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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 041804
AFDGSP

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

.SYNOPSIS...
Low pressure will cross our area this afternoon into early Saturday
before developing into a Nor`easter off the Mid-Atlantic coast,
bringing rain to most areas and light snow to the mountains. Another
fast-moving system could bring light precipitation to the region
Monday. Seasonably cool and generally dry weather is otherwise
expected for much of the next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1247 PM Friday...the leading edge of the precip band has
broken up somewhat over the intervening few hours, but expect as
the forcing increases rapidly late today, it will blossom and fill
back in, so the trend toward very high precip probs still looks
good. Have made a few adjustments on the back end this evening as
the upper/sfc low deepen rapidly as they exit the fcst area. Will
re-evaluate thunder chances across the CLT metro area in light
of the updated SPC Day 1 moving the wrn edge of the Marginal
perilously close. Still like the idea of a small thunder chance
over the Lakelands/lower Piedmont for elevated storms. There is
increasing concerns over a period of strong wind gusts this evening,
which will be evaluated for the afternoon package as well.

Otherwise, an upper low centered over the mid-Mississippi
Valley will slowly swing east thru the Tennessee Valley while
gradually phasing with stronger northern stream energy. Improving
upper-lvl dynamics and fetch off the Gulf of Mexico will support
cyclogenesis along a cold front over the northern Gulf. The low
pressure system will race NE today, with the center of the low
expected to move right over the CWFA this evening. Rain ahead of
this feature will overspread the area from the SW thru the day,
possibly becoming moderate to heavy at times this aftn/evening
across the NC Mountains and Piedmont. Temps should remain too warm
to support anything but rain, and the majority of guidance has
minimal instability at best, so thunder remains unlikely. As the low
lifts NE of the CWFA early Saturday, the gradient wind increases
out of the W and NW, and we can expect some light NW-flow snow
showers over the NC Mtns. The higher peaks could see some wind
gusts approaching Advisory criteria, but these speeds should be
relegated to elevations of roughly 5000ft and above. In addition,
snow accums are not expected to exceed an inch for most of the
impacted areas, although higher amounts are possible over the
northern mtns and along the TN border thru 18z Saturday.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
As of 200 AM Fri: Moisture profiles in the NW upslope areas near the
Tenn border suggest that any snow showers will continue for a portion
of the day Saturday, tapering off as the moisture is depleted. Winds
will continue to be brisk through the day at those higher elevations.
A relatively weak sfc high will fill in thereafter; the upper flow
becomes quasi-zonal if not slightly anticyclonic on Sunday.

It gets a little complicated late Sunday into Monday as one or more
shortwaves rotate around the departed low to our north, and into the
Southeast. The major models remain in some disagreement on how these
waves phase, but the orientation of the upper jet would seem to
support good dynamic lift across the CWA even if the phasing isn`t
perfect; this looks likely to result in the shortwaves congealing
into an upper low that reaches its deepest state over the CWA around
midday Monday. A remaining appreciable difference is in how much
moisture is brought in ahead of the southernmost shortwave, which
originates in the ArkLaTex region. The ECMWF and Canadian GDPS are
wetter in this sense, suggesting deep moisture is replenished sooner
and beats the cold air into the area, so to speak. The GFS is a bit
drier and colder to start, but a bit wetter with the upslope precip
on the backside of the system, so it ends up a bit snowier overall.
It also hints at a touch of Piedmont snow as the low departs. The NAM
does not yet encompass the whole event but appears to feature a
moisture trend more like the EC/CMC camp while also being colder. The
NAM/GFS differences however may be attributable to their higher
resolution. Went with a blend of PoP and temp trend guidance that
favors mainly slight-chance values due to the low confidence, with a
more EC/CMC-like timing. This keeps almost all of the snow mention
over the mountains and foothills; it looks like another event where
the same forecast trends that warrant higher PoPs likely will warrant
generally warmer temps. Even with the EC/CMC solutions having better
moisture initially, the continental airmass preceding the system
doesn`t allow for really good moisture fetch and the event is a
pretty quick hit. Thus QPF, and the resulting snow accums, are
currently small.

Temps initially will run just a little below normal. Above-normal
mins are expected Sunday night under increasing moisture, and
significantly below-normal temps Monday and Monday night with the
ongoing precip giving way to CAA.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
As of 310 AM Fri: Sfc high pressure looks to dominate the
pattern over the Southeast from Tuesday through early next Friday,
suggesting a relatively quiet period. The initial East Coast upper
trough will shift offshore late Wednesday. The Euro does show
another trailing shortwave diving toward the Gulf around this time,
but it depicts cyclogenesis too far south and east to affect us;
this depiction may be something to watch as that could change,
but for now not worthy of any PoP. Temps moderate back above normal
by Day 7.

&&

.AVIATION /18Z 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...

            18-24Z        00-06Z        06-12Z        12-18Z
KCLT       High  83%     High  80%     High 100%     High 100%
KGSP       Med   79%     High  89%     High 100%     High 100%
KAVL       High  87%     High  82%     Med   77%     High  95%
KHKY       High  88%     High  90%     High 100%     High 100%
KGMU       High  82%     High  90%     High 100%     High 100%
KAND       Med   75%     High  87%     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...Wimberley
NEAR TERM...JPT/PM
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...PM