Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171939

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
239 PM EST Fri Nov 17 2017

High pressure will linger over the region before a moist cold
front crosses from the west Saturday night. Another large area of
dry and cool high pressure will build in Sunday and remain into next


As of 215 PM Friday: The remainder of the afternoon should be sunny
with temps topping out around a category above normal. Fair weather
will prevail tonight as a flat upper ridge moves overhead and
pushes surface high pressure off the Carolina coast. High/mid-level
moisture will move in late tonight, which combined with the onset
of warm advection, should keep min temps on the warm side of normal.

Changes will set in on Saturday. Clouds will already be increasing
from above at daybreak as moistening isentropic upglide commences. A
vigorous and fast moving mid/upper wave will cross the Plains
during the day, with the trof axis reaching the MS River by the
end of the period. The bulk of the mid/upper forcing ahead of the
trof is expected to remain just to the west of the mtns by sunset,
unless the trof accelerates. What we lack in mid/upper forcing we
make up in low level warm advection and isentropic upglide as we
go through the afternoon. The upglide will result in a continued
increase in cloudiness and will probably start to produce precip
aloft by late afternoon. Problem is the initially dry boundary
layer. It may take a few hours for the light precip to work its way
down to the ground. The mtns will have the best chance of seeing
measurable precip before sunset as the deeper moisture moves in
around 21Z. The fcst will feature an increasing chance there late
in the day. Still think the Piedmont will remain dry. Wind will
increase late in the day as the leading edge of the low level jet
translates eastward and reaches the mtns, but at this time think
the extent of strong winds will be limited to elevations above 5k
feet, so a wind advisory is not warranted at this time.


As of 220 PM Friday: Strong/sharp cold front and associated shower
band should be entering the southern Appalachians at the start of
the period. Short term model guidance depicts positive (but minimal)
sbCAPE within the frontal convergence zone, while low level wind
shear is forecast in the 40-50 kts range. As such, the convection-
allowing models that provide data that far out depict a narrow,
low-topped QLCS moving into the forecast area during the evening. I
would not completely rule out a brief damaging wind event or perhaps
even a brief/weak spin-up in association with this feature,
especially across our western zones. With time, short term guidance
depicts a weakening frontal convergence zone/circulation as the
boundary moves out into the Piedmont, thus a diminishing trend is
anticipated as the frontal band crosses the forecast area. Tomorrow
night`s pops range from categorical across the mtns to solid chance
across the I-77 corridor.

Snow levels will quickly fall behind the front late Sat night, while
a moist NW flow is expected to briefly develop during the pre-dawn
hours on Sunday. Ingredients are such that accumulating snowfall
will be possible, but mainly only in the high elevations (above
4000`, if not 5000`) along the TN border. Otherwise, very gusty NW
winds will develop in the wake of the front, especially in the
vicinity of the Blue Ridge, as mountain wave scenario unfolds in
response to strong/deepening NW flow. If anything, the setup appears
less impressive than in previous model runs, but the potential for a
wind advisory-level event still appears to be high enough to include
a mention in the HWO.

The remainder of the short term will feature chilly and very dry
conditions, with temps expected to average a solid 10-15 degrees
below climo from Sunday afternoon through Monday.


As of 200 PM EST Friday: A shortwave ridge will move off the
southeast coast Monday night, quickly followed by a southern stream
shortwave lifting from the Deep South across the Carolinas on
Tuesday. Deep moisture should be very limited with this feature, but
slight chances of light rain appear possible during a brief period
of low level upglide on Tuesday.

Some degree of a split flow pattern then develops over the east
Wednesday through Friday, but with continued significant differences
continuing in the model details. The ECMWF is more robust on closed
low development with the southern stream over the northern Gulf of
Mexico Wednesday into Thursday. Both models have a more progressive
northern stream, but with the GFS more amplified and less out of
phase with the southern stream system. All solutions keep moisture
associated with the southern tier/Gulf system suppressed generally
to the south through Thursday, but the ECMWF brings more moisture
into the region Thursday night through Friday while the GFS confines
it to the immediate southeast coastline. A blend will be featured
that brings low end PoPs for rain into the region, especially the
southeast half, Thursday night and Friday. This system bears
watching since the drier GFS thicknesses are rather chilly during
this period.


At KCLT and Elsewhere: VFR at all TAF sites through tonight. High
pressure will translate across the area today, setting up over the
Carolina coast tonight. This will lead to more or less continually
veering but light winds, becoming SE early this afternoon and
eventually becoming S or SW tonight. The only clouds that will
be seen today will be occasional cirrus passing through within
relatively fast and northwesterly upper level flow. This evening,
as a potent upper trough approaches from the west, clouds will
begin to thicken and high/mid levels. Around daybreak Saturday,
some low level upglide will begin and moisture will increase from
the S and SW. This is expected to form an MVFR-level cloud base
in the mid-morning at most TAF sites. The TAFs include a broken
MVFR ceiling to account for this, but it is by no means certain,
as the upglide is not especially strong. Could easily be limited
to scattered low clouds. Flow will increase by midday and thus
occasional wind gusts are possible from the SW.

Outlook: Potential for MVFR/IFR goes up Saturday afternoon as
moisture increases ahead of an approaching cold front. The front
should push thru Saturday night with a period of low clouds
and rain.  Expect VFR from Sunday morning daybreak onward into
the middle of next week.

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:




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