Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180610

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
110 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 100 AM Sat:  A fast moving upper trof is moving out of
the Rockies with surface cyclogenesis now underway over KS/MO.
With that, wly/wnwly H5 flow continues to carry mid/high level
moisture downstream across the mid MS and TN valleys as well as
across the southern Appalachians. Major changes to the fcst with
this update are all related to TAF issuance. Wind gusts at high
elevations tonight still look to meet advisory criteria so an
advisory is being considered for there. Will make a decision for
morning package around 330-400 AM, if neighboring offices agree.

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.


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: 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:




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