Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191957
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
257 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 230 PM: seeing some evidence of light shower activity across
the Smokies and along the TN border as of mid-afternoon, and that
should continue through the rest of the day. We are carrying a small
precip chance in that area and feel that should be adequate as it is
unclear if any of the showers will produce measurable rainfall.
Clouds topping the upper ridge continue to diminish after sliding
down off the higher terrain. This has allowed temps to climb higher
than expected again this afternoon.

Over the next 12-18 hours, a preliminary round of shower activity
will occur over the western Carolinas and northeast Georgia...sort
of a preview of what will happen over the weekend. The culprit will
be an upper low over the Plains that will lift northward tonight
while dragging a short wave up the back side of the upper ridge on
the east coast. The short wave nudges the ridge axis to the coast
while deforming the upstream half with its passage tonight. Although
the short wave deamplifies, it still brings a swath of dpva across
the region on Friday morning. Coincident with this feature, an area
of weakening upper divergence will move overhead. Low level
isentropic upglide will improve ahead of the wave, with a maxima
just before the wave moves through, but will continue thru Friday
afternoon. The combination of forcing and deep moisture supports
ramping precip probability up into the categorical range across the
higher terrain late tonight, with likely reaching the western
Piedmont of NC in the pre-dawn hours on Friday. However, the
model guidance suggests a slight delay in arrival, which has been
imposed upon the new forecast. Expected amounts do not look high
enough to be concerned with flooding. There is a very small concern
about severe storms early Friday with the passage of the main short
wave and its surface low reflection along the I-85 corridor. The
short range models roll a small blob of strong shear/helicity across
the I-85 corridor like a little ball of hate in the 09Z to 15Z time
frame. Fortunately, the time of day is such that surface-based
instability will be non-existent, even with the mild overnight
temps. Best to keep one eye on this feature, though. Once the wave
passes Friday morning, the upper ridge re-asserts 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.
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 /20Z THURSDAY THROUGH TUESDAY/...
At KCLT and Elsewhere: VFR conditions are expected into the early
part of the evening, with gradually deteriorating conditions
expected to overspread the area from the west beginning late evening
and continuing through the overnight. Wind in the process of
switching from light NE to something more south of east, so think
KCLT will have a southeast wind by 21Z at the latest. The new model
guidance imposes a slight delay in the arrival of the precipitation
from the west, and thus the arrival of the restrictions are also
delayed by another hour or two. See no reason to think that precip
will not eventually move in from the W/SW, as low/mid level forcing
an moisture look excellent. Some of the guidance continues to be
pessimistic and depicts IFR or LIFR conditions developing by the
start of operations on Friday morning, but confidence in that
scenario still low at this point. Prefer to keep most TAFs MVFR as
the precip moves through, and then allow the precip to bring the
ceiling down to IFR and have that condition linger thru mid-morning.
Ceiling should lift back to MVFR in the 15Z to 16Z time frame, but
it may scatter out right around that time as well. so have opted to
leave conditions at the low end of MVFR for the time being. Wind
should veer more to the S overnight, and then SW late in the period.

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...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High  83%     High 100%
KGSP       High 100%     High 100%     High  90%     High  95%
KAVL       High 100%     High 100%     High 100%     High  89%
KHKY       High 100%     High 100%     High  93%     High 100%
KGMU       High 100%     High  95%     High 100%     High  91%
KAND       High 100%     High 100%     High  97%     High  89%

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...PM
SHORT TERM...Wimberley
LONG TERM...JAT
AVIATION...PM


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