Area Forecast Discussion
Issued by NWS Greer, SC

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988
FXUS62 KGSP 152350
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
750 PM EDT Sun Oct 15 2017

.SYNOPSIS...
Light rainfall chances increase tonight and linger through Monday
morning as a fast-moving cold front approaches and moves through
the area.  Dry conditions and below-normal temperatures will
follow the front beginning Monday and persist through mid-week.
Warmer temperatures will return later this week as deep-layer
ridging sets up over the Southeast US.

&&

.NEAR TERM /THROUGH MONDAY/...
As of 745 PM EDT Sunday:  Updated discussion for 00z taf issuance.
A weakening/broken line of showers ahead of a cold front continues
to move east across the TN valley at this time.  These showers
will arrive in the mtns of western NC over the next hour thus pops
were allowed to continue their previous upward trends on schedule.
The latest SPC Mesoanalysis favors a relatively stable airmass
across the mtns and much of western NC, however with some elevated
instability still in play along/south of the I85 corridor.
Thus am expecting the southern flank of this convective line
to move into ne GA and the western Upstate closer to midnight,
possibly with a few rumbles of thunder to accompany.  All in all,
the current fcst trends look good with only a few minor tweaks to
near term t/td/sky made.

A major upper air pattern change and attendant cold front will
result in much cooler and drier weather across the forecast
area beginning Monday. As of 205 pm Sunday: the wind shift
representing the leading edge of this cold front extends from
the lower Mississippi Valley into the western Ohio Valley. Due to
the anafront nature of this boundary, the bulk of shower activity
is west of the wind shift. However, a narrow convective band has
developed within an instability axis along the convergence zone,
and this activity is expected to begin moving across the NC mtns by
mid/late evening. In the interim, isolated showers will continue
across portions of the high terrain of southwest NC and northeast
GA in response to strengthening SW flow/increasing mechanical
lift. The thunderstorm potential through tonight looks to be very
low, and only showers will be forecast in most areas.

Having said that, later tonight, short term model guidance does
depict some (mostly elevated) instability increasing across the
Piedmont. This could result in a few lightning strikes across the
Piedmont late tonight, and a slight chance of thunder has been
introduced in some of those areas. However, the main impact of this
instability may be to sustain shower activity east of the mtns,
despite increasing downslope flow and weakening convergence along
the boundary. Nevertheless, by far the highest precip chances should
exist along the TN border, where categorical pops appear warranted.
Pops taper off east of there, with mainly solid chances forecast
east and south of the Blue Ridge.

Min temps tonight will remain well above normal in most areas,
but most mtn locations should see temps fall rather quickly around
daybreak/sunrise.  Based upon the orientation of the post-frontal
surface high,  winds are expected to shift to the N or even N/NE
rather quickly in the wake of the front Monday morning. This will
preclude the amount of substantial downslope warming that one would
typically associate with autumn fropas. Thus, even areas east of
the mountains will probably see maxes reached before or near noon,
with temps remaining steady through the day. The result will be
forecast maxes around 5 degrees below climo Monday afternoon.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY/...
As of 2pm EDT Sunday:  By Monday evening, cold front and
precipitation will be well past the area to the southeast with dry
air advection on-going. Winds on Monday will be somewhat brisk out
of the north and will feel cold. By Tuesday morning, dewpoints
plummet nearly 20 degrees into the low-30s in the higher elevations
to upper 40s in the piedmont, and PWATS plummet to .4 inches.
Clouds also become scarce Tues. and Wed.  Following passage of upper
synoptic wave Monday night, region goes into a weakly forced regime
that is not much affected by passage of clipper-system through the
Great lakes on Wednesday.

Main issue Tuesday and Wednesday will be low temperatures in the
mornings which could give some patchy frost to the higher elevations.
Calm winds, clear skies, and dry air all combine for lows Tuesday
and Wednesday mornings that are several degrees below normal, and
well below the past few weeks.  Highs Tuesday and Wednesday are
anticipated to be near normal, which will be a substantial change.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
As of 200 PM EDT Sunday: A southern stream wave will move off the
southeast U.S. coast Wednesday night, with a ridge building from the
MS River Valley to the southern Appalachians Thursday through
Friday. Surface high pressure will set up under the upper ridge
axis, with very dry profiles in place. The dominant ridge will
remain in place over the southeast for the first half of the weekend
before breaking down from the northwest on Sunday in response to a
full latitude trough sweeping east through the plains. Slightly
better low level moisture return from the Atlantic, along with
deeper layer SW flow aloft, should just be getting started at the
end of the forecast period Sunday night. Will feature a gradual
increase in clouds on Sunday, but the forecast will remain dry.
Temperatures will steadily rebound to about two to three categories
above climatological values through the long term period.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
At KCLT and Elsewhere:  VFR conditions are expected to prevail
at most sites through the period, with an occasional lowering to
MVFR/IFR as a cold front intrudes.  As such, a line of showers
continues to work through the TN valley, likely to move into
the western NC mtns over the next hour two.  This line and its
associated front will sweep through the region overnight leading
to lowering of cigs and visb, as well as chances for showers.
Thus all tafs feature similar trends with timing adjusted by site
location relative to frontal proximity as well as CAM guidance.
Although it`s likely that all sites will see precipitation,
confidence really wasn`t high enough to prevail wx therefore tempos
were used where appropriate.  One thing to note is that guidance
does seem to be a bit more optimistic with regards to MVFR cigs
across the NC/SC piedmont sites around daybreak, therefore such was
no longer prevailed and thus added to the aforementioned tempos.
Otherwise behind the front, winds will veer nwly then nely through
the mid/late morning hours as high pressure builds in.  Winds will
be a bit stronger on Monday with some low end gusting possible
thanks to increased PGFs.

Outlook: Much drier and cooler air will filter in behind the front
Monday and linger through the week.

Confidence Table...

            00-06Z        06-12Z        12-18Z        18-00Z
KCLT       High 100%     High  90%     High  96%     High 100%
KGSP       High  99%     High  86%     High  94%     High 100%
KAVL       High  93%     High  83%     High  87%     High 100%
KHKY       High  97%     High  93%     High  97%     High 100%
KGMU       High  98%     High  96%     High 100%     High 100%
KAND       High  99%     High  96%     High  96%     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...JMP
NEAR TERM...CDG/JDL
SHORT TERM...WJM
LONG TERM...HG
AVIATION...CDG



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