Area Forecast Discussion
Issued by NWS Greer, SC

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080
FXUS62 KGSP 290841
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
441 AM EDT Thu Sep 29 2016

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

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 315 AM EDT Thursday: A surface low was just west of Charlotte
at AFD time, with a nearly stationary front stretched southwest into
N GA from there. Tight dewpoint gradient along the front has allowed
an area of low sbCAPEs to linger across the northern Piedmont and
foothills. Aloft, the parent deep upper low was centered over the
Midwest, and is progged to continue diving slowly S/SE through the
day today, basically spinning over KY for the remainder of the near
term. Dry slot rotating around the upper low is dominating the
western half of our forecast area, and with the surface dry air in
place should see much cooler temperatures this morning as sites are
able to radiate down closer to normal overnight lows for late
September. However, with relatively little change to the airmass
east of the front, despite slightly cooler lows this morning, high
temperatures today should surge once again to above normal levels
(so much for that cooling we were originally hoping for!).

The bigger issue for today, along and ahead of the front, will be
convective potential. With the moist airmass still in place plus
another round of above-normal temperatures, sbCAPE will be on the
rise today as well. As usual the NAM is much higher...like almost an
order of magnitude...than the GFS, but along the I-77 corridor and
then spreading west into the northern foothills, should see an area
of at least moderate (1000-1500 J/kg) instability. Upper jet setting
up over the northern foothills with impressive speed diffluence and
large area of DPVA will contribute to concerning deep-layer shear
values of 50-60kt, with some pockets of 70kt (but luckily these
higher shear values are not collocated with the best instability).
These combined give Significant Tornado Parameter values in the NE
portion of our forecast area of almost 5 on the NAM and just under 1
on the GFS (though the GFS`s STP is maximized before peak heating
and drops off after that). The limiting factors will be the dry slot
which will continue to push NEward and a wee bit of WAA around 500mb
that contributes to weak mid-level lapse rates. SPC Day1 Outlook has
Marginal Risk in those areas with mention of close watching for
possible upgrade to Slight. Forecast pops for those areas today are
slight to chance, but personally am thinking that the conditional
probability of severe (given convection to begin with) is pretty
darn high. Damaging winds would be the main threat, and though the
lapse rates would limit hail threat, strong shear would likely lead
to at least some supercell formation and with sustained updrafts
could still result in large hail. Brief tornado also not at all out
of the question.

Not much movement expected with the front tonight but still do
expect some dry/CAA, so should see overnight lows dropping to near
or just slightly blow normal west of the general I-77 corridor. But
the trend has been to warm the forecast ever so slightly each night
so wouldn`t be surprised if that happens again with the afternoon
package.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 230 AM EDT Thursday...The large upper low will wobble/meander
over the Ohio Valley during the Short Term, keeping the CWFA under a
pronounced dry slot on its SW side. Given the trend in the upper low
track, thicknesses don`t fall as much as previous model runs had
suggested. So temps were nudged up slightly for Friday and Saturday.
Max temps will be about one or two categories above normal (except
slightly cooler in the western half of the NC mountains). Min temps
will be around normal. Dry conditions are expected and skies should
be mostly clear thru the period, but the stalled front will linger
just to our east on Friday.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 300 AM EDT Thursday...An upper low should gradually fill and
open up into the westerlies somewhere over the Great Lakes sunday
and monday. From there model disagreement becomes quite large, with
how deep the northern stream trough digs in the Northeast states.
The 00Z ECMWF has a consolidated trough become progressive and exit
off the East Coast on Tuesday. The GFS and Canadian keep a large, yet
weak closed low, and drift it much more slowly. This also has an
impact on the possible track of T.C. Matthew. The GFS continues to
favor a quicker track out of the tropics and taking it somewhere
along the East Coast. While the ECMWF has the trough miss Matthew,
causing Matthew to only drift slowly north to the Bahamas at Day 10.
What this means for our forecast area is despite the large
disagreement in the upper level pattern, the sensible weather
forecast is similar. Basically, a large surface high will gradually
build in under the filling upper low across the Great Lakes, and
wedge down into the Carolinas by Monday thru Wednesday. Northeasterly
low-level flow should remain fairly dry (though cannot rule out some
stratus and a small chance for precip). I will keep a dry forecast
with temps remaining slightly above normal.

&&

.AVIATION /09Z THURSDAY THROUGH MONDAY/...
At KCLT: No changes to the KCLT TAF for the 09z AMD. Very quiet
early this morning with a cold front still to the west of the
airfield. Convection yesterday has saturated the ground and expect
MVFR fog to form in the next couple of hours, with the possibility
of IFR restrictions as well, and this is hinted at in a TEMPO.
Should see typical clearing after daybreak. For now, convection
chances at the airfield are not high enough to include in the TAF at
this point but will reevaluate for the 12z TAF. Southerly winds
should pick up this afternoon to near 10kt, and lower again later
tonight.

Elsewhere: Clearing aloft has already allowed VLIFR vsby/cigs to
develop at KHKY and expect typical mountain valley fog to develop as
well (to impact KAVL), with clearing after daybreak. Otherwise, like
with KCLT, convection chances not high enough to mention yet in the
TAFs. Wind trends similar in speed/direction to KCLT as well, but
with a wind shift late in the period at KAVL/KHKY bringing winds
around to the W and NW.

Outlook: Dry high pressure will build in slowly behind the front
through Friday, which should bring VFR conditions in most places
through the end of the week.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       High  81%     High  91%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     Med   68%
KHKY       Med   77%     High  80%     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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...ARK
NEAR TERM...TDP
SHORT TERM...ARK
LONG TERM...ARK
AVIATION...TDP



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