Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 200627

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
227 AM EDT Tue Mar 20 2018

A complex low pressure system will cross the region from the west
today. Colder air will spill southward into the region behind the
system, with mountain snow showers returning tonight, and scattered
snow showers possibly extending east of the mountains early
Wednesday. Drier high pressure will briefly return over the area on
Thursday, but with another moist low arriving from the west for the


As of 215 AM Tuesday: Water vapor imagery shows the closed upper low
moving up the Ohio River Valley early this morning. With the
associated complex low pressure system arriving from the west,
instability has remained fairly modest and kept severe weather
largely at bay. The latest HRRR has the strongest convection moving
east of our area circa 09Z, with the mid level dry slot wrapping in
quickly from the southwest 09Z to 12z and persisting overhead
through late morning. The convection-allowing models then redevelop
showers from the west under the approaching passing upper trough
axis this afternoon and evening. Some embedded thunder, and possibly
small hail, are expected.

The closed upper low will move from the central Appalachians tonight
to the Delmarva coast. The complicated surface low pressure system
will continue to occlude overhead, with steadily cooling
temperatures from the high terrain down toward the lower valleys
overnight. Snow levels will crash quickly across the mountains from
the west tonight. Cold air will continue to spill southward west of
the low through Wednesday as northwest upslope flow moisture becomes
rather robust and winds increase. Will likely migrate the Winter
Storm Watch to a Warning in most areas (perhaps not Jackson Co.) and
then tack on an advisory tier for the rest of the NC mountains.
Moisture breaking containment east of the mountains, along mainly
the I-40 corridor but possibly into Charlotte, could produce some
snow showers early Wednesday. It appears too early for an Advisory
there. Further details will be available in the next AFD pending


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: Anticipate occasional thunderstorms through 09Z, with the
convection allowing models moving the activity quickly east after
09Z. IFR cigs will likely fill in this morning, then slowly lift to
VFR through early afternoon. Another round of scattered showers is
likely this afternoon and evening under the passing upper trough.
Expect SW to WSW winds through the day, becoming northerly tonight
with restrictions returning.

Elsewhere: Thunderstorms appear largely done at KAVL and the
foothill sites, but with lower restrictions likely to fill in
through the morning hours before lifting by early afternoon.
Additional showers will redevelop from the west with the appraoching
trough, but confidence is currently too low to advertise
thunderstorms. Expect mainly SW to WSW winds east of the mountains,
but with NW flow at KAVL and KHKY. Winds will turn NW to N
throughout tonight, with gusts developing over the high terrain.
Mountain snow showers may start to mix in near KAVL at the end of
the end of the period.

Outlook: Colder air wrapping around the exiting system will bring
snow to the mountains through at least Wednesday, and potentially
mixed precip to KHKY and KCLT into Wednesday morning, with rain
showers expected elsewhere. MVFR/IFR is anticipated, with VFR
returning gradually Wednesday night through Thursday. Another moist
system will arrive from the west this weekend.

Confidence Table...

            06-12Z        12-18Z        18-24Z        00-06Z
KCLT       Med   75%     Med   76%     High  94%     High  87%
KGSP       Med   77%     Med   73%     High 100%     High 100%
KAVL       Med   78%     High  88%     High 100%     Med   67%
KHKY       High  82%     Med   69%     Med   77%     Med   70%
KGMU       High  87%     Med   75%     High 100%     High 100%
KAND       Med   76%     Med   72%     High 100%     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...Winter Storm Watch from late tonight through Wednesday evening
     for NCZ033-048>052-058-059.


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