Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180843

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
343 AM EST Sat Nov 18 2017

Moisture will increase over the region today, with increasing
southerly winds, until a moist cold front crosses from the west
tonight. Another large area of dry and cool high pressure will
build in Sunday and remain in place well into next week, bringing
cool but settled weather.


As of 300 AM Sat: A weak upper disturbance crossing NC/VA is
driving some high-based radar returns over the I-77 corridor
attm. These have not resulted at any precip reaching the ground
at the numerous obs sites they have passed over in the last couple
of hours, and should depart soon anyway.

The axis of an upper level trough extends from the Canadian prairie
south through the eastern ranges of the Rockies. A seasonably
strong low pressure system has developed in response; its center
is near Kansas City. A cold front associated with this system
will swing quickly east and is progged to enter the CWFA this
evening, and be nearly out of it by daybreak Sunday. Accordingly,
brisk southwesterly low-level flow is ongoing across the
Southeast. Isentropic lift will occur this morning, though the
winds are not ideally oriented to get a great deal of low cloud
cover developing as a result. More of interest for precip during
the day will be upslope forcing along the SW portion of the Blue
Ridge Escarpment, so it is there that PoPs ramp up first. Mostly
cloudy skies should limit insolation; will shy away from the warmer
MOS-based temps and stay a little closer to raw values. Enough
mixing is expected to bring down some noticeable wind gusts across
the whole area; gusts should easily top 20 kt in the Piedmont with
peak values closer to 30 kt. In the mountains--particularly high
elevations--the prefrontal LLJ will be tapped and gusts to 40 kt
develop as soon as 21z. We decided to hoist a Wind Advisory at that
time; we could have argued for a high-elevation product, due to a
lack of model/algorithm support for strong gusts in the valleys,
but in collaboration with neighbors it was decided to include
the entirety of the mtn zones. This advisory will run through
the night and capture the strongest post-frontal cold advective
gusts as well. The peaks above 5000 ft may see gusts reach warning
criteria at times, so there is a small chance we could end up with a
high-elevation split for a High Wind Warning on a subsequent shift.

The timing of the front has been adjusted mainly using the NAMNest,
which is quite close to the HREF mean (it being a member of the
HREF). Frontal precip is a near certainty in the northwestern
couple tiers of zones. Very strong low-level shear will accompany
the front. HREF members depict a narrow line of convection pushing
into NE GA and SW NC in the late evening. Its ensemble max SBCAPE
does exceed 100 J/kg in the vicinity of this line, so we plan to
monitor this feature closely if such a scenario verifies. That said,
Bufkit profiles indicate the convective layer will be very shallow
and thus not much of that shear will be realized, so the severe
threat is small (and may help explain why no members show high
updraft helicity). The consensus is that this line will fall apart
as it moves across the midpoint of the CWFA, so the high likely
to categorical PoPs currently do not extend further east from there.

Based temp trends tonight on hourly guidance given near-steady
trends ahead of front and rapid cooling behind it. So finally,
I`ll note there is a shallow layer of moisture that remains banked
against the mountains and thus some snow accumulations are fcst
along the Tenn border. Winds are the most impressive part of the
setup, with no instability and temps not particularly cold at the
top of the layer. Hence I went with a 7:1 SLR, and snow totals
are below advisory criteria.


As of 200 AM Saturday: Cold high pressure builds in behind the
departing cold front Sunday. NW flow precip along the TN border ends
by afternoon, but clouds linger through the day. Should see clearing
skies elsewhere, but some clouds could break containment and spread
across the foothills and piedmont. Very windy conditions to start the
day across the mountains slowly diminish during the afternoon as the
pressure gradient relaxes. Should be a breezy day outside of the

High pressure slides east across the area Monday, then offshore
Tuesday as a weak cold front crosses the MS River valley. Expect
mostly clear skies Monday with increasing clouds Monday night into
Tuesday. Weak moisture return and isentropic lift increase Tuesday
in the developing south to southwesterly low level flow. There may
be some light showers across the area as a result. For now, precip
chances remain in the slight chance category. Any precip develops
late enough in the day to be all rain.

Temps remain below normal through the period.


As of 130 AM EST Saturday: An upper shortwave will approach the area
Tuesday night as longwave troughing propagates towards the east
coast. At the surface, a weak wedge of high pressure will remain
over our forecast area, with the center of the surface high off the
east coast. A weak wave will pass by to our south, though it will
provide just enough of a moisture plume into Georgia and the
Carolinas for some upglide-induced precip Tuesday night over the
remnant surface wedge. The moisture will be quite shallow, but
models are in good enough agreement on some QPF response that slight
chance pops were maintained.

The evolution of the next system is again in relatively poor
agreement in global model solutions. The ECMWF, mimicking the GFS
from a few days ago, keeps the next upper impulse more open, though
it does close off a surface low over the Gulf of Mexico and
progresses it over north Florida, keeping our area dry. The GFS, now
mimicking the ECMWF solution from a few days ago, closes off a more
intense surface and upper low over the Gulf of Mexico and tracks the
low farther north, bringing precipitation to our area on Thursday
and Friday. Uncertainty is high as global models continue to poach
each others` solutions, so confidence on the forecast for late on
Thanksgiving and Black Friday is low. For now, slight chance to
chance pops were added late Thursday and Friday. With longwave
troughing approaching, temperatures will start the period near
average before dropping to near 10 degrees below average through the
end of next week.


At KCLT and Elsewhere: Thick AS/CI deck is spreading into the area
from the NW, downstream of a developing cyclone in the middle of
the country. Low level warm upglide will set up later this morning
as southerly flow strengthens overhead. Guidance differs as to how
low cigs go with this pattern, and furthermore some guidance does
not develop cigs at all until much later in the day. Knowing that
models often over-forecast cigs at onset of such an event, and the
overall weak pressure advection noted on isentropic charts, led me
to favor the less restrictive GLAMP guidance this morning. Gusty
winds will develop by late morning and more or less will last
through the rest of the period. The winds will promote mixing and
cigs are expected to mix up to low VFR where they do develop at
lower levels. The cold front associated with the aforementioned
system will push rather quickly thru the area overnight tonight,
bringing a brief round of SHRA. The wind shift is timed at all
sites very near or a little after 06z and is not mentioned in this
issuance. A couple of the SHRA could be associated with highly
localized strong to even severe wind gusts, and isolated incidents
of LLWS are possible ahead of the front.

Outlook: Cold front will push east of all sites by around daybreak
Sunday, with rapid improvement in flight category behind it. Expect
VFR from then into 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 from 4 PM this afternoon to noon EST Sunday for
NC...Wind Advisory from 4 PM this afternoon to noon EST Sunday for


NEAR TERM...Wimberley
LONG TERM...Carroll
AVIATION...Wimberley is the U.S. government's official web portal to all federal, state and local government web resources and services.