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fxus62 kgsp 072144 

Area forecast discussion
National Weather Service Greenville-Spartanburg SC
444 PM EST Sat Dec 7 2019

dry high pressure will be in control of the weather for the
East Coast through Sunday then unsettled weather returns for the
first half of the week. A cold front crosses our region late
Tuesday. Rain is likely Tuesday, possibly changing to snow in the
North Carolina mountains early Wednesday. Dry and cold high pressure
will return behind the front.


Near term /through Sunday/...
as of 440 PM EST saturday: quiet weather continues across the area
early this evening. Skies are a bit more clear than expected across
the foothills, but clouds should be on the increase this evening as
high pressure nudges east of the mountains and support better
easterly upslope flow. A few sites are running a few degrees warmer
than forecast, but expect cooling associated with sunset to allow
current conds to catch up with the forecast.

Otherwise, fairly quiet near-term period with dry onset cad
developing today. Broad troughing aloft with trough axis moving
offshore this afternoon, and surface high pressure setting up over
the northern mid-Atlantic states. The surface high will ridge down
the eastern slopes of the Appalachians tonight, remaining locked in
place as the parent surface high slowly slides east and eventually
offshore by Sunday afternoon. Uplope flow will bring some moisture
to the mountains, really only in the form of clouds; not enough deep
moisture for any precipitation to form, though with the clouds over
the mountaintops and below freezing temps tonight some typical rime
ice may form at the highest elevations. Temperatures tonight will be
pretty close to seasonal normals, and the damming and low-level cold air advection
will keep highs tomorrow 5-7 degrees below normal. Expect increasing
clouds late in the period as moisture starts working over the cold
dome from the SW and a cold front approaches from the Ohio Valley, but
rain should hold off till after the end of the period.


Short term /Sunday night through Wednesday/...
as of 130 PM Sat: the center of surface high pressure moves east
away from the East Coast Sunday night as a frontal system approaches
from the west. A moist south to southwesterly flow develops between
the two systems. However, isentropic lift remains weak and the
moisture shallow. The better forcing and deeper moisture, along with
upslope flow, will be over the mountains Sunday night and Monday.
Keep the highest pops over those locations. Low temps Sunday night
should be early with slow warming given the flow and warm air advection. Therefore,
do not expect anything but rain. Highs Monday will be tricky as the
pattern looks like an in-situ type cad would develop. However, the
lack of continuous precip outside of the mountains would limit the
strength of any cad. There will be quite a bit of clouds and weak
northerly surface winds, so temps could still be held down.

Precip chances increase slowly Monday night as the front cold front
slowly moves toward the area and the low level flow remains more
southwesterly. Chances increase more quickly across the area Tuesday
as the front moves in. Some heavy rainfall is possible across the NC
mountains west of the French Broad valley. While an isolated
thunderstorm can't be ruled out, any instability is very weak
keeping any severe chances low despite strong shear. Temps will be
above normal despite the precip as any wedge erodes.

High pop continues early Tuesday night as the front only slowly
crosses the area. However, pop quickly diminishes from west to east
overnight and early Wednesday as the front picks up speed and dry
high pressure moves in from the northwest. Can't rule out a brief period of
northwest flow snow across the mountains as colder air moves in behind the
front. However, no significant accums expected. That said, lows
Tuesday night remain above normal, but highs Wednesday fall below


Long term /Wednesday night through Saturday/...
as of 240 PM Sat: quite a bit of run to run inconsistency with the
model solutions for the medium range. This leads to a high
uncertainty and low confidence forecast, especially for the end of
the period.

The GFS is on its own showing cyclogenesis over the Gulf Coast
Wednesday night and Thursday spreading precip into the area as the
low moves east. It only slowly diminishes precip Friday as the low
moves up the Atlantic coast. It then has another round of
cyclogenesis near the Georgia/Florida coast which spreads precip back into the
area for Saturday. The GFS also shows an odd and generally unlikely
feature of cold air developing into the area from the NE after
precip has already begun and without an associated continued dry
fetch after any initial evaporational cooling. Forecast soundings
show a strong warm nose, making precip more freezing rain, possibly
mixed with or starting as sleet at onset, eventually changing to
rain after an the seemingly too long in duration cold surface temps.

The ECMWF, the Canadian generally agrees with this scenario, has
dry high pressure over the area Thursday. It does show cyclogenesis
in the Gulf of Mexico, but much farther south than the GFS. This low
then slowly moves east on Friday with energy transferring to the
Atlantic coast. This spreads precip north into the area as early as
late Thursday night but more likely on Friday. Precip then remains
over the area through the end of the period. The European model (ecmwf) does start
out cold and dry with a strong warm nose. It would also suggest more
of a freezing rain, possibly mixed with or starting as sleet at
onset changing to rain as the atmosphere slowly warms. This temp
trend certainly seems more reasonable and fits the pattern
recognition. Therefore have trended toward this scenario with the
forecast. Still, given the uncertainty, any deterministic forecast
has a low chance of occurrence. Will not put any mention of wintry
weather in the severe weather potential statement for now, but this will need to be monitored.


Aviation /22z Saturday through Thursday/...
at kclt and elsewhere: VFR through the period for all but kavl,
where MVFR cigs may develop in the predawn hours. NE to east-northeast winds 5-
10mph will continue through most of the period as damming continues
to develop, though down-valley southeast winds will prevail at kavl.
Upstate sites should see the strongest winds, closer to the
gradient, whereas kclt/khky might go more lgt/vrb overnight. Expect
increasing clouds tomorrow but should remain VFR through the end of
the period.

Outlook: restrictions are likely to return late Sunday into early
Monday with occasional rain developing until a cold front pushes
through the area Tuesday. Dry high pressure should return by midweek.

Confidence table...

21-03z 03-09z 09-15z 15-18z
kclt high 100% high 100% high 100% high 100%
kgsp high 100% high 100% high 100% high 100%
kavl high 96% high 91% Med 78% high 100%
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 and ensemble forecasts
are available at the following link:



Gsp watches/warnings/advisories...



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