Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 220154

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
954 PM EDT Wed Mar 21 2018

Mountain snowfall will gradually end through Thursday morning as a
deep low-pressure system departs the East Coast. Dry and
unseasonably cool high pressure dominates the weather through
Friday. The next low-pressure system arrives for the weekend with
light precipitation and even cooler temperatures.


945 PM Update...Diminishing radar returns are noted across the NC
mtns as large scale lift decreases on the back side of the upper
low. There remains decent upstream moisture and nw/ly llvl winds are
continuing quite strong. However with moisture waning overnight and
a s/w ridge working in from the northwest...snowfall rates will
continue to drop and expect arnd an inch additional accum thru
the overnight...mainly across the Smokies. A mtn wave has developed
evident in a standing Ci on GOES-16 and a gap wind could develop
overnight bringing gusty conds back over the Upstate.

730 PM Update...No major changes needed to the grids outside of
minor tweaks to hr/ly temps. Light snow continues to fall across the
NC/TN spine region with good ulvl forcing and a llvl moisture
supply. Have noticed a lowering trend in intensity on the 88D...but
still expect light accumulating snowfall thru the evening. It/s hard
to get observed snowfall amts for the far wrn high elevation
portions of these counties...but a a county canvass is now being
conducted...which will be included in the next PNS. Still seeing
isol light returns across the adv areas so this will be allowed to
continue. The HWO snow mention may be dropped due to lack of
snowfall seen across the far ern mtn zones.

As of 245 PM...latest RADAR and satellite imagery are showing good
snowfall returns across the NC/TENN Border region as deep moisture
and strong NW flow winds persist over the area. We`re also still
seeing some light returns across parts of the NC Foothills and
Piedmont, however much of this appears to be virga. NWLY winds
will remain vigorous well into the evening with winds tapering off
later tonight as the main sfc low lifts farther NE and the pressure
gradient weakens across the fcst area. Winds will pick up again
tomorrow afternoon as we mix out, but they should not be as strong
as today, especially wrt to gust values. Otherwise, the upper trof
axis will mix east of the fcst area later tonight and lift offshore
early tomorrow. At the sfc, the main low will lift farther northward
and up the Atlantic Coast tonight and tomorrow. As it does, broad
sfc high pressure will gradually overspread the region and dry out
the profiles. After roughly 12z or so, most of the snow showers
should be ending across the higher terrain.


As of 220 pm Wednesday: Heights will be on the rise across the area
through at least the first half of the short term, as upper low/
trough axis pull away from the East Coast, and a high-amplitude,
albeit dampening ridge builds in from the west. The ridge will
continue to dampen toward the end of the period, as a mid-level
speed max rides the top of the ridge, shearing out as it moves into
the Ohio Valley and surrounding areas by the end of Saturday.

In terms of sensible weather, the period will begin dry and
unseasonably cool, with temps expected to average around 10 degrees
below climo Thu night and Fri. Precip chances will then increase
late Fri into Fri night, as warm front activates to the E/SE of
weakening cyclone (associated with aforementioned dampening short
wave). By Sat morning, pops range from a slight chance across the
Piedmont, to solid chances near the TN border. If precip does
develop during this time, forecast soundings suggest light snow, or
at least a rain/snow mix would be possible for all areas roughly
north of the French Broad Valley and mainly across the high
elevations south of there, with light accums possible across the
northern mtns. However, warm advection flow would likely allow for a
transition to rain during the day Saturday, at which time a
Miller-B scenario would likely be unfolding across the Southeast.

The highest pops for Saturday (likely) will be favored across the
western mountains with the approach of the dissipating inland
surface low, and across the I-40 corridor in the Piedmont/foothills,
where low level convergence and isentropic lift will develop along
and above an advancing back door cold front/developing surface
wedge. Uncertainty regarding the timing of this back door front
makes for a problematic temp forecast for Saturday, but for now,
maxes are expected to range from the 40s across the northern tier of
zones, to the lower/mid 60s across the upper Savannah River Valley/
Lakelands areas of the Upstate and northeast GA.


As of 235 PM Wednesday: Seeing a bit more clarity with regard to
the fcst early next week. The model guidance is fairly consistent
with the approach and passage of a Miller Type-B sfc low on Saturday
night and Sunday. The forcing and deep moisture generally support
the idea that precip will be likely overnight and during the morning
hours. The main concern is the shallow cold air the models show
diving southward to the east of the Blue Ridge as the secondary
low forms off the Carolina coast early Sunday. The models suggest
low level thickness supportive of mixed precip types (read: sleet
or freezing rain) but that would be way out of season east of the
mtns. Fortunately, guidance also suggests that temps will only
make it down to the upper 30s outside the mtns, so only rain will
be fcst. Not so at the really high elevations, where it seems like
winter does not want to give up, as profiles show the potential
for snow above about 5k feet. Precip should slowly diminish on
Sunday as the sfc low pulls away rapidly and the moisture dries
up, while a classic cold air damming wedge develops by the end of
the day. Temps are kind of tricky for Sunday and will depend on
how quickly the precip ends. Either way, they will be well below
normal. From that point onward, the guidance shows the cool and
dry air mass to be quite expansive as it is driven all the way
down to N Florida and westward into Alabama on Sunday night and
Monday. The trend in the models is for the dry air mass to win
out and both models have what amounts to a dry wedge for that
period. This far out, confidence is usually better on temps than on
precip chances, so this trend plays well into my fcst preference,
which is to eliminate any small chance of precip (and thus any
small chance of wintry precip east of the mtns) for Sunday night
and early Monday. The rest of the period remains dry and cool
thru Tuesday night as a chunk of the sfc high moves down off the
southeast coast by Wednesday morning. The wedge will get some weak
periodic reinforcement from weak warm advection and isentropic lift,
but it will stick around thru Tuesday not so much because of that,
but because there won`t be much to destroy it, and the edges will
be so far away that it won`t be able to mix out up into our region
until after Tuesday. The high center being off the southeast coast
on Wednesday should finally allow that to happen, so a big warm
rebound is planned for that day. Precip chances will begin to
work their way back in from the west over the mtns in the warm
advection pattern out ahead of the next system in the afternoon.


At CLT and elsewhere: No flight restrictions thru the TAF period
across all sites. The atmos will continue to dry thru the mid-levels
overnight and leave generally FEW/SCT Ci during the daytime Thursday.
Winds will remain strong as a reinforcing sfc high builds across the
Deep South and maintains a rather tight p/grad over the area. Expect
gusts to be maintained overnight at KAVL and possibly across the
Upstate terminals as a gap wind looks to develop. Good mixing
tomorrow afternoon will again allow nw/ly moderate gusts at KAVL and
lower-end gusts elsewhere.

Outlook: VFR conditions expected through Friday, with increasing
precipitation chances and possible flight restrictions this weekend.

Confidence Table...

            02-08Z        08-14Z        14-20Z        20-00Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High 100%     High 100%     High 100%     High 100%
KHKY       High 100%     High 100%     High 100%     High 100%
KGMU       High 100%     High 100%     High 100%     High 100%
KAND       High 100%     High 100%     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 Weather Advisory until 8 AM EDT Thursday for NCZ053-059-
     Winter Storm Warning until 8 AM EDT Thursday for NCZ033-


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