Area Forecast Discussion
Issued by NWS Jackson, KY

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000
FXUS63 KJKL 120327
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
1027 PM EST Mon Dec 11 2017

.UPDATE...
Issued at 1027 PM EST MON DEC 11 2017

The latest surface analysis shows a low pressure now across
northern Ohio and a cold front extending southward from that. This
dry cold front is moving across central Kentucky and leading to
gusty winds along and behind the front. Given this have updated
winds and gusts to better deal with the trends. This has also
further complicated hourly temperature forecast with mixing
leading to a 15 degree jump at Quicksand Kentucky Mesonet site.
There remains some question on the degree of mixing that will
remain in the sheltered valleys spots, as an area of clearing
pushes toward the area. Given the uncertainty related to how the
temperatures will react plan on keeping the isolated spots of
patchy freezing rain going towards dawn. Despite this thinking the
overall trend would be for little impacts in relation to the
freezing rain based on road temperatures. Otherwise this is more
of a minor update to deal with the latest obs and trends.

UPDATE Issued at 750 PM EST MON DEC 11 2017

The evening surface analysis shows a double barrel low pressure
across the Great Lakes and Ohio Valley regions, with one across
the UP of Michigan and the other closer to home across central
Illinois. These lows are associated with two different upper level
shortwave that will help drive the weather over the next 24 hours
or so. The initial struggle in the short term will be
temperatures, with some valleys seeing quick drop offs this
evening. However, satellite and obs have indicated an area of mid
to high level clouds are streaming across central and eastern
Kentucky and subsequently the temperatures have leveled off. This
leads to the next challenge this evening, there is a some
clearing on the backside of this initial cloud deck before lower
clouds move eastward behind the clearing. This will be critical
for potential wintry precipitation toward dawn, but will wait see
how temperatures trend through the evening for any required
changes. At this point will only need to make minor changes with
SKY grids for potential clearing. Also updated with latest obs and
trends for temps/dews/winds.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 415 PM EST MON DEC 11 2017

The main forecast concern remains the light wintry precipitation
potential late tonight and on Tuesday along and behind a cold
front associated with a clipper system to track to the north of
the OH River tonight across the southern Great Lakes reaching
western NY by sunrise on Tuesday.

Temperatures on precipitation arrival in most locations will
likely be near freezing if not above, with many locations above
freezing for the entire night. This is following above normal
highs mainly in the 50s today across the entire region. The
exceptions to this will be elevations above 3000 feet elevation
near the VA border which should fall below freezing late tonight
and some of the deeper valleys that may fall below freezing around
or prior to midnight before temperatures possible rise to near if
not above freezing before dawn as clouds arrive and pressure
gradient and winds increase and the boundary layer becomes more
mixed.

Also at precipitation arrival/development, the clouds may contain
supercooled water and little if any ice for the first hour or two
late tonight particularly with southwest extent toward the TN
border. This makes the initial precipitation type uncertain
between needle or small flakes, drizzle or even a touch of
freezing drizzle on some elevated surfaces in the highest
elevations and perhaps a few normally colder valley locations at
onset. Marginal to warm road and ground surfaces anticipated on
onset also makes the extent of impacts uncertain. As it stands
right now, most of the scattered precipitation that does fall for
the bulk of the area should fall between 5 AM and 11 AM or during
parts of the morning commute with snow showers and flurries
lingering in the southern and easternmost locations into Tuesday
evening. The model consensus is that any precipitation from about
8 AM on should be all snow.

QPF for the event will be light and some locations may not
receive measurable precipitation at all. However, given the
convective nature of the event with falling temperatures aloft
steepening up lapse rates, feel that MOS guidance and model blend
pops were underdone for much of the central and western parts of
the area and that good chance (50 percent) to likely pops (60
percent) were in order for this event. At this point, it appears
that most locations should receive a dusting to a half of an inch
of snow accumulation due to much warmer ground going into this
event compared to the last one. A light glaze of ice could occur
at onset one the highest elevations and few of the coldest
valleys. In addition to the falling snow, winds should gust as
high as 20 to 30 mph at times late tonight and on Tuesday.

The temperature curve for tonight will be complex as already noted
with most locations dropping into the low to mid 30s by 9 AM EST.
Otherwise, temperatures should gradually fall on Tuesday with cold
advection continuing. The coldest of the airmass should be over
the area on Tuesday night with widespread lows in the teens. The
top of Black Mtn should drop into the upper single digits above
zero.

.LONG TERM...(Wednesday through Monday)
Issued at 406 PM EST MON DEC 11 2017

There are multiple systems to contend with, including rain and snow,
and model discrepancies ballooning late in the period. Forecast
confidence takes a hit as time goes by.

Wednesday should be a quiet day, before a clipper type system is
expected to pass by to our north on Wednesday night. Any precip of
substance is expected to be to our north on Wednesday night into
Thursday, with just a potential for flurries over the northeast part
of the JKL forecast area.

After a break in the weather Thursday afternoon into Thursday night,
an upper level impulse rotates through the persistent eastern CONUS
trough late Thursday night and Friday. The GFS is more aggressive
with precip than the ECMWF, but still light. A compromise yields
chance POPs. Any precip would probably be snow.

After another quiet day on Saturday, the final system affects the
area on Sunday. The GFS has a low track through Kentucky and taps
into southern stream moisture, resulting in generous precip in our
area. Temperatures support primarily rain. Meanwhile, the ECMWF low
track is north of the Ohio River and the system does not tap into as
much moisture, and it`s precip for our area is much lighter. For
now, have used high chance POPs Sunday into Sunday night. Will
expect changes to be necessary as models eventually converge on a
solution.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
ISSUED AT 622 PM EST MON DEC 11 2017

An area of mid to high clouds continues move east across western
and central Kentucky and starting to move into the Bluegrass this
evening. Overall VFR mid to high clouds will remain the story till
we move toward 8Z to 10Z timeframe when we will see CIGS begin to
lower toward MVFR conditions. A clipper system will work into the
area toward dawn and the onset of precipitation will be tricky,
with the potential of early drizzle and/or freezing drizzle. The
best shot of freezing drizzle will be in the shelter valley
locations that manage to go below freezing before thinker cloud
cover move in tonight. This clipper precipitation will
transition to all snow through the morning into the afternoon as
temperatures drop throughout the day. Some of the more robust
snow showers coupled with gusty winds will lead to IFR or lower
VIS at times.

The winds tonight will remain light to begin the TAF period
before becoming gusty toward the 8Z to 10Z timeframe. Some
locations will see gusts of 20 to 25 knots at times Tuesday
morning into Tuesday afternoon. Winds will begin out of the south
and southwest and veer to west and northwest through the period.
Also added a period of LLWS early tonight, as the low level jet
increases before surface winds.

&&

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

$$

UPDATE...DJ
SHORT TERM...JP
LONG TERM...HAL
AVIATION...DJ


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