Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 291105

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

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.


As of 645 AM Thursday: Sunrise updates are mainly for aviation
parameters, but did issue SPS a couple of hours ago for patchy dense
fog around the I-77 corridor. Not widespread enough for a dense fog
advisory. Made some minor tweaks to pops based on new CAM guidance
but nothing major.

Otherwise, 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 this morning.

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 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


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.


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.


At KCLT: Widespread IFR conditions around the airfield and have held
onto this until 14z. Convection chances at the airfield are not high
enough to include in the TAF but will continue to reevaluate.
Southerly winds should pick up this afternoon to near 10kt, and
lower again later tonight.

Elsewhere: IFR conditions continue at KHKY but VFR at other sites.
Have included VCTS for KHKY briefly this afternoon, but no
convection expected at other sites. Wind trends similar in
speed/direction to KCLT as well, but with a wind shift late in the
period bringing winds around to the W and NW. Conditions favorable
tonight/early Friday morning for restrictions at KAVL/KHKY and have
trended this way in the TAF.

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

Confidence Table...

            11-17Z        17-23Z        23-05Z        05-06Z
KCLT       Med   66%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     Med   62%
KHKY       High 100%     High  90%     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:




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