Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 300833

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
333 AM EST Wed Nov 30 2016

Deep moisture will return from the west along an approaching cold
front through the morning. The cold front moves thru this afternoon
and evening. In its wake, dry high pressure fills in tonight,
persisting through Saturday. Moisture will return Sunday into Monday
ahead of another developing low pressure system to the west.


As of 300 AM Wed: A cold front extends from low pressure over
the western Great Lakes to the central Gulf Coast; the warm front
associated with this system is aligned just north of I-40 across
western NC, and should edge slowly northward this morning. The
warm sector is teeming with convection from LA into East TN,
in a tight upper height gradient ahead of a potent shortwave
over TX. Instability is much more abundant over there, but SPC
mesoanalysis indicates 100-300 J of MLCAPE extending as far
northeast as the I-26 corridor. This diminishes over the next
few hrs per RAP. In the meantime, there is a very small chance
we could see a t-storm develop, given the continued lo-lev WAA
as well as the possibility of upper level eddies moving in from
upstream convection. Otherwise, a few showers will persist mainly
invof the upslope areas of the Blue Ridge early this AM.

Strong southwesterly low to midlevel flow continues today as the
cold front encroaches. Upglide doesn`t really look any better until
just prior to its arrival, so PoPs are slow to increase before it
reaches the mountains early this aftn. Timing has been determined
based on a compromise between the 4km NAM and SREF probs. Precip
rates are not expected to be great enough to cause hydro issues
(soils still have a lot of capacity even after yesterday`s rains,
anyway). We`re more concerned about the possibility of damaging
winds, given the impressive shear parameters and strong forcing
from the front itself. Instability will be the limiting factor,
with overcast skies expected over most areas thru the day, mediocre
lapse rates, and possibly even a midlevel cap associated with the
WAA. The majority of SREF members fail to generate even 250 J/kg of
SBCAPE, though a majority also produces above 40 kt of effective
shear. Thunder has been limited to a "scattered" mention. SPC has
rightfully included the CWFA in a Marginal Risk area for today,
with a small portion of our southern SC/GA zones in the Slight
Risk. They highlight the fact that instability will be better to
our southwest and that the Slight was extended this far north mainly
in case a cell develops there and survives until it gets here.

An additional concern will be the prefrontal winds--the LLJ being
directly exposed to the mtn ridges and potentially mixing down over
the Piedmont. As also occurred yesterday morning, 50 kt winds may
be present as low as 4-5 kft MSL. We already have issued a wind
advisory for the NC mountains, and this will be maintained. Prog
soundings suggest enough mixing will occur in the Piedmont that
gusts may exceed 30 kt at times. Gusty, but currently it appears
those will remain below advisory criteria.

Drying and clearing is expected to occur pretty quickly once the
front passes. The moist layer that develops in NW flow against
the mountains looks too shallow for a mentionable PoP. However,
in the ensuing CAA, winds will ramp back up again at the high
elevations. Another windy night, but not quite to the same
degree. Except perhaps on the highest peaks, gusts do not warrant
an advisory.


As of 245 AM EST Wednesday: the late part of the work week looks
fairly quiet. A cold front will be long-gone to the east by daybreak
on Thursday. Precip chances look very low, even in the upslope areas
near the TN border. The front will usher in cooler and drier high
pressure across the Southeast for Thursday and Friday. The flow will
go nearly zonal on Thursday as the high settles over the Deep South.
Temps will be on the order of 10-15 degrees cooler, and just
slightly above normal, for Thursday. An upper ridge starts to
amplify to our west on Friday in response to the next srn stream
trof digging over the Southwest. This will help keep the sfc high
center to our west thru Friday night. Temps should be a bit cooler
on Friday, maybe just a degree or two below normal.


As of 225 AM EST Wednesday: the medium range fcst picks up at 12z on
Saturday with flat upper ridging in place over the southeast and a
large H5 low cutting off from the southern stream flow over the Baja
Peninsula. Overall, the long range models appear to be coming into
much better agreement wrt the large scale pattern evolution. The flat
upper ridging is expected to remain in place over the region into early
next week as the closed low eventually becomes absorbed by the mean
flow to its north. By Monday afternoon, heights begin to increase again
as stronger ridging overspreads the region from the west.

At the sfc, the pattern evolution still appears more uncertain compared
to the upper levels. High pressure will be in place over the region to
start the period. As we move into Sunday and then early next week, the
sfc pattern becomes more muddled. The models try to develop some sort
of coastal low over the Gulf of Mexico, but it remains unclear exactly
where and when it will take shape. All of the models have a broad area
of deep moisture moving into the CWFA on Sunday from the west. The models
dry things out by late Monday as a central CONUS high moves over the fcst
area from the west. Over the next 24 to 36 hrs, the models move the high
up over New England and a CAD regime appears to setup by the end of the
period. I kept likely POPs on Sunday and then tapered them back to solid
chance for Monday and Tuesday. The temperature fcst is especially tricky
and could change quite a bit over the next few days as confidence in the
pattern solidifies.


At KCLT and elsewhere: IFR/LIFR cigs remain over most of the area,
with warm frontal boundary bisecting the area. Light showers are
occurring north of the boundary. Some improvement has occurred
in spots over the Upstate following the departure of the showers,
but it is believed any periods of improvement will be short-lived
given continued WAA and moist boundary layer. The setup favors cigs
over fog, but some areas of MVFR will persist. The poor conditions
are expected to persist into the day, with a cold front pushing
in from the west this afternoon. Ahead of the front, instability
will be limited, but a few isolated TSRA are expected (chances
too low for TAF mention at this issuance). Expect frequent gusts
of 20-25 kt with peak winds perhaps exceeding 30 kt, due to strong
flow prior to the fropa. The front moves thru the area during the
late afternoon to mid-evening timeframe; following its departure
VFR returns. Wind shift to NW occurs in the evening at KAVL,
and after 06z at the other sites.

Outlook: Drying occurs behind the front Wed night, with dry high
pressure returning for Thursday thru Saturday.

Confidence Table...

            08-14Z        14-20Z        20-02Z        02-06Z
KCLT       Low   24%     Med   69%     High 100%     High  94%
KGSP       Med   76%     Med   72%     Med   72%     High 100%
KAVL       High  91%     Med   76%     High  91%     High 100%
KHKY       Med   69%     Med   73%     Med   67%     High  91%
KGMU       Med   77%     Med   77%     Med   77%     High 100%
KAND       Med   61%     Med   65%     Med   76%     High 100%

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:


NC...Wind Advisory until 7 PM EST this evening for NCZ033-048>053-


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