Area Forecast Discussion
Issued by NWS Greer, SC

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FXUS62 KGSP 232317
AFDGSP

Area Forecast Discussion
National Weather Service Greenville-Spartanburg SC
617 PM EST Mon Jan 23 2017

.SYNOPSIS...
A strong upper level low will move northeast of the region before
warm high pressure builds into the region through late Wednesday. A
cold front will push east of forecast area Thursday...with below
normal temperatures returning Friday through the weekend.

&&

.NEAR TERM /THROUGH TUESDAY/...
2315 UTC Update: Pops and QPF were adjusted based on radar trends.

As of 225 PM EST: Water vapor imagery shows the closed upper low
center moving east across the central Carolinas piedmont, with
surface observations indicated the pressure falls near coastal North
Carolina. This system will continue to slowly pivot away to the east
through late day, with lingering deformation zone forcing moving
east across the I-77 corridor through late afternoon. Elevated PoP
values will follow this area of deeper moisture and forcing. As the
banded forcing slides farther east, northwest flow upslope forcing
and moisture will increase across the western mountains this evening
and overnight. Thermal profiles continue to support mainly rain
across the mountains, however, snow levels should continue to fall
across elevations above about 4000 to 5000 feet. This should allow
the higher ridges to see some light snow accumulations through the
overnight areas, but any advisory level accumulations would be
mainly at 5000 feet and up - with upwards of 3 inches possible
around the Smokies. No Winter Advisory products appear needed given
the very high elevation nature of any accumulations. In addition,
the low level cold advection jet during the peak of the northwest
flow may reach around 50 kt along the Blue Ridge toward daybreak.
The surface based mixed layer should just briefly the strong winds
near the Blue Ridge, so we high end advisory criteria still looks
good there. Any uptick in the 850 mb cold advection jet overnight
could require a small High Wind Warning near the northern Blue
Ridge.

The 500 mb low center will move east off the Mid-Atlantic coast
overnight, with broad ridging building over the southern
Appalachians from the west through Tuesday. NW upslope moisture will
steadily dry up as well through the day on Tue with mountain winds
diminishing after Noon as the pressure gradient relaxes. Expect lee
troughing to develop east of the mountains through Tuesday
afternoon. Downsloping off the mountains should yield plenty of max
temps in the 60s over the foothills/piedmont Tuesday afternoon.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/...
As of 245 PM EST Monday: High pressure will be across the area on
Tuesday night. That feature will pull away on Wednesday as the next
in a series of systems impacts the region. Temperatures in the
southwesterly flow will top out around 70 outside the mountains
Wednesday afternoon.

A cold front will push across the region Wednesday night. A band of
showers is expected with this feature. Amounts look to be a quarter
to a third of an inch in the mountains and around a tenth of an inch
in the Piedmont.

The front will push east of the region on Thursday leaving most of
the area dry and a bit cooler. Northwest flow showers will develop
along the Tennessee border by Thursday afternoon. The showers will
be snow in the higher elevations and rain lower down with a gradual
trend to more snow by late in the day. Little accumulation is
expected.

&&

.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
As of 230 PM Monday...looks like a return to winter for the western
Carolinas and northeast Georgia at the end of the week, not so much
because of precipitation, but in terms of temperatures. We have been
anticipating an upper pattern change for the end of the month and
the new guidance continues to show a mean upper trof over the
eastern half of the country for the period from Friday onward to the
end of the 10-day model runs, with perhaps a brief break in the
middle of next week. The mean upper trof axis should be centered
over land for most of the period, which should afford us the passage
of several weak short waves dropping down into the trof and more
than one surge of low level moisture from the north/west. This bodes
well for the chances of snow near the TN border, but the timing of
any periods of better probability are difficult to nail down because
of timing differences between the GFS and ECMWF.  Suffice to say
that we should expect more than one NW Flow Snow event during the
medium range period, the first of which may be underway Thursday
night and into Friday morning. The GFS in particular shows moisture
surges Friday evening, then another Saturday afternoon, and a third
Sunday night and Monday. The ECMWF is not as impressed with
magnitude and duration. The forecast more or less follows the idea
of the operational GFS, while limiting precip to the lower part of
the chance range due to uncertainty this far out in the forecast. No
precipitation is expected east of the spine of the Appalachians.
Temps should be at or slightly below normal. The last surge on
Monday could be the interesting one, as the GFS shows a stronger
short wave with a vort center diving down and crossing northeast
GA/Upstate SC in the afternoon with a deep cold air mass in place.
This sort of mid/upper feature is typically what we need to cause
some of the snow showers/flurries to break containment and move out
east of the Blue Ridge. It is way too far in the future to start
including chances of snow outside the mtns, but it is fair to say
there is a non-zero chance of wintry precip as seen in fcst
soundings from the GFS over the NC Piedmont/Foothills and GEFS plume
diagram at CLT on Monday. This will hold some of our interest going
forward.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
At KCLT: Lingering moisture under the slowly departing closed low
pressure system will produce mainly MVFR ceilings and occasional
light rain showers through late afternoon. There is fairly good
consensus among the models/MOS on toggling winds more WNW after 21Z
and this has been followed in the latest TAF. Expect the ceilings to
then gradually lift to VFR and scatter overnight. Winds could toggle
back toward WSW with lee troughing Tuesday, but this is uncertain
through 18Z so will keep the flow WNW for now.

Elsewhere: Quite a range in ceilings is still observed this
afternoon, with slowly scattering IFR from KHKY to KGSP and much
higher VFR at KAND and KAVL. Anticipate slow improvement throughout
the region through late afternoon and this evening as the low
pressure system pulls away to the east. The main exception could be
KAVL where improving NW flow moisture may help regenerate MVFR
ceilings for most of the night. Anticipate the gradient tightening
and WNW to NW flow developing throughout, with late afternoon gusts
across the western Upstate of SC and strong gusts right through
tonight at KAVL. Expect any lingering rain showers to pull east with
the low pressure system through late afternoon, with the main
exception being the potential for lingering VCSH near KAVL. The
pressure gradient will slowly start to relax near the end of the
period.

Outlook: Dry conditions are expected late Tuesday into Wednesday.
Brief moisture return is possible ahead of the next cold front
Wednesday night into Thursday, with colder air and NW flow moisture
setting up by late week into the weekend.

Confidence Table...

            23-05Z        05-11Z        11-17Z        17-18Z
KCLT       High  83%     High 100%     High 100%     High 100%
KGSP       High 100%     High 100%     High 100%     High 100%
KAVL       High  89%     High  85%     Med   75%     High  94%
KHKY       Low   47%     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...Wind Advisory until noon EST Tuesday for NCZ033-048>053-058-
     059-062>065-501-503-505-507-509.
SC...None.

&&

$$
SYNOPSIS...SBK
NEAR TERM...HG/JAT
SHORT TERM...SW
LONG TERM...PM
AVIATION...HG



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