Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 281951

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
351 PM EDT Wed Sep 28 2016

A stationary front will remain over the Midlands of South Carolina
and the Sandhills of North Carolina through Thursday.
Meanwhile, an upper level low will drop south into the Ohio Valley
and linger there through the end of the week.


As of 340 PM Wednesday: A healthy cumulus field has developed
across the area east of the Blue Ridge. Convective initiation
is underway as well, but better coverage was noted across the
SC Midlands. Still think the fcst will catch up over the wrn
Piedmont during mid afternoon. A prime location might be in
the vicinity of the old differential heating boundary running
across the I-85 corridor from w of Charlotte to the Piedmont
Triad. The environment across the Piedmont is mildly compelling,
with sfc-based CAPE in the 2000 to 2500 J/kg range, dCAPE above
800 J/kg, and SR helicity on the order of 100-150 m2/s2. Cannot
rule out the development of a few supercells over the Piedmont
thru late afternoon and the Supercell Composite Parameter on the
SPC mesoanalysis agrees. Our NC counties east of the Blue Ridge
(plus Avery) have been included in a Severe Thunderstorm Watch
until 1000 PM. So far the storms that have developed in this area
have grown very quickly and several have developed large icy cores.

No major surprises in the new model guidance, with the center of
a large upper low moving slowly down across the Midwest/OH Valley
tonight and then over the Cumberland Plateau on Thursday. The trof
axis remains to our west the entire time, with spokes of vorticity
moving around the circulation. This will have a difficult time
pushing the surface front off to the east. Think most of the
convection will die off in the mid/late evening. Dry air will
attempt to filter in from the SW in the dry slot, which should allow
for some cooler temps in northeast GA and the wrn Upstate. However,
there are indications that low level moisture will remain high
across the wrn Piedmont, which could lead to some dense fog around
daybreak. Confidence way too low for any advisories for now.

On Thursday, the upper low is close enough to the mtns such that
more than one fairly strong short wave will pass overhead, along
with some upper divergence. Meanwhile, colder air aloft should
provide some buoyancy. Expect some low-topped convection to develop
in the afternoon over the mtns, so a chance probability will be
kept there. Meanwhile, will also hold onto a chance east of I-77
where the surface boundary will remain. What is interesting is the
lingering severe storm potential seen on the new Day 2 Outlook. The
drier air filtering in from the SW should keep the convection at
bay over northeast GA and most of Upstate SC. Will bump up high
temps a few degrees for Thursday afternoon on general principles.


As of 150 PM Wed: Deep upper low remains centered over Kentucky
Thursday night into Friday, then gradually wobbles northward
through Saturday. Heights aloft will have already bottomed out
over our area during this period; upper lapse rates become quite
poor by daytime Friday. Though small precip chances linger over
the northern portion of our CWFA thru Fri mrng, the whole area
is left dry thereafter, with light southerly return flow around
Atlantic high pressure. Mins are expected to be around normal,
with max temps a tad above normal due to plentiful sunshine.


As of 155 PM EDT Wednesday...At the starting point of 00Z Sunday,
the cut off upper low is forecast to be over NE corner of Indiana.
Other than some cloudiness mainly over the NC Mountains, no sensible
weather for our area. The low will begin to move NE for the rest of
the weekend and open up to the upper westerlies as the low fills.
Our region will be under the influence of a high amplitude upper
ridge and surface high pressure. 500MB trough will be crossing the
West Coast Sunday and the Rockies Monday and Tuesday as our ridge
gets pinched east.

According to the 06Z GFS, Tropical system Matthew will be a powerful
hurricane making landfall on the NC coast Wednesday evening. This
track varies a little from day to day as yesterday the GFS had it
just a little farther east. Over the last 2 to 3 days this model has
been consistent in stating their would be some East Coast impact
from this system. The new 12Z GFS now has the track just slightly
farther east and still close enough to the western wall of
circulation for some affects to Outer Banks.  Still the timing for
passage of NC coast still Wed evening. The GFS has the wind from the
east for our area beginning Tuesday around the north end of the
circulation of Matthew.  The ECMWF is much slower with Matthew just
reaching the Bahamas at the end of the week. The end result for us
at this time is we will stay dry except for perhaps a few showers
late Wednesday north of CLT.

Temps a couple categories above normal Sunday through Tuesday then
Max Temps down near normal Wednesday due to passing Matthew.


At KCLT...cumulus field developing across the Piedmont with bases in
the 025-035 range. This could result in a temporary ceiling close to
the upper end of MVFR, but this will be left out of the TAF in favor
of keeping the TEMPO for thunderstorms. The latest CAM guidance has
thunderstorms developing nearby as early as 19Z and moving through
as late as 23Z, so this will be the time period covered by the
temporary restriction. Could have some lingering shower activity
into the early evening, then convective debris clouds overnight. The
actual cold front, as it were, does not appear to ever move across
the region, or at least there isn`t much of a wind shift behind it.
In fact, the guidance has the winds coming back around to a more
southerly direction. The guidance hits the fog hard again in the
pre-dawn hours, with some guidance going down as low as VLIFR. Will
compromise and introduce another MVFR fog restriction starting at
11Z. This could develop earlier and could also be down in the LIFR
or worse range, but those details are left out for now. Any fog
should burn off quickly. New convection could blow up again across
metro CLT by midday Thursday, but this was left out for brevity.

Elsewhere...could be a few showers that develop near the other TAF
sites but coverage should be isolated, thus only a VCSH was
included. Drier air comes in this evening which should be enough to
keep clouds scattered. Over the mtns, looks like a good scenario for
valley fog in the pre-dawn hours, thus KAVL has a good chance to see
an IFR restriction, perhaps VLIFR by sunrise. KHKY has the next best
chance for seeing low stratus/fog based on its proximity to trapped
low level moisture near the front. Dry air is expected to wrap
around the mtns from the SW, so no fog is anticipated at the Upstate
TAF sites. Otherwise, VFR on Thursday after the fog burns off.
Showers are possible by early afternoon over the mtns, but this was
left out of KAVL for brevity.

Outlook: Dry high pressure will build in slowly behind the front
Thursday afternoon, which should bring VFR conditions in most places
through the end of the week, although some instability could lead to
showers over the NC part of the area on Thursday afternoon.

Confidence Table...

            20-02Z        02-08Z        08-14Z        14-18Z
KCLT       High 100%     High 100%     High  80%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High  91%     Med   75%     High  91%
KHKY       High 100%     High  90%     High  83%     High  96%
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:




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