Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 201537
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1037 AM EST Sat Jan 20 2018

.SYNOPSIS...
High pressure will move offshore this weekend and allow a warm
southerly flow through Monday. A rainy cold front will cross the
region Tuesday followed by dry high pressure persisting through the
end of the work week.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 1030 AM EST Saturday:  Made a few tweaks to sky and temps as
moisture along the TN line continues to yield widespread upslope
stratus, while elsewhere temps are warming nicely.  Therefore
adjusted sky along the NC/TN border by increasing such along the
ridges/peaks, and also bumped temps up a few degrees.  Otherwise the
prevailing fcst is in good shape.  Will let the black ice SPS go on
schedule at the top of the hour given temps in the upper 30s to
lower 40s across the piedmont.

Previous Discussion: A split flow pattern persists across the SE
CONUS today/tonight while a weak srn stream h5 low traverses south
of the FA. Upper heights will rise ahead of the low which will help
keep the area dry along with above normal warmth by a couple cats.
The NAM continues to be the only guidance indicating upslope precip
across the far wrn NC mtns. Soundings across the spine indicate any
precip that falls would be frozen in the form of fzdz or sleet this
morning as a weak warm nose forms within a shallow moist layer. Have
adj the going slt PoPs by limiting the potential to the nrn mtns. If
any precip occurs it will transition to rain arnd 14z with no sigfnt
accums thru the day. The raw model guidance and some MOS output is
showing a distinct minima in max temps across the NC Piedmont today.
This is confusing considering rising heights...persistent llvl
theta/e adv and a downsloping h85 flow. So...went with the warmer
ADJMAV and even bumped this up a degree or two with good insol on
tap. With increasing llvl moisture...mins will remain abv freezing
over most areas for the first time in a week.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
As of 230 AM EST Saturday: Short wave ridging builds over the area
Sunday and Sunday night ahead of a deep upper low moving into the
central plains. At the surface, the center of high pressure moves
across the area and offshore. Southwesterly flow and rising heights
will lead to above normal temps both periods. After a foggy start
and lingering clouds across the mountains skies clear during the day
with returning clouds overnight.

Heights fall through Monday night as the upper low moves into the
lower Great Lakes and the associated trough south of the low moves
into the southern Appalachians. Short wave energy rounds the low and
moves across the area. However, the best energy moves by to our
north. A strong upper jet moves toward the area as well but the best
divergence also moves by to our north where the left exit region of
the jet is located. At the surface, low level south to southwesterly
flow strengthens Monday as a cold front moves into Middle TN. This
brings increasing moisture into the area. With the best forcing
remaining to our west during the day, precip will be slow to move in
but will increase in the southerly upslope flow areas during the
afternoon. The front moves into the area Monday evening then east of
the area overnight. A strong southerly low level jet moves in with
the front increasing wind speeds and moisture and creating strong
shear. Helicity values will be quite high as well. The NAM has
MUCAPE values up to around 300 J/kg while the GFS is much less. The
SREF mean does show a good chance of over 100 J/kg MUCAPE values.
This creates the potential for a high shear/low CAPE scenario Monday
night as this system moves through. Heavy rain will be a possibility
as well with copious moisture and strong southerly flow. Temps will
be above normal.

The front quickly moves east Tuesday taking the moisture and forcing
with it. There will be some lingering moisture along the TN border
in the developing NW flow during the morning, so cannot rule out
some light snow showers. Highs will be near normal mountains and
around 5 degrees above normal elsewhere.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
As of 315 AM EST Saturday: A weak clipper-type short wave moves
across the east coast Wednesday and Wednesday night. Do not expect
any precip with this system but an increase in clouds and continued
windy conditions across the mountains is likely. Temps drop back to
near normal for Wednesday and Thursday. Surface high pressure
remains over the area Friday as an upper ridge builds overhead. This
keeps precipitation to our west. Temps remain steady or rise slightly
on Friday.

&&

.AVIATION /16Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: No restrictions thru the TAF period all sites
outside of some possible MVFR VSBY over the NC terminals arnd
daybreak Sun. Otherwise...the atmos will remain very dry thru a deep
layer as an area of upper forcing and moisture crosses south of
area. A low-level ridge and sfc high pressure will maintain weak sfc
winds aligned w/ly to sw/ly non/mtns and up-valley across the mtns
today with decoupling expected aft midnight. Some increase in llvl
clouds across the mtns this morning/afternoon while FEW/SCT Ci
crosses from the southwest all sites.

Outlook: Potential for MVFR Monday into Tuesday with an approaching
cold front. Otherwise expect VFR.

Confidence Table...

            15-21Z        21-03Z        03-09Z        09-12Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High  96%     High  96%
KHKY       High 100%     High 100%     High 100%     High  94%
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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...SBK
NEAR TERM...CDG/SBK
SHORT TERM...RWH
LONG TERM...RWH
AVIATION...SBK



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