Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 181148
AFDGSP

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

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
As of 640 AM Sat: The axis of an upper level trough extends
from the Canadian prairie south into the central High Plains. A
seasonably strong low pressure system has developed in response;
its center is over Missouri. 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. A small area of cloud did form just
south of the Blue Ridge Escarpment over NE GA and far NW SC. This
is expected to persist thru late morning.  The same upslope forcing
may will allow precip to form, so it is there that PoPs ramp up
first. However, latest hi-res guidance suggested a later onset to
precip than previously expected. 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. A
Wind Advisory begins at that time; model/algorithm derived gusts
really appear confined to the high elevations above 3500 feet,
but in agreement with neighbors we opted 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 reflects mostly the NAMNest, which is quite
close to the HREF mean (it being a member thereof). 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
taper at that point.

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. This produces numbers near the 50th
percentile on WPC probabilistic snow plots.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY/...
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
mountains.

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.

&&

.LONG TERM /TUESDAY NIGHT THROUGH FRIDAY/...
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.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and Elsewhere: Patchy AS/CI deck is spreading into the area
from the NW, downstream of a developing cyclone in the middle of
the country. At the sfc, we are basically in the warm sector of
that system, with cold front set to cross the area tonight. Weak
warm upglide is occurring overhead attm. With the aid of the Blue
Ridge Escarpment, MVFR level cigs are expanding over the NW Upstate
and KGSP/KGMU. Knowing that models often over-forecast cigs at
onset of such an event, and the overall weak pressure advection
noted on isentropic charts, lend confidence to this deck mixing
out by late morning. Gusty winds will develop around that time
and more or less will last until cold fropa. The cold front will
push rather quickly thru the area overnight tonight, bringing
a brief round of SHRA followed by a wind shift. Kept PoPs close
to the expected time of fropa, with PROB30s saved for after that
time in case front slows down as can happen nocturnally. The SHRA
look likely to bring periodic MVFR, with spotty IFR not out of the
question. A couple of the SHRA also could be associated with highly
localized strong to even severe wind gusts, and isolated incidents
of LLWS are possible ahead of the front. SHSN will develop along
the Tennessee border in the ensuing NW flow following the front
but this is not expected to affect KAVL or any other TAF site.

Outlook: Expect VFR from Sunday morning 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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...Wind Advisory from 4 PM this afternoon to noon EST Sunday for
     GAZ010-017.
NC...Wind Advisory from 4 PM this afternoon to noon EST Sunday for
     NCZ033-048>053-058-059-062>065-501-503-505-507-509.
SC...None.

&&

$$
SYNOPSIS...Wimberley
NEAR TERM...Wimberley
SHORT TERM...RWH
LONG TERM...Carroll
AVIATION...Wimberley



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