the southern stream system that impacts our area late
Sunday night through early Tuesday morning.  Disagreement amongst
the model guidance has not changed appreciably from the overnight
runs with notable differences in timing and forcing still reducing
overall confidence in the sensible weather impacts.  Therefore,
the forecast still advertises a period of mountain snow beginning
overnight Sunday as the attendant surface low treks across the
Deep South within the southern stream jet.  This light snowfall
will transition to a modest northwest flow event Monday afternoon
as a potent 540mb closed low drops into the central Appalachians.
East of the mountains, without appreciable CAA ahead of the southern
stream system, and wet bulb temperatures above freezing, any precip
that develops on the northern extent of the system early Monday
morning is expected to be a chilly rain at this time.  Of course,
this will continue to be watched.  The ECMWF/Canadian/NAM solutions
remain faster than the GFS and moves southern stream precip by
the area by early Monday afternoon which is about 3-6 hours faster
than the GFS.  The forecast generally supports the consensus with
regards to timing.  All models are in agreement that QPF will be
very light for most.

With the Ohio Valley closed low dropping rapidly SE into the
Appalachians, northern stream phasing still looks to occur generally
east of our area Monday afternoon.  As northwest upslope flow
forcing establishes itself over the mountains Monday afternoon,
lapse rates may be steep enough, when combined with favorable
vorticity advection, to support convective shower activity over the
Piedmont, primarily where any convergence zones set up downstream
of the mountain ridges.  These zones will be highly dependent on
wind direction, but the GFS and the NAM are suggesting fingers of
very light precip developing from the Savannah River Valley to the
I-26 corridor Monday afternoon and evening.  By this time, with CAA
starting to develop, NAM/GFS profiles and wet bulb temperatures
suggest that a transition to snow cannot be ruled out within
these showers should they develop, especially north and east of
the Saluda River Valley.  Overall, low-level dry air is rather
unimpressive at this time, but we are still several days out and
this will bear some watching.

For the short-term period, we are currently expecting light snow
accumulations over the mountain ridges through early Tuesday morning
of up to 2-3" in isolated locations with no accumulations elsewhere.
Temperatures will remain a few degrees below normal Sunday and
chillier on Monday, especially where CAA will be strongest behind
the southern system over the mountains, where temperatures will peak
about 10 degrees below normal.  East of the mountains Monday`s high
temperatures will peak in the upper 40s north and lower 50s south.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
As of 230 PM Fri: With the departing upper low on Tuesday,
conditions will improve quickly and set us up for a seasonably cool
and dry remainder to the work week.  Models are in general agreement
that another potent shortwave will dive SE within the NW flow aloft
and cross the region sometime Tuesday evening through Wednesday
morning. However, there will be no moisture to work with and
outside of the possibility of some gusty winds mixing down over the
higher elevations Tuesday night, the shortwave will be a non-event.
Thereafter, widespread ridging will overtake the eastern US, and
allow our temperatures to moderate nicely into Thursday and Friday,
with highs possibly reaching 4-6 degrees above normal, which is
equivalent to the upper 50s to near 60 east of the mountains.

&&

.AVIATION /20Z FRIDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: Busy for the next 12 hours as a very dynamic
upper low forces the deepening of a surface low moving across
the western Carolinas this evening. Conditions will generally be
MVFR due to a low cloud ceiling this afternoon ahead of a band of
showers moving in from the west. The shower activity is expected
to expand and fill in as the strong mid/upper forcing comes in
to play this afternoon. The convection-allowing models are in
good agreement with the timing of this band of mainly showers
moving east across the fcst area between roughly 21Z and 03Z,
with some shower activity lingering over wrn NC into the early
morning hours. Will employ a 4-hr TEMPO group to account for the
higher intensity of precip and low clouds bringing conditions
down to IFR at least temporarily. We lack sfc-based instability,
which will limit thunder chances, and the stable shallow layer of
air near the sfc will also limit our wind gust potential. In this
situation, the winds will be howling from the S/SW this evening,
but from roughly 3k feet and higher. Note this is not low enough
to incorporate in the TAF as a low level wind shear, which by
definition is in the lowest 2k feet. However, it will be very
close, and that strong wind shear will exist somewhere in the
2k to 5k foot layer, with SW winds on the order of 40-50kt for a
period ramping up around 21Z and then dropping off rapidly with
the back end of the precip after 01Z. We should remain MVFR for
a few hours this evening. East of the mtns, a rapid clearing will
take place followed by a wind shift to WNW or NW behind the cold
front. Right now this is timed at 06Z at KCLT, but could occur
earlier. Meanwhile, at KAVL, the low cloud ceiling may persist
through the early morning hours as low clouds blow up the valley,
bringing a few showers. All sites should be VFR from roughly the
start of morning ops onward. Winds will be gusty from the NW over
the mtn valleys, but gusts are not as likely outside the mtns.

Outlook: VFR for most of the weekend. Moisture will return Sunday
night with a ceiling restriction likely on Monday as another low
pressure system lifts out of the Gulf and up the Atlantic Coast.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High  81%     High  87%     High 100%     High 100%
KGSP       Med   77%     High  91%     High 100%     High 100%
KAVL       High  83%     Med   77%     Med   67%     High  97%
KHKY       High  85%     High  93%     High 100%     High 100%
KGMU       Med   71%     High  93%     High 100%     High 100%
KAND       Med   73%     High  97%     High 100%     High 100%

The percentage reflects the number of guidance members agreeing
with the scheduled TAF issuance flight rule category. Complete hourly
experimental aviation forecast consistency tables are available at
the following link:

www.weather.gov/gsp/aviation

&&

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

&&

$$
SYNOPSIS...JMP
NEAR TERM...PM
SHORT TERM...JMP
LONG TERM...JMP
AVIATION...PM