Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 091831 AAB
AFDJKL

AREA FORECAST DISCUSSION...UPDATED
National Weather Service Jackson KY
131 PM EST Sat Dec 9 2017

.UPDATE...
Issued at 129 PM EST SAT DEC 9 2017

As of early afternoon, an eastern Conus and North American trough
was in place with a western Conus ridge. A clipper system is
moving through this trough with the surface low centered over
southwest MI and the trailing cold front extending into western
KY. Some light prefrontal returns have been moving east across the
OH Valley and the Commonwealth and some of this scattered very
light snow is likely starting to reach the ground across the
westernmost counties. Meanwhile across the southeastern part of
the area the mid and low level clouds have not yet arrived and
temperatures are in the mid to upper 30s with some valleys near
the 40 degree mark.

The scattered mainly light snow showers should continue east this
afternoon and then as cold air moves in aloft and along and just
behind the cold front an increase in intensity and coverage of the
snow showers is anticipated as it should take on more of a
convective nature. This will be arriving later in the afternoon to
the early evening and could be more impactful to travel with
reduced visibilities and have a better chance of accumulating as
temperatures drop toward sunset and after. This is mentioned in
the HWO and an SPS and Graphical nowcast may also be needed later
this afternoon.

Hourly grids have been updated to time in snow shower chances
from the west and to have pops peaking during the mid afternoon to
early evening, when snow shower intensity should peak and have a
more convective nature.

UPDATE Issued at 630 AM EST SAT DEC 9 2017

The forecast is on track so far this morning. Mid and high level
clouds will continue to stream across the area, before finally
scattering out by mid morning. Snow showers should begin moving
into the area from the west and northwest by early this afternoon.
The latest obs were ingested into the forecast grids to establish
new trends. No forecast update is planned for this morning.

&&

.SHORT TERM...(Today through Sunday)
Issued at 300 AM EST SAT DEC 9 2017

Extensive cloud cover will continue to stream into the skies over
eastern Kentucky through out the morning, as an area of low
pressure moves across the southeastern CONUS and eventually up the
east coast. The weather system of concern for our area for today
and tonight is an area of low pressure that is currently spinning
over the Great Lakes region. This fast moving system is forecast
to move southward across the Ohio Valley and central Appalachian
regions this afternoon through tonight. This little clipper system
will bring scattered snow showers to eastern Kentucky today and
early tonight. Even though surface temperatures may not always be
at or below freezing today, there will be more than enough cold
air in place above the surface to allow all snow or mostly snow to
fall. Some light accumulations will be possible and will range
from a dusting to four tenths of an inch. The highest snow fall
totals will occur on the taller ridges along the Virginia border.
Any snow accumulations will most likely be confined to grassy
areas and elevated surfaces. The snow showers should taper off
beginning late this afternoon, with snow flurries lingering once
the snow showers exit the area this evening. The snow should
finally be out of the area by early Sunday morning.

Temperatures will not make it out of the 30s today and probably
will not reach 40 for most locations on Sunday either. Another
shot of very cold air is expected to hit the region tonight,
which will allow temperatures to plummet into the upper teens and
lower 20s by early Sunday morning. Extensive cloud cover will
remain in place across the area through late tonight as well,
before giving way to partly cloudy skies on Sunday. Winds will
shift from the south to the west today, as the clipper system
mentioned earlier moves quickly across the area. In fact, winds
are forecast to increase to 10 to 15 mph sustained by mid morning
and could gust to around 20 mph at times.

.LONG TERM...(Sunday night through Friday)
Issued at 330 AM EST SAT DEC 9 2017

The models are in generally good agreement aloft through the long
term portion of the forecast as they all depict a dominant trough
over the northeastern portion of the nation. However periodic
shortwaves moving through this flow bring with them more model
discrepancies at this smaller scale. The first of these waves
will be exiting the area at the start of the extended portion of
the forecast though another one will cut through Kentucky on its
heels early Monday. A more potent impulse follows for later Monday
as it targets the lower Ohio Valley. This takes place as the
height anomaly over the Great Lakes deepens with the GFS and ECMWF
quite similar and the GEM an outlier to the south and east. By
late in the day Tuesday another stream of energy will pass over
eastern Kentucky with a significant fall in the heights throughout
the Ohio Valley into Wednesday morning. After that, the Great
Lakes trough will shift east with heights rebounding over Kentucky
into Thursday morning. However, the ECMWF does bring another wave
to the area for Thursday but there is little support from the GFS
or Canadian with this feature as the latter favor a slower and
broader trough that will not come through the area until later
that night. The differences here lowers confidence in any specific
solution and leaves much uncertainty in the forecast specifics.
This trough will shift east on Friday with heights again on the
rebound - more substantially in the ECMWF than the GFS. Given the
concerns with the smaller scale features throughout the extended
portion of the forecast the blend was a reasonable starting point
for most grids but caution was used for any specifics heavily
influenced by solely one model.

Sensible weather will feature a fairly cold period this upcoming
week along with periodic chances for light snowfall - highly
influenced by upslope flow and terrain enhancements in lieu of
decent moisture content. For Monday night and Tuesday the JKL CWA
will be targeted by these snow showers as a sfc low passes by
well to the north and a cold front presses into the area early
Tuesday with a reinforcing shot of arctic air and a prolonged
period of upslope. However, with limited moisture available the
QPF will be low for this time frame with just a series of dustings
expected from the terrain affected snow showers. Following a very
cold Wednesday morning, dry and quiet weather will follow through
mid week before the next wave brings additional pcpn chances on
Thursday into Friday with more upslope sfc winds prolonging the
light snow shower chances in the wake of this latter system`s
cold front.

Made only minor adjustments to temperatures through the period
mainly for terrain effects in the non CAA patterns of Sunday and
Wednesday nights. Made more substantial adjustments to PoPs and
snow chances through the week - beefing up the potential in our
higher terrain Monday night into Tuesday and again Thursday into
Friday.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
ISSUED AT 105 PM EST SAT DEC 9 2017

Middle and low clouds continue to spread east in advance of a cold
front with southwest or west southwest winds generally in the 10
to 15KT range with some gusts up near 20kt for most ridgetop and
western locations. Some scattered very light snow is likely
reaching the ground in the far west as well, but CIGS and VIS
remain in the VFR range. CIGS should continue to lower as the cold
front approaches consistent with upstream obs over central KY and
coverage of snow showers should also increase during the first 6
hours of the period. Some at least brief MVFR to IFR VIS and MVFR CIGS
are possible in this. Winds should shift to the west and northwest
behind the front with gusts as high as 20 to 25kt with snow
shower coverage diminishing from west to east from 0Z to 6Z.
Northern and eastern locations will also experience a few hours of
MVFR CIGS during that time. Clouds should begin to clear and CIGS
improve by the 6Z to 12Z period, giving way to VFR for all
locations by the last 6 hours of the period.

&&

.JKL WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...JP
SHORT TERM...AR
LONG TERM...GREIF
AVIATION...JP



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