Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 171424

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
924 AM EST Mon Dec 17 2018

Dry high pressure will linger through midweek. Another low pressure
system is then expected to develop to our west and bring widespread
precipitation to the region for Thursday and Friday. As the low
moves out on Saturday, high pressure will return for the weekend.


As of 922 AM EST Monday: Some patches of thicker mid-level clouds
moving overhead this morning, and that appears to be slowing the
warmup across parts of the region. Will modify the temperature
trend thru late morning, but no need to give up on the high temps
just yet.

Otherwise, the near-term will be very quiet in the wake of the
weekend system, with NW flow continuing over the area with a
little wiggle shortwave pushing over the region today. A deep
system will be gearing up over the Rockies, inducing downstream
ridging over the Plains, which in turn will keep our flow from the
NW today. With downsloping NW winds at the surface and nearly full
sun (expect some lingering NW flow cloudiness in the mountains
through early afternoon or until the moisture finally dries up),
temperatures will warm nicely, a 5-7 degrees above seasonal
normals, and the warmest day since December 3. Really absolutely
beautiful out there today, but with possibly some occasionally
gusty winds especially in the mountains as the wiggle in the flow
moves across aloft. Lows tonight will be pretty close to normal as
cool high pressure across the Great Lakes gets pushed this way as
the wavetrain works its way east. A dry front on the leading edge
of the surface high will move in toward the end of the period,
to affect temps as we move into the short term.


As of 100 AM Mon: The short term period looks largely uneventful. On
Tuesday sfc high pressure will migrate from the Ohio Valley to
the East Coast. Temperatures will remain a few degrees above
normal Tuesday under sunny skies. While return flow sets up over
the high Tuesday night, it is not until late in the day Wednesday
that moisture becomes sufficient to expect a sharp uptick in cloud
cover. Temps will fall back to about normal. Precip chances do
return Wednesday night in the ensuing upglide, though the moist
layer remains shallow thru daybreak Thursday. Models have trended
later with the precip onset, so Wed night PoPs have been lowered
from the previous fcst. The precip is expected to be all rain
except on the very highest ridges of the northern mountains, but
most likely they will warm Thursday morning before precip begins.


As of 250 AM Mon: As a sharp upper trough pushes across the
Mississippi Valley, possibly with an embedded cutoff low,
cyclogenesis will occur Thursday to our southwest. Model consensus
suggests the resultant low generally will track rapidly up the
western side of the Appalachians, ending up near Lake Ontario by
the end of Friday.

The moist upglide already underway at the start of the period will
strengthen and peak late Thursday, and categorical PoPs already look
like a good bet thru early Friday now that the model solutions have
converged. The incumbent airmass does not look particularly cold,
and furthermore the track of the low and prog profiles suggest the
precip in this timeframe will be all liquid over the populated areas
of the CWFA (snow could occur on the very highest peaks). There is
a bit of discrepancy in the models in how they depict wraparound
precip east of the mountains, if any, mainly timing differences of
a few hours. This has been handled by delaying the decline of PoPs a
bit on Friday. By early evening, with cold advection starting on the
back side of the low, snow levels will fall over the higher terrain
as the primary forcing becomes northwest flow. Saturation remains
over a rather deep layer during this time, with some Great Lakes
fetch, implying light accumulation is possible where it is cold
enough. Brisk winds will also be experienced across the mountains,
but current indications suggest they won`t be unusually strong
for this time of year.

Moisture diminishes late Friday night into Saturday as high pressure
builds in from the middle of the country. This will help bring low
temps back to around normal, but highs will stay above normal east
of the mountains in downslope flow. The ECMWF depicts a clipper
bringing another chance of precip to the mountains at the very
end of the period Sunday night, with temps supportive of snow. The
GFS shows no such feature, at least not yet, so the chance is kept
very small in the forecast.


At KCLT and elsewhere: VFR through the period, except for some
lingering low MVFR clouds at KAVL that should continue to erode over
the next hour or so this morning as the NW flow moisture exits.
Winds varying around W this morning favoring WSW early but should
veer to WSW by mid-morning for all but KAND (where lee troughing
might keep winds more on the S side). Winds will continue veering
around to NE toward the end of the period. KAVL winds will remain NW
up-valley through the period, with occasional low-end gusts today.

Outlook: Expect VFR conditions through midweek. A wet low pressure
system looks likely to bring restrictions Thursday, perhaps lasting
into Friday in some areas.

Confidence Table...

            14-20Z        20-02Z        02-08Z        08-12Z
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:




SHORT TERM...Wimberley
LONG TERM...Wimberley
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