Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 190000

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
700 PM EST Sat Nov 18 2017

A cold front crosses our region tonight with breezy to Windy
conditions expected on Sunday.  Cool high pressure will be in
control for much of the week. Another cold front arrives next


As of 655 PM EST Saturday:  Updated discussion for 00z taf issuance.
Light upglide induced ra continues to produce echoes across portions
of the Upper Savannah and French Broad river valleys at this time.
The front continues to work its way across Middle TN with some
strong convection seen over the last few hours.  That said,
latest SPC mesoanalysis indicate little/no cape across the TN
valley eastward into the NC high terrain.  Upper forcing does look
to be slightly enhanced on the northern end of the line moving
into the Cumberland Plateau, which will eventually slide east
into the central/northern mountains.  Therefore although think
thunder chances are slim, a few rogue strong/severe gusts could
mix down from the enhanced llj along the convective line into the
late evening hours.  Thus no adjustments are planned for the HWO
through at least early morning.  Otherwise, tweaked pops/t/td
and sky to reflect recent ob/radar trends as well as camguide,
and left the remainder of the fcst as it was post fropa.

Previous Discussion:  Increasingly moist/strengthening SW flow will
continue into the evening hours in advance of a strong cold front
that will march across the TN Valley over the next few hours. Still
wouldn`t rule out some showers developing in response to mechanical
lift near the intersection of GA/NC/SC late this afternoon into
the early evening, and small pops will continue to be featured
there through that time.  Pops then ramp up steeply across western
areas by late evening, as strongly forced frontal band should be
knocking on the back door of the CWA by 10 pm-ish. If anything,
short term guidance has backed off a bit on the already-low sbCAPE
values forecast within the frontal convergence zone as the band
begins its trek across the southern Appalachians. However, shear
values will be more than adequate for a marginal high shear/low
CAPE severe threat, with perhaps a damaging wind gust or two,
and/or even a brief QLCS spin-up not out of the question. This
threat should mainly be confined to areas roughly west of I-26
this evening, as veering flow across the frontal zone will result
in weakening convergence as well as shear parameters, while the
sbCAPE is also forecast to diminish even further with time as the
front crosses the CWA.

While winds will be howling (gusts as high as 60 mph) this evening
on the exposed peaks and ridge tops of the southern Appalachians,
winds elsewhere should be quite tame until fropa occurs btw 03-09Z.
Strong winds should then descend into the lower elevations within
the cold advection regime. While advisory criteria gusts will likely
be hard to come by in the mtn valleys even then, guidance tends to
under-do the winds for the first 1-2 hours post-fropa during strong
cold-advection surges, so would expect at least occasional advisory
criteria gusts in the lower elevations during that time. Meanwhile,
the high peaks and ridge tops should continue to howl, with gusts
as high as 60 mph likely occurring through tomorrow morning.
That being the case, see no reason to deviate from the current
wind advisory.

Snow levels drop quickly across the high terrain from late evening
on, while a moist NW flow  quickly develops into the southern
Appalachians. Expect scattered showers, with high elevations snow
showers to develop late tonight/early Sunday. However, ingredients
are such that accumulating snowfall will be difficult to come by,
with moisture depth and residence time of moisture coincident with
cold air being the primary limiting factors. As such, any accums
should be limited to elevations above 4000` near the TN border,
if not 5000`, and even there less than an inch is expected.

Clearing quickly occurs Sunday morning, with most locations
(the TN border area being the primary exception) likely seeing
clear/mostly clear skies by sunrise. Max temps will be 10-15
degrees below normal in most areas.


As of 130 PM Saturday: Not much going over the short range period.
The models are in good agreement with the synoptic pattern which
features a 1026 mb sfc high traversing the area west to east while
subs remains strong thru most of the period. This high will center
off the the Atl coast early Tues...however strong llvl ridging will
remain over the FA Tue morning into the early afternoon. Thus...very
little moisture flux will be available before the ridge breaks down
in the afternoon as a low pressure trof advances from the west.
Soundings show little precip potential across most areas except
within an isent lift area aligned over the srn BL and srn Balsams
later Tue. Not expecting much precip with this scenario as deep
layered lift remains low-end as a broad area of vort energy
producing mainly upper and mid level clouds crosses a slowly
moistening BL.

Max temps will be held about 5-7 degrees below normal both days as a
llvl thermal airmass is slow to modify. Mins will also be about 10
degrees below normal with freezing conds supporting a wintry mix
over the sw/ly upslope zones early Tue...however advecting llvl
moisture will likely be too late for any sensible wx to materialize.


As of 215 PM EST Saturday: The medium range period around the
Thanksgiving holiday remains uncertain across the southeast due to
the continued run-to-run inconsistencies among the models. Nearly
all solutions have a broad upstream trough at the start of the
period over, or just east of, the MS river valley Tuesday night.
Differences then develop quickly through Wednesday. In a continued
role reversal from previous day solutions, the 12Z GFS now features
a strong cutoff low developing over the Deep South. The 00Z ECMWF
solution featured a more progressive open trough, however, the 12Z
data has trended back toward the deeper southern tier GFS low for
Wedneday night through Thursday. Fortunately, even the deeper,
slower solutions keep the abundant moisture shunted mainly toward
the coastline Thursday as the northern Gulf of Mexico surface low
redevelops over the southeast coastal waters. Will keep mainly
slight chances for light rain going across the southeast parts of
the forecast area for Thanksgiving day.

Deep layer northwest flow should set up behind the departing system
on Friday with lee troughing developing in surface high pressure
over the southeast. A strong cold front will then approach from the
northwest late next Saturday but with dry conditions likely
persisting through late day. Temperatures will gradually trend back
up toward climatology just ahead of the approaching front.


At KCLT and Elsewhere: A mixture of MVFR possibly IFR leading into
solid VFR through this taf period at all sites.  A fast moving
cold front continues to approach the region from the west with
a rather dynamic llj out ahead.  With that carried llws at all
sites through midnight, tapering behind the front as flow veers.
Precip timing at each site was based on camguide which seems
to be in good agreement across the board.  Swly sfc flow will
remain gusty ahead of the front with low stratocu increasing in
coverage with cigs lowering into the MVFR range, possibly IFR in
association with any frontal shra.  Behind the front skies will
clear out rather rapidly with nwly winds increasing yielding low
end gusting amidst improving caa.  That said, with moist upslope
flow favored along the TN line, did hold onto MVFR stratus at KAVL
as upstream clouds advect in through around midday.

Outlook: Expect VFR from Sunday morning through at least the middle
of next week, under the influence of cold continental high pressure.

Confidence Table...

            17-23Z        23-05Z        05-11Z        11-12Z
KCLT       High  87%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  94%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     Low   58%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High  94%     High 100%     High 100%     High  83%

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:


GA...Wind Advisory until noon EST Sunday for GAZ010-017.
NC...Wind Advisory until noon EST Sunday for NCZ033-048>053-058-059-


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