Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 180530
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
1230 AM EST Sat Jan 18 2020

.SYNOPSIS...
Cool high pressure over New England will move east as a cold front
crosses the Mississippi Valley tonight. The cold front will cross
Tennessee Saturday as precipitation develops over our area ahead of
the front.  As the cold front moves off the Carolina coast on
Sunday, very cold high pressure will drop south out of central
Canada. Expect this high pressure to bring below normal temperatures
through mid week.

&&

.NEAR TERM /THROUGH TODAY/...
As of 1230 AM: Thickening mid-level clouds will return early this
morning. Have had to adjust temp curve as a result of warmer temps
due to the earlier mid clouds. Uncertain whether this will yet have
an impact on the potential for a brief period of precip-type issues.

Otherwise, tonight, the latest guidance continues to prog sfc high
pressure building in back across the Carolinas as it shifts eastward
into the New England region, with a cold air damming wedge well in
place on Saturday. With increasing cloud cover and moisture in ahead
of an approaching cold front, driven by upper troughing swinging
through the northern Plains, colder temperatures within the wedge
and moisture will allow for wintry precipitation to move into the
area on Saturday morning. However, the biggest challenge with this
forecast period will be the onset of precipitation reaching the
ground, with the already dry air in place, and the rate at which the
atmospheric column saturates by Saturday morning, overlapping with
the colder temperatures, all while fcst soundings show a fairly well
defined warm nose over western NC/along the Blue Ridge Escarpment,
with the wedge in place. Around daybreak on Saturday through late
morning, continue to anticipate the likelihood of a wintry mix,
with diabatic cooling/wet-bulbing becoming a note of interest,
as to where there will be snowflakes over the northern foothills,
with freezing rain/possible sleet mainly across the NC mountains. As
of right now, in going with WPC guidance and going temperatures,
it looks like snow accumulation will be none attm. Not to say that
snowflakes won`t fall, but given the wet/gradually warming grounds,
accumulations will be hard to come by. As for ice accumulation,
latest fcst has anywhere from a light glaze to a tenth of an inch,
mainly across the southern mountains, but not to say that there
couldn`t be a light glaze across the northern mountains. Especially
across the higher elevations. Precipitation will shift eastward
through the morning hours, as temperatures gradually begin to rise
above freezing, after a night with temperature across NC in the
lower 30s, to mid 30s across the Upstate. By the time precipitation
reaches the I-77 corridor/CLT region, do expect precipitation to
fall as all rain, though light wintry precipitation could linger
through late morning, across the higher elevations across the
northern mountains. However, confidence is low attm, with the
possibility of temperatures being slightly colder than currently
forecasting.

By Saturday afternoon into evening, expect all precipitation to
fall as rain, with temperatures in the lower 40s within the wedge.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...
As of 200 PM EST Friday: The short term picks up Saturday night as a
surface cold front is moving through the area. Precipitation will be
widespread, but temperatures by this point should be warm enough
that everything is liquid in form. Precip will taper off by 12Z
Sunday morning, with dry and cold surface high pressure moving in
during the day as the cold front exits the area. The only weather of
note will be windy conditions on Sunday, though the pressure
gradient will relax somewhat and winds will taper off by Sunday
night despite the continuing cold air advection. Upper troughing
will deepen a bit on Monday, with both max and min temps expected to
be about 5-10 degrees below average and dry conditions through the
day.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
As of 220 PM EST Friday: The medium range begins Monday night in the
midst of cold and dry period, with upper troughing amplifying over
the eastern CONUS and dry Canadian high pressure at the surface.
This will keep max and min temps below average (finally) Tuesday and
Wednesday. An upper low will swing through the upper flow on Tuesday
into Wednesday, but with no available moisture, it`ll propagate
through the area with no impact until it reaches the SC coastal
Plain and causes cyclogenesis offshore. Late Wednesday and Thursday,
a small upper ridge will amplify over the southeast, with
temperatures recovering to near/just above average Thursday and
Friday. The next system will approach the area on Friday as an upper
low causes surface cyclogenesis somewhere in the south/central
CONUS, though models disagree on exactly where this will happen. The
placement of the eventual surface low will have quite a lot of
influence on the expected precipitation type for our area, though
there is plenty of time to work out the final details. Regardless of
the exact low track, moisture increases ahead of the next front, an
pops were kept to chance on Friday before increasing to low-end
likely on Saturday as the better upper forcing/deep moisture/surface
front arrive in the area.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
At KCLT and elsewhere: VFR conditions should prevail from now until
around mid morning. A light ESE wind continues at KCLT with an
easterly wind for all but KAVL where SSE wind continues. Light
precip will spread eastward after daybreak but will have a hard time
reaching the ground through midday, and thus the ceiling should only
gradually drop through VFR and into the MVFR range. This should not
happen at KCLT until maybe 16Z at the earliest. The only precip-type
problem might be at KAVL if it arrives early enough but have left
out for now as it is inside the 9 hour PROB30 rule. Outside of the
mtns, the precip at onset should be rain. Will not rule out a very
brief period when a few sleet pellets reach the ground, but think it
will not last long enough to cause any problems. Temps should be at
least 36F by the time any light precip would reach the ground at
KCLT. As this happens, the wind should come around more southerly.
Eventually, the steady rain will arrive...probably not until 18Z at
KCLT, and that should gradually bring the ceiling down thru the MVFR
category and into the IFR category by nightfall. Similar but earlier
trends elsewhere. IFR cigs and MVFR vsby continue overnight even as
rain ends. Winds turn SW overnight, NW at KAVL.

Outlook: A cold front should clear the region by daybreak Sunday.
After the front passes, expect VFR for several days.

Confidence Table...

            05-11Z        11-17Z        17-23Z        23-00Z
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:

www.weather.gov/gsp/aviation

&&

.GSP WATCHES/WARNINGS/ADVISORIES...
GA...None.
NC...None.
SC...None.

&&

$$
SYNOPSIS...DEO
NEAR TERM...PM/RWH/SGL
SHORT TERM...17
LONG TERM...17
AVIATION...RWH



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