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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS62 KGSP 040304
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1004 PM EST Thu Dec 3 2020

.SYNOPSIS...
Low pressure developing across the Deep South tonight will cross
our area late tomorrow into early Saturday before developing into a
Nor`easter off the Mid-Atlantic coast. An upper air disturbance could
bring light precip to the region Monday. Seasonably cool and
generally dry weather is otherwise expected for much of the next
week.

&&

.NEAR TERM /THROUGH FRIDAY/...
-- Changed Discussion --
1000 PM Update...The fcst is in good shape. Made some minor temp
adjustments, but otherwise values are falling slowly in high and
increasingly thicker cloud cover. Some upslope drizzle, light rain,
and perhaps a few snow flurries abv 3.5 Kft will fall across the NC
mtns and NE GA with no sigfnt accum thru the next update.

As of 200 PM Thursday: The main weather interest into Friday is
associated with an upper low that was centered over the mid
Mississippi Valley. The upper low will slowly swing east trough the
Tennessee Valley while gradually phasing with stronger northern
stream energy. Continued weak warm air advection and increasing
cloud cover ahead of this system tonight will keep most areas above
freezing with 30s to near 40 common. Improving upper dynamics and
fetch off the Gulf of Mexico will support cyclogenesis along a cold
front over the northern Gulf tonight. The low pressure system will
race northeast through the day, pushing into our portion of the I-85
corridor early in the evening. Rain ahead of this feature will
overspread the area from the southwest through the day, possibly
becoming moderate to heavy at times in the afternoon across the
mountains. Fortunately, guidance keeps wet-bulb values above
freezing by the time precipitation would start so it appear we will
be dealing with plain ole rain. Even though the warm sector of the
system tries to work into far eastern areas during the afternoon and
evening, the most aggressive guidance has minimal instability, so in
coordination with neighboring offices have removed thunder mention
from the forecast. Warm air advection and increased cloud cover will
roughly cancel out for highs again in the 50s to near 60 east of the
mountains Friday with mainly 40s across the mountains.
-- End Changed Discussion --

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY/...
As of 225 PM Thursday: The aforementioned surface low will continue
to rapidly intensify as it departs the area Friday evening. Rain
will gradually come to an end east of the mountains during this time
as drier air filters in behind the departing cold front, enhanced by
downsloping flow off of the mountains. Rainfall totals will
generally be less than an inch, with one to locally two inches
possible across the escarpment and southwest mountains. A brief
northwest flow event may occur on the back-side of this system into
the first part of Saturday. However, only marginally cold air will
be especially chasing the best moisture out of the area. In fact the
moisture may become shallow enough that a period of sleet or
freezing rain may mix in at times focused across the highest
elevations due to a lack of ice nuclei. An inch or two of snow will
be possible above 5000 feet with a dusting possible due around 3500
feet nearest the TN border. Breezy but sub-advisory northwest winds
and drying conditions expected to continue through the day Saturday.
Weak high pressure will briefly build into the area Saturday night
into Sunday. Good radiational cooling conditions will support lows
generally in the mid 20s to low 30s. Daytime highs will continue to
be mainly in the 50s, although 30s to 40s across the mountains on
Saturday. Clouds may begin to increase late in the day Sunday ahead
of our next potential system.

&&

.LONG TERM /SUNDAY NIGHT THROUGH THURSDAY/...
As of 150 pm Thursday: It appears likely that the complex split flow
pattern will become phased across the East either late in the short
term or early in the extended...with the timing of this process
depending upon which global model one prefers. The timing and
details of this evolution will have implications on the sensible
weather for our forecast area early next week. The hot-off-the-
presses ECMWF looks very similar to the 12Z Canadian in phasing a
northern stream wave into the southern stream across the Deep South
Sunday night/early Monday. The result of this far-south trajectory
is cyclogenesis that occurs almost in the central Gulf, with a track
into south Florida and minimal precip impacts for our area. While
the GFS upper air evolution looks quite a bit different, with
phasing of the two streams occurring later and farther northeast, it
too depicts surface development well to the south. The difference
lies in the fact that the GFS has a much more robust/clipper-like
upper low crossing the forecast area on Monday, resulting in a round
of northwest flow showers, some of which could break containment
into the foothills and Piedmont considering how far the mid-level
speed max is depicted.

Having said all that, either of these scenarios support some
potential for light precip across the forecast area on Monday, with
the GFS scenario being a little more favorable for possible wintry
precip. Even so, this looks to be a very low-at-best qpf event, and
slight chance pops are about as far was we`re willing to go at this
point. Otherwise, the extended period will be dry and seasonably
cool, with some moderation in temps likely by Day 7, when global
models generally agree in building an upper ridge across the East.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
At KCLT and elsewhere: Flight conds will deteriorate during the TAF
period as a highly moist and dynamical low approaches the area and
interacts with an insitu wedge of hipres. Expect areas of showers to
begin across the wrn terminals shortly aft daybreak and then become
more widespread thru the early afternoon over all sites. Lowering
CIGS into the IFR and LIFR range will become widespread by the
afternoon and persist thru the period. Some lowering of VSBY into
the MVFR/IFR range will also occur within heavier rain bands. The
gust potential will be low as sfc mixing remains very shallow while
winds generally align se/ly to sw/ly thru the evening.

Outlook: SHRA will come to an end Saturday with a return to VFR over
most sites by the afternoon. Breezy northwest winds will develop
Friday night and continue into Saturday.

Confidence Table...

            03-09Z        09-15Z        15-21Z        21-00Z
KCLT       High 100%     High 100%     High  81%     High  81%
KGSP       High 100%     High  96%     High  95%     Med   76%
KAVL       High 100%     High 100%     High  83%     Med   74%
KHKY       High 100%     High 100%     High  85%     High  86%
KGMU       High 100%     High  86%     High  96%     Med   78%
KAND       High 100%     Med   66%     High  98%     Med   78%

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...JDL
NEAR TERM...Munroe/SBK
SHORT TERM...Munroe
LONG TERM...JDL
AVIATION...SBK