Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200545

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1245 AM EST Fri Jan 20 2017

A vigorous low pressure moves from Texas Sunday to the Carolinas
Monday bringing abundant moisture and the potential for storms.
Mild high pressure will be in control Tuesday and Wednesday until a
cold front crosses our region Wednesday night.


As of 1245 AM EST: Water vapor imagery shows a deep low pressure
center lifting northeast across the central plains into the midwest
early this morning. 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 Atlanta
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.


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


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.


At KCLT and Elsewhere: Conditions will gradually deteriorate from
the southwest early this morning with the approach of a strong upper
level shortwave and associated deeper moisture. Should see a
transition down into MVFR then IFR as showers move in and intensify.
Cannot rule out an isolated TSRA, but chance very low with little in
the way of instability. Expect winds to become SE before daybreak
then SW after as showers taper off behind the wave. Although mid and
upper levels will dry quickly behind the departing wave this
morning, 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...

            05-11Z        11-17Z        17-23Z        23-00Z
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 and ensemble forecasts
are available at the following link:




SHORT TERM...Wimberley
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