Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 082342
AFDJKL

AREA FORECAST DISCUSSION
National Weather Service Jackson KY
642 PM EST Fri Dec 8 2017

.UPDATE...
Issued at 642 PM EST FRI DEC 8 2017

The evening surface analysis is showing a small influence from
surface high nosing northward. Otherwise we remain stuck between
two systems this evening. This keeps us mostly driven by the
strong jet energy aloft and disturbance to the east that continue
to render mid to upper level clouds across eastern Kentucky. This
being to case did increase the overall cloud coverage for portions
of the night. There remains some concern on weaker moisture
making it into the far southeast tonight, however, the short term
models are overdone based on current trends. These models have
been most robust with the QPF and given this confidence remains
low this evening for any snow. Therefore, did lower POPs and QPF
in the southeast and this subsequently leads to lower snowfall
potential. The cloud cover also leads to less confidence in the
low temperatures tonight and did lean slightly higher based on
that idea. Otherwise loaded the grids with latest obs and trends.

&&

.SHORT TERM...(This evening through Saturday night)
Issued at 247 PM EST FRI DEC 8 2017

We have two weather systems we are monitoring this afternoon. The
first one is pushing northeast across the southeastern states with
a large plume of moisture extending across the southern
Appalachians. The second system is diving southeastward across the
far western great lakes. Presently, eastern Kentucky is sandwiched
between these two systems, with nothing more than a bit of mid to
high level cloud cover. This will change for a few folks tonight
as moisture associated with the system to our south brushes the
Kentucky counties bordering Virginia. RAP/HRRR have been wavering
around with the northward extent of precipitation over the past 3
or 4 runs, but its looking more likely that some light snow will
push into our counties bordering Virginia late this evening or
overnight. RAP/HRRR/GFS all indicate about 0.05 to 0.10 inch of
liquid equivalent tonight, which would support around an inch of
snowfall with a snow ratio near 15:1. The highest totals will
likely reside in the highest terrain, with drier air eating away
at totals at lower elevations. For all other locations, expect
skies to gradually clear overnight. The clearing skies and
expected light winds will allow for temperatures to plummet
tonight. Lows will fall well into the teens in many areas. The
exception will be in the far southeast where precipitation and
clouds keep temperatures in the 20s.

Any snowfall will end around daybreak Saturday with a mostly sunny
start expected. However, the second system will roll into eastern
Kentucky from the northwest by midday or early afternoon. This
will bring an increase in cloud cover and increased chances for
snow showers. Low level lapse rates will steepen up quite a bit
through the early evening hours, allowing for a few intense snow
showers that could lay down a quick coating of snow. This is
despite highs expected to reach into the mid 30s. We will likely
see temperature falls within the snow showers. However, given the
air temperatures near or above freezing, travel conditions should
not be all that impacted. Expected liquid equivalent is only going
to be a few hundredths of an inch for most, so this won`t support
that much snow. Perhaps up to a half inch in favored westerly
upslope regions. A few snow showers or flurries may linger into
Saturday evening before coming to an end. Most of the moisture
with this system will fall in the dendritic growth zone, so we
could see some decent dendrites with the snow on Saturday. This
could aid in some instances of reduced visibilities, impacting
travel in the heavier snow showers.

Temperatures Saturday night will drop back into the teens again
with another strong push of cold air moving in behind the clipper
system.

.LONG TERM...(Sunday through Friday)
Issued at 410 PM EST FRI DEC 8 2017

The long term period is expected to begin with a western Conus
ridge and an eastern Conus/eastern North American trough. The axis
of this trough is expected to have moved east of the area by the
start of the period or shortly thereafter. At that point, surface
high pressure is expected to be centered over the Southern Plains
and southeastern states. The clipper system/shortwave that is
expected to affect the region in the near term will have moved
east of the area with another clipper system working through the
Great Lakes at that point.

Height rises are generally anticipated from Sunday into Monday
with the eastern Conus trough becoming more broad in general and
the airmass modifying. In fact, 850 mb temperature are expected to
recover from near -9C or -10C to start the period to around 3C or
so by Monday evening with southerly flow between high pressure
departing to the south and east and the next clipper system
dropping into the MS Valley on Monday.

Like the previous clipper system, the one for Monday night through
Tuesday is again expected to track north of the area a track
typical of minimal wintry precipitation. The cold front
associated with this system should move across the area on Monday
night with a few snow showers and flurries with additional
coverage of snow showers possible on west to northwest flow and
increased instability due to solar insolation on Tuesday. At this
point, isolated to scattered coverage of this is expected with any
accumulations mainly above 2000 feet and limited to an inch or
less as vertical motions should peak below the dendritic growth
zone with limited moisture. The coldest of the airmass should move
across the area on Tuesday night with 850 mb temperatures
bottoming out near -15C.

The period from late Tuesday night through Wednesday night appears
to be in between clipper systems at this time, with cold high
temperatures and a fair amount of mid and high level clouds.
There is uncertainty regarding the track and timing of the last
clipper system of the extended period which should arrive late
Thursday or Thursday night with light precipitation chances
lingering into Friday. What falls, if anything with this system
should primarily be snow.

After a cold start, highs should average 10 to 15 degrees below
normal on Sunday. Clouds and some mixiness due to a moderate
pressure gradient should keep temperatures from bottoming out on
Sunday night. Highs should moderate to near normal levels for
Monday. As colder air moves in, daytime highs may struggle to
climb above the mid 30s on Tuesday in the cold advection pattern.
With the heart of the cold airmass in place for much of the night
on Tuesday night, widespread teens are expected for lows.
The cold airmass will still be in place for Wednesday with many
locations likely struggling to climb above freezing. Temperatures
will likely moderate ahead of the the last clipper system of the
long term period on Thursday to around 5 degrees below normal,
with similar highs are expected on Friday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
ISSUED AT 622 PM EST FRI DEC 8 2017

We continue to see mid and upper level clouds from strong upper
level jet and disturbances aloft. The good news is we remain VFR
and this will be the story for the TAF sites through much of the
period. There is a chance of seeing moisture sneak into our far
southeast tonight which could lead to light snow. While this
could lead to localized issues, the models are not handling this
very well based on current observations leading to less
confidence. Another upper level disturbance drops southeast
Saturday afternoon, and we will see snow showers develop and move
across the area. This could lead to localized drops in CIGs and
VIS, but kept the TAF sites VCSH for now. Given some influence by
nosing high pressure think the winds will remain calm tonight, and
then winds will increase through the day Saturday out of the west
at 10 knots with gusts of 15 to 20 knots.

&&

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

$$

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



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