Area Forecast Discussion
Issued by NWS Jackson, KY

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FXUS63 KJKL 121852

National Weather Service Jackson KY
152 PM EST Tue Dec 12 2017

Issued at 152 PM EST TUE DEC 12 2017

There are still some heavier brief bursts of snow present, but
overall, snow showers have peaked and will be on an overall
decline going into the evening. The update blends early afternoon
obs into an updated night time forecast in which clouds break up
faster in our western counties, and precip winds down faster.

UPDATE Issued at 934 AM EST TUE DEC 12 2017

Have updated temperatures and precip type for today and tonight
based on latest obs and model data. Once cloud ice production
begins, the steep low level lapse rates headed in will result in
rain vs. snow temps being fairly warm in the upper 30s today.

UPDATE Issued at 710 AM EST TUE DEC 12 2017

The snow is starting to develop over the area. Have updated the
forecast mainly to add in the latest T and Td obs/trends. Did fine
tune the PoPs and will likely wipe out the freezing pcpn with an
update before 8 am. Look for a new set of zones, updated SPS, and
HWO with that issuance. The updated grids have been sent to the
NDFD and web servers.


.SHORT TERM...(Today through Wednesday)
Issued at 305 AM EST TUE DEC 12 2017

07z sfc analysis shows a fairly strong area of low pressure moving
east through northwest Pennsylvania. A cold front from this system
is poised to enter Kentucky from the northwest. Much colder air
trails the front along with brisk northwest winds. Out ahead of
it, good mixing through the night thanks to a moderately strong
low level jet is keeping the temperatures up even in the sheltered
valleys where readings are down to the low and mid 40s. However,
a hole in the cloud cover is pushing east into eastern Kentucky
and would expect to see better cooling beneath this for the next
few hours - but the winds mixing warmer air from aloft and
locations