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-- Discussion containing changed information from previous version are highlighted. --
909
FXUS62 KGSP 280834
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
334 AM EST Sat Nov 28 2020

.SYNOPSIS...
-- Changed Discussion --
An upper disturbance will move east of the area today. Dry high
pressure will then briefly return through tonight. A strong low
pressure system approaching from the southwest may support a threat
for heavy rainfall from late Sunday into early Monday. Northwest
flow snow is expected to develop across the mountains in the storm`s
wake from late Monday through Tuesday. Another strong low may cross
the region at the end of the week.
-- End Changed Discussion --

&&

.NEAR TERM /THROUGH TONIGHT/...
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As of 305 AM EST: Area of light showers moving across the CWFA
faster than guidance expected. Have increased PoP numbers and area
covered for early morning in response. QPF will be light. There will
be some patchy fog along and east of the I-77 corridor as well.

Precipitation will still exit quickly after daybreak with the
eastward departure of the upper short waves that helped initiate the
showers. Another sfc high will build in following the departure of
the shortwaves. Cloud cover should diminish quickly thru the late
morning as well. Plentiful sunshine and downsloping will partially
offset the cooler airmass. Highs should be near 5 to 10 degrees
above normal.

High pressure continues to build into the area as the center of the
high moves into VA. Low level easterly flow develops and could bring
some moisture back into the area. The guidance disagrees on how much
moisture moves in, and the amount of any clouds. Do show an increase
in clouds over the western CWFA, but fog could be more of a concern
in some locations where clouds don`t develop. Of course, there`s
disagreement in fog development as well. Lows will be around 5
degrees above normal, but could be cooler if clouds are slow to
develop and winds don`t keep the atmosphere slightly mixed.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
-- Changed Discussion --
As of 200 AM Saturday: The first half of Sunday will start out dry
as sfc high gradually pulls offshore as our next system begins
to push into the cwa around 00Z Monday. This is all in part of
a cutoff low that is pushing eastward and eventually gets caught
up with the southern stream energy that meanders along the Gulf
Coast through the period. Both systems will carry the momentum
into the Southeast with deep layer moisture and perhaps modest
instability, especially in the southern fringe of the cwa. Model
guidance has been spitting out 100-300 J/kg of SBCAPE and 0-6
km effective bulk shear of 40-50kts. The better environment for
any strong to severe thunderstorm development is south and east
of the I-85 corridor. Confidence is on the upward trend on the
the overall severe threat staying confined to that area, while
confidence remains steady with a low-end hydro threat over the
southern Blue Ridge Escarpment due to upslope enhancement.

As the sfc low and attendant frontal boundary pushes upstream the
backdoor longwave trough that eventually absorbs the cut off low,
cold NW flow will push into the area and the transition to winter
weather is likely over parts of the mountain community. Heaviest
amounts will be at the highest peaks and TN-NC border along the
spine of the Blue Ridge where the transition is likely to occur
much sooner than other areas in the higher elevations. A sharp drop
in thicknesses around 18Z Monday will drop snow levels overall,
but the most favorable locations will be elevations at or above
3500 ft. With a lingering moisture fetch over the OH Valley and
stretching into the southern Appalachians on the backside of
the low. Expect the snow to stick around through the end of the
forecast period for the higher peaks in NC.

Last hazard that could present weather troubles will be the
tightening of the pressure gradients over the mountains once
the NW component fully becomes established. CAA will enter the
environment which will only increase the overall momentum at the
sfc. Gusty winds and possible advisories could be considered in this
scenario to go along with the expected snowfall in the mountains
of NC. Near-normal temperatures are expected through Monday, but
a huge dip is expected Monday night with lows in the 20s for the
mountains and low 30s every where else.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
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As of 300 AM Saturday: The evolved upper low sets up shop
over the eastern CONUS with the center of it just north of the
area. Very cold air will be in store with thicknesses <540 dm
and deep radiational cooling, expect the coldest max and min
temps of the season so far with highs not getting out of the 40s
anywhere and lows in the 10s across the mountains and low to mid 20s
elsewhere. During this part of the medium range, temperatures will
be 10+ below climo for this time of the year. Any lingering NW snow
should taper off by Tuesday afternoon and dry air will filter into
the cwa. Temperatures begin to rebound by the middle part of next
week as upper ridging pushes higher heights into the region and the
upper low ejects to the north and east. Broad sfc high controls the
overall weather pattern through Thursday before more changes take
place. Model guidance really diverge past Thursday due to the
location and track of the next cut off low that generates over the
southwestern CONUS just like the previous. Both the GFS and ECMWF
show a second cut off low diving south over the High Plains while
the other develops over the Southwest. The ECMWF actually phases the
two together over the Southern Plains and forms a very potent upper
low that moves across the southeastern CONUS by the end of the
period. However, the GFS keeps the initial cut off low over the
Southwest while the second cut low dives in from the High Plains and
slides into the Southeast near the same time frame as the ECMWF. Not
as potent, but keeps the area unsettled to say the least and kept 20
PoPs in the forecast for now due to the lack of confidence this far
out. Looks as if the unsettled and amplified pattern will continue
as we run through the first week of December. Outside of Tuesday,
Tuesday night, and Wednesday night expect temperatures to make a
return to near-normal or slightly below-normal values on the back
half of next week.
-- End Changed Discussion --

&&

.AVIATION /09Z SATURDAY THROUGH WEDNESDAY/...
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At KCLT and elsewhere: Moisture will increase ahead of two passing
upper waves early this morning, with lowering VFR bases expected
along with areas of sprinkles or vicinity rain showers at or near
most of the TAF sites. Clouds will scatter quickly behind the
departing upper waves late morning through the afternoon. VFR
conditions will continue. Light and variable winds early will become
N to NE at less than 10 kt through the day, except continued NW flow
at KAVL. Winds become light and variable again during the evening
with a return of mid and high clouds, especially across the west.

Outlook: Increasing chances of rainfall and restrictions expected to
develop Sunday and continue into early next week. Wintry
precipitation will be possible in the mountains Monday into Tuesday
with VFR returning elsewhere.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     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 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...RWH
NEAR TERM...RWH
SHORT TERM...CAC
LONG TERM...CAC
AVIATION...RWH