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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 041151
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
651 AM 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/...
-- Changed Discussion --
As of 635 AM Friday...the latest RADAR imagery has the broader
area of heavier showers still over Central Tennessee and Kentucky.
The lull in precip activity will continue over the next few hours
as the lighter precip moves farther NE of our fcst area and the
more significant precip takes its time approaching our western
zones. Otherwise, temps should remain above freezing thru the
morning hours as cloud cover will remain bkn to ovc.

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.
-- End Changed Discussion --

&&

.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 /12Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
At KCLT and elsewhere: Conditions expected to deteriorate thru
the 12z TAF period as a moist and dynamical low develops over the
fcst area and interacts with an in-situ wedge of high pressure.
Showers will become more widespread across the fcst area thru the
afternoon/evening with CIGS lowering into the IFR and possibly
LIFR range. Some lowering of VSBY into the MVFR/IFR range will
also likely occur within heavier rain bands. The gust potential
will be fairly low-end as sfc mixing appears shallow with winds
generally aligning SELY to SWLY thru most of today. They will
eventually veer around to WLY and increase in speed early Sat.
The main exception will be KAVL, where winds will take on a more
NWLY direction with stronger gusts in the 20 to 26kt range.

Outlook: VFR conditions should return over most sites by late Sat
morning. Breezy WLY to NWLY winds will continue thru late Saturday.
Another round of restrictions is likely early next week as another
low pressure system lifts out of the Gulf and up the Atlantic Coast.

Confidence Table...

            12-18Z        18-24Z        00-06Z        06-12Z
KCLT       High  85%     Med   78%     Med   78%     High 100%
KGSP       High  95%     High  98%     High  82%     High 100%
KAVL       High  91%     Med   79%     Med   78%     High  88%
KHKY       High  89%     High  92%     High  81%     High 100%
KGMU       High  95%     High  90%     High  81%     High 100%
KAND       High  85%     High  81%     High  89%     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...Wimberley
NEAR TERM...JPT
SHORT TERM...Wimberley
LONG TERM...Wimberley
AVIATION...JPT