Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 191938

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
338 PM EDT Mon Mar 19 2018

A complex low pressure system will approach from the west tonight
bringing showers and thunderstorms to the region. The low will cross
the region tomorrow through tomorrow night. In its wake, mountain snow
showers are likely late Tuesday through Wednesday. Drier high pressure
will return over the area on Thursday with another moist low arriving
from the west for the weekend.


As of 230 PM EDT Monday: Potent little ball of vorticity is working
its way toward the Mid-MS Valley this evening. Surface pattern is a
bit of a mess with the primary low over the Ozarks, a secondary low
(almost a Miller B looking thing) over extreme SW VA, with a warm
front stretched between the two and then around the Appalachians
where a bit of damming remains, though little bits of sunshine that
peeked through today have done a good job with eroding some of that.
High clouds have been on the increase today which have kept
temperatures down across the western half of the area after some
initial sunshine this morning. Ahead of the approaching
surface/upper low, isentropic lift is on the increase as well, and
will result in a gradual blossoming of showers and possibly drizzle
(or at least low clouds) this evening.

The surface low will lift NE into S KY, but with the damming wedge
still weakly in place, a secondary low should form somewhere across
N GA or possibly even the Upstate. The front will be draped like a
ribbon between the two, eroding the damming from the SW as the two
lows lift NE. With the diffluence aloft ahead of the upper low, the
incoming upper jet, plentiful dPVA, plus the lift associated with
the cold front itself, we`re looking at a strongly forced synoptic
system pushing through tonight and tomorrow. Deep-layer shear will
be impressive as the system moves across, with 0-6km shear of at
least 50-60kt (though higher naturally closer to the upper low). Low-
level helicity of at least 200m2/s2, in some cases 300-400m2/s2, will
result in beautifully curved hodographs tonight, peaking in shape
around 06z (give or take an hour or two depending on the site).

To counteract this scary-looking shear, as is typical in damming
events (not to mention just the overnight frontal passage), CAPE
will be limited. We seem to be in a bizarro world because the NAM is
actually on the low side of CAPE for once, not bringing any into the
northern Upstate or NC overnight, but with a very strong gradient
right on our southern CWA border. The GFS, however, brings a good
200-400J/kg in a finger up the foothills and I-26 corridor
overnight. That said, though, even with the lack of instability, if
supercells really get going over N AL and N GA, the immediate storm
environment might certainly be enough to overcome the surrounding
environment, and SPC HREF does bring streaks of concerning max
updraft helicity across the Upstate (I-85 and south) overnight. SPC
Slight Risk was expanded to include all of our NE GA counties and
Greenwood/Abbeville, with the Marginal risk extending almost to I-
26. Severe thunderstorms with damaging winds/large hail and possibly
isolated tornadoes certainly can be expected, though the threat
would diminish the farther east you go. CAMs do still have a greater
area of thunderstorms pushing across the area overnight so isolated
strong winds will be possible just about anywhere, with of course a
widespread lightning concern. The thunderstorm threat will be
pushing out in the predawn hours.

Heavy rain, of course, will be a threat with the convection moving
through, but it should move through fast enough that most areas will
see no more than an inch, basin-average, though with convection it`s
certainly possible that isolated areas might see more than that. Any
flash flood threat would be minimal.

A secondary shortwave diving down the NW flow behind the initial
upper system will bring a secondary surge of lift to the area on
Tuesday. Lapse rates will increase and with temperatures warming
behind the eroded CAD (to fairly seasonal levels), can`t rule out
some isolated thunderstorms especially across the NC foothills and
Piedmont, but for now have kept wording at showers.


As of 3:00 PM EDT Monday: An upper level low pressure system roughly
located over northern Tennessee and Kentucky, will wobble into our
region Tuesday night while low pressure develops along the Mid
Atlantic Seaboard.

