Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 130721

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
321 AM EDT Tue Mar 13 2018

Colder and drier high pressure will build across the area today and
persist through midweek. Expect a warming trend later in the week to
near or above normal temperatures. A cold front and area of low
pressure will bring showers to begin the weekend.


As of 300 AM EDT Tuesday: Upper low that caused all the ruckus
yesterday is off the NC coast and will take its problems with it up
the Eastern Seaboard. Longwave trough/cutoff low continues to spin
over the Great Lakes/southern Ontario. Another little lobe of
vorticity will rotate around the main low, sliding down the NW flow
aloft to push into the Appalachians tonight. At the surface, lee
trough of low pressure will remain across the western Piedmont
today, but the lee trough will weaken as high pressure works its way
down the Plains and tries to start ridging into the area.

Overall, for the Piedmont, expect a mostly clear and cool day, with
mostly cloudy and definitely cooler for the mountains. Might see a
little bit of sunshine midday in the wake of the exiting system and
before the next shortwave starts really impacting the area, but
should be enough mid-level moisture for at least some cloudiness,
spilling over occasionally into the Piedmont as stratocu. With the
shortwave bringing a reinforcing shot of CAA to the region, lows
tonight should drop at least a couple of degrees lower than what
we`ll see this morning; luckily continued mixing overnight will
prevent us from bottoming out at the dewpoints (near 20 across most
of the Piedmont near sunrise Wed). Wind chills will be an issue;
still a little high for an advisory but especially in the mountains
it`ll be darn cold at the bus stops Wednesday morning.

Other concern with the shortwave diving down and its attendant
moisture is another round of light snow overnight. QPF seems to pile
up more significantly in the Smokies than the rest of the TN border
mountains, and so expect a quick shot of up to 3" at the highest
elevations there (the rest of the mountains look like maybe 1-2").
Will issue another high-elevation advisory for Swain and Haywood
Counties to account for this. Of course, the caveat remains that if
snow ratios increase, which is always a concern with NW flow
situations, so always possible that the advisory might need to be
expanded (or upgraded across the Smokies). Sometimes it seems like
this system will never end...


As of 145 AM Tuesday: The short range begins with continued NWF
-snsh across the wrn NC mtns. Soundings are fully below freezing
with no warm nose nor freezing precip concerns. An additional  half
inch is probable thru the early afternoon. Dry air works  in rather
quickly as a cP airmass continues to mix in  throughout the day.
This will drop sfc td/s into the lower  teens and possibly the
single digits over most of the  FA....thereby making RH values less
than 25% over the non/mtns.  With deep mixing to h8...gusts shud
reach the 20-25 mph range  during the afternoon. Fire danger will be
a concern Wed  afternoon...mainly across NE GA as soil moisture
levels have  already dropped 15 gm in the past 6 hrs to 12-15 gm.
With  another 36 hrs of evaporation...expect fuel moisture to be less
than 10 gm over NE GA.

Upper level heights begin to rise and will continue to increase thru
the period...however a llvl thermal trof will be slow to modify as a
component of sfc-h92 CAA continues. Thus...max temps will be held
abt 15 degrees below normal. With clear conds...overnight lows will
likely drop a few degrees below normal as well. On Thu...winds back
sw/ly across the non/mtns in response to a dry llvl trof crossing
the FA from the northwest. This along with good insol will allow max
temps to reach normal levels. There is an adv level wind concern
across the NC mtns in the afternoon as the trof tightens the sfc
p/grad to 6 mb. This possibility will be mentioned in the HWO.


As of 220 AM Tuesday: The ext range remains active with some
uncertainty still existing with the Sat system. Fri will be a
rather pleasant day overall as Atl ridging dominates the SE CONUS
and allows max temps to reach a cat or so abv normal. There is a low
chance of precip across the NC mtns as a frontal band and ulvl
energy begins working in from the west. This front will be associated
with a weakening low pressure system moving out of the srn Plains
and heading directly toward the srn Apps. Good moisture adv will be
available ahead of the system with the majority of precip falling
west of the FA.

The frontal band shud back-door into the area Fri night and
continue south across the srn Upstate thru daybreak Sat. This is
when the models begin to have varying ideas with the pattern. The
GFS develops a rainy and moderately strong insitu wedge thru the
day Sat...while the ECMWF agrees somewhat but is drier...and the CMC
is much drier yet. Thus...PoPs have been maintained in the low
chance range. No good chance of thunder as the models keep the
sigfnt instability SW of the FA thru the day. Max temps will be
tricky...but have continued to lower values based on the possibility
of precip coverage and the expected airmass mix associated with the
cP high pushing south. With the cooler temps...light snow showers
are probable across the nrn tier of zones Fri night...with low-end
measurable likely only across the far nrn NC mtns.

The atmos should dry out deeply Sun as the sfc high centers
off the NC coast and ulvl ridging crosses overhead. With increasing
sw/ly flow...max temps will return to arnd or abv normal levels.
Another weak system approaches the area Mon and PoPs have been
maintained in the low/mod chance range due to the uncertainty with
the mass fields and forcing.


AT KCLT and elsewhere: VFR through the period. NW flow moisture
piling up against the NC mountains may spill over for the next
couple of hours and lead to flurries or very light -SHSN at
KAVL/KHKY. Increasing moisture again this afternoon will likely
result in some low VFR Cu/StratoCu across the area, possibly
lingering into the overnight hours especially at KAVL. Otherwise
mainly a wind forecast, with NNW winds to start the period. Low-end
gusts continue at KGMU/KGSP, with more significant gusts, 25-30kt at
KAVL (this will continue pretty much through the period). Should see
some decrease in the next few hours, but afternoon mixing may lead
to low-end gusts at all TAF sites. Winds may shift more W to even
WSW briefly this afternoon as lee toughing develops before veering
back WNW to NW; have kept KCLT on the north side for now.

Outlook: Northwest flow moisture could keep some clouds in at KAVL
from time to time through mid week. Otherwise, VFR conditions are
expected to persist through the end of the work week.

Confidence Table...

            07-13Z        13-19Z        19-01Z        01-06Z
KCLT       High 100%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  98%     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 10 AM EDT this morning for
     Winter Weather Advisory from 8 PM this evening to 11 AM EDT
     Wednesday for NCZ051-052.
     Wind Advisory until 8 AM EDT this morning for NCZ053.


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