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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 042341
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
641 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/...
-- Changed Discussion --
630 PM Update...A line of moderate to heavy rain is pushing east
across the FA. Due to the quickness of the system, not seeing great
precip totals with a few tenths of an inch across the non/mtn and
amts so far across the wrn NC mtns of arnd 1.5 inches. No expecting
any hydro issues as the system will continue pushing east of the FA
by 04z. The thunder threat isn/t great as sfc-based instability
remains low, however, the CLT area and south could see dynamically
driven cells capable of producing lightning over the next few hrs.
Moderate gusty winds are also possible over the ern zones as a
developing llvl jet crosses the area and showers may bring down
stronger winds to the sfc.

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.
-- End Changed Discussion --

&&

.SHORT TERM /SATURDAY NIGHT THROUGH MONDAY NIGHT/...
-- Changed Discussion --
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 light rain at this time.
Of course, this will continue to be watched.  The ECMWF/Canadian/NAM
solutions remain faster than the GFS and move southern stream precip
by the area early Monday afternoon which is about 3-6 hours faster
than the GFS.  The forecast generally supports the consensus with
regards to timing.  Lacking appreciable moisture flux, all models
are in agreement that QPF will be light.

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 to support appreciable wet bulbing, and
SBCAPE values remain below 100 J/kg (a decent threshold to watch
for stronger dynamic cooling), 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 are forecasted to peak in the upper 40s north
and lower 50s south, but this was below model blends and we expect
the forecast highs to trend cooler with successive packages.
-- End Changed Discussion --

&&

.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 /00Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --
At KCLT and elsewhere: A band of light to moderate showers crossing
the fcst area is lowering CIGS and VSBY into the MVFR/IFR range
while areas across the west are seeing lowering CIGS in a highly
moist airmass behind a sfc low. Expect these CIGS to improve as
drier air mixes in from the NW thru 04z or so. Not expecting a great
thunder threat at KCLT as the main convg line moves in, but sw/ly to
nw/ly winds will likely gust in the low to mid 20 kt range thru 03z.
Improving flight cats across all TAF sites after midnight, with
moderate nw/ly gusts likely at KAVL. Winds shud remain strong enuf to
offset a morning fog threat over most terminals, yet MVFR to IFR
VSBY may occur occasionally before sunrise at KAVL.

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...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       Med   78%     High  90%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  89%     Med   75%     High  80%     High  94%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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
-- End Changed Discussion --

&&

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

&&

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