As spokes of energy move through the area, crashing heights aloft,
coupled with a potential deformation zone, cold air being drawn in
from the north (via CAD like structure) and cold air aloft, should
set-up a banding or a deformation zone of precipitation.

Precipitation will be ongoing Tuesday night, with the greatest
values over the northern three quarters of our forecast area (FA).
Considering thermal profiles, rain will change to snow over the NC
mountains and perhaps foothills and mountains of NE Georgia Tuesday

It appears the focus of precipitation will remain over the northern
half of the FA Wednesday, before being drawn back to the mountains,
as low pressure shifts north and northeast up the coast, and
lingering upslope conditions remain.

Considerable collaboration with surrounding offices and National
Centers concerning snow amounts and timing. It appears we are
looking at about a 24 hour event. We have issued a Winter Storm
Watch from midnight Wednesday to midnight Thursday for 3 to 6 inches
of wet heavy snow. Although we might fall just a tad short on the
deterministic numbers, wind gusts increasing to 35 to 45 mph and
even higher on the mountain tops, (combined with the potential heavy
snow), could lead to impact issues. We would not be surprised if the
model trend remains that a slice of Winter Weather Advisories might
be needed across the foothills of NC, perhaps fanning out along the
I40 corridor and the mountains of NE Georgia.

As the surface and upper level energy move away Thursday, weather
conditions should quiet.

We are looking at below normal temperatures during this part of the
forecast cycle.


As of 220 PM Monday: the extended fcst picks up at 00z Friday with
broad upper trofing moving off the Atlantic Coast while upper ridging
spreads over the Central CONUS. As the upper ridge approaches our
region, it is expected to deamplify and flatten. This more zonal
pattern is expected to persist over the area thru the weekend and
into early next week. At the sfc, cool and dry high pressure will be
in place to start the period. As we move into the weekend, another
low will deepen over the plains and move eastward. As this happens,
low-lvl flow will become more southerly and deeper moisture will
overspread the fcst area as a warm front lifts northward. Beyond this
point the track and evolution of the low remains fairly uncertain
with the ECMWF moving the system to our north late Sat/early Sun and
then offshore later on Sunday. The GFS, on the other hand, dissipates
the low by early Sunday and develops a wedge over our fcst area that
lingers for the remainder of the period.


At KCLT and Elsewhere: Skies are chaotic at TAF time with pockets of
VFR and MVFR across the area, but will start off generally MVFR.
Expecting a slow deteriorating trend through the evening and early
overnight hours as moisture increases. Cold front pushing through
overnight will bring -TSRA chances to the SC TAFs, and have
continued mention of those in PROB30 but this will have to be
adjusted to TEMPO with the next issuance if not earlier. Cigs should
drop to IFR and possibly LIFR everywhere overnight, with rapid
clearing from W to E in the morning (as early as 13z at KAND),
though at least MVFR conditions should hang on at KHKY through the
end of the period. Low-end gusts may pick up behind the front as
well. Wind shift at KCLT expected around noon on Tuesday and will
narrow this timing down with later issuances.

Outlook: Colder air wrapping around the exiting system will bring
snow to the mountains, and potentially mixed precip to to KAVL, KHKY
and KCLT Tuesday night into Wednesday morning, VCSH/-SHRA is
expected elsewhere. MVFR/IFR anticipated, with VFR returning late

Confidence Table...

            19-01Z        01-07Z        07-13Z        13-18Z
KCLT       Med   69%     Med   73%     Med   75%     High  85%
KGSP       High  85%     Med   60%     Low   47%     High  88%
KAVL       High  90%     Low   59%     Low   53%     High  90%
KHKY       Med   65%     Med   79%     Med   79%     High  84%
KGMU       Med   78%     Med   64%     Med   63%     High  88%
KAND       High  86%     Med   65%     Low   52%     High  92%

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...Winter Storm Watch from late Tuesday night through Wednesday
     evening for NCZ033-048>052-058-059.


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