Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200234
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
934 PM EST Thu Jan 19 2017

.SYNOPSIS...
A warm front will lift north across the region tonight and tomorrow
bringing more rain to the area. A more significant low pressure
system is expected to track across the region this weekend, bringing
showers and thunderstorms for Saturday, Sunday, and Monday. Drier
high pressure is expected to build back over the region by the
middle of next week.

&&

.NEAR TERM /THROUGH FRIDAY/...
As of 933 PM EST: Water vapor imagery shows a deep low pressure
center lifting northeast across the central plains into the
midwest late this evening. The associated trough axis is
steadily acquiring a more negative tilt with time, with a sharp
axis of higher vorticity lifting quickly northeast across
Alabama. The near term models have this strong upper forcing
lifting northeast through the region during the overnight hours
and PoPs will rise to high likely to categorical throughout as
the deeper moisture and forcing arrive from the southwest after
midnight. Surface observations continue to indicate a strong
boundary draped across central Georgia, with lower 60s dewpoints
near the GA/AL line while locations north of Alanta and
northeast into our forecast area feature dewpoints mainly in the
lower 40s. Little to no SBCAPE is currently indicated and most
numerical models have little SBCAPE arriving overnight during
the period of better forcing. However, it remains possible that
some higher dewpoints could wrap into the lower piedmont and
elevated convection could be possible under the passing upper
trough, so at least slight chance thunder will be retained
overnight for the southeast half of the area. A transitory low
level jet of 35 to 40 kt will also impact the region from 06Z to
12Z and any modest convection that develops will need to be
monitored overnight for a very low end strong to severe
thunderstorm chance.

Once the wave passes Friday morning, the upper ridge reasserts
itself in the afternoon. The general absence of forcing and the
shallow moisture suggest a lull in the action through the afternoon
hours, so precip chances are allowed to diminish down into the
slight chance range in most area. Temps will be tricky again in the
afternoon, with warm advection likely into northeast GA, but
lingering cloudiness over the NW Piedmont delaying the warmup in
that location. Have stuck close to the raw blend of models, but
expect another tough outcome like the past few days.

&&

.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 230 PM Thu: Deepening low pressure will move northeastward
through the central Plains Friday night. The lull in active weather
over our area will continue through this time, until a warm front
associated with that low approaches the area during the day Saturday,
as well as weak DPVA resulting from a shortwave. This brings another
somewhat remarkable plume of moisture overhead, with PWAT
standardized anomalies of +2-3 SD. Abundant cloud cover seems likely
to limit destabilization in what would otherwise be an environment
that would make us a bit uneasy about convection, with favorable
lapse rates aloft and a modest LLJ crossing the area resulting in 40
kt of effective shear by afternoon, per NAM and EC.

A shortwave entering the Desert Southwest Saturday morning will shift
eastward and deepen into a cutoff low over TX/OK Saturday night, with
a strong surface reflection. This will set the stage for another
round of significant weather across the Southeast. Ahead of the main
low, a shortwave is depicted swinging from the western Gulf Coast up
into the Carolinas by midday Sunday. The LLJ and moisture advection
reamplify ahead of that feature. The little shortwave, combined with
the continued approach of the main low, cools the mid and upper
levels and leads to much more significant instability despite max
temps similar to those Saturday. Also, upper level winds become
anomalously strong. Sunday still appears to bear a notable severe
weather risk for at least the lower Piedmont; if the NAM and GFS are
correct in predicting several hundred joules of CAPE for the majority
of the CWFA, the threat area may be larger than that currently
depicted on SPC Day 4 outlook. Shear parameters are basically
through-the-roof on the NAM, less so on the GFS but still
significant. As an example, SHERBE peaks above 1 on the GFS and close
to 2 on the NAM. NAM would also suggest a tornado threat due to
exceedingly high 0-1km SRH values. Damaging winds would be the most
likely severe weather, though low freezing levels suggest marginally
severe hail is also possible with the stronger updrafts.

In addition to any severe threat, we are anticipating at least a
small possibility of flooding. Area streamflows are below normal, but
with repeated rounds of rainfall poorly drained areas and smaller
streams could rise with time. Late Saturday night and Sunday morning
is when we will have the best overlap of deep moisture and forcing,
so any t-storms that develop in that period might cause some
localized flash flooding, but that too does not look particularly
likely.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
AT 200 PM EST Thursday...On Monday morning evening a deep upper low
will be closed off over the Carolinas, with ridging to the east and
west. This low moves off the East Coast on Tuesday, while the ridge
upstream approaches the crosses the MS River Valley. On Wednesday
the ridge moves off the East Coast, while a positively tilted upper
trough extends from the Great Lakes to NM. By Thursday the model
solutions diverge, with the GFS still showing a positive tilt to the
trough from Eastern Canada to the Southern Rockies, while the ECMWF
is less less progressive in the north, and more so in the south,
resulting in a trough from the Great Lakes to TX.

At the surface, on Monday morning a low pressure system will be over
the central Carolinas, with a band of low level Atlantic moisture
sweeping around behind it over the Western Carolinas and Georgia.
The low moves off the East Coast on Monday night, ending
precipitation everywhere but the NC Mountains neat the TN Border.
Temperatures and vertical profiles support an all liquid event.
Precipitation should end on Tuesday morning as a Gulf surface ridge
moves over the area. Rainfall may return as early as Tuesday evening
as Gulf Moisture ahead of another front reaches the Southern
Appalachian. Although Gulf Moisture is greatest on Wednesday morning
as the front approaches from the west, weakening gulf inflow will
somewhat limit precipitation amounts. As the front moves south of
our area on Thursday, precipitation should diminish. Temperatures
will run above normal.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
At KCLT and Elsewhere: VFR conditions late this evening will
gradually deteriorate from the southwest overnight with the approach
of a strong upper level shortwave and associated deeper moisture.
Anticipate a transition down into MVFR with the onset of the better
shower coverage from 05Z to 08Z, with IFR conditions in moderate
showers becoming increasingly possible from 08Z through daybreak.
There remains a small thunderstorm threat, but instability is
currently lacking across the region so no mention will be made in
the TAF for the overnight hours. Easterly flow may adjust from ENE
to ESE from time to time early but quickly adjust to SW by late
Friday morning as the showers taper off behind the wave. Although
mid and upper levels will dry quickly behind the departing wave on
Friday, low levels will remain moist.  Thus, expect slow improvement
from MVFR early to VFR by mid to late afternoon in most places.

Outlook: The very unsettled pattern will continue through the
weekend, bringing periods of rain and/or restrictions. Heavy rain
showers and perhaps thunderstorms are possible Sunday.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     Med   69%     Med   77%     High  83%
KGSP       High 100%     Med   73%     Med   70%     High 100%
KAVL       High 100%     Med   67%     Med   79%     High 100%
KHKY       High 100%     Med   77%     Med   71%     High  82%
KGMU       High 100%     Med   66%     Med   69%     High 100%
KAND       High  98%     Med   65%     Med   73%     High  91%

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...JPT
NEAR TERM...HG
SHORT TERM...Wimberley
LONG TERM...JAT
AVIATION...HG


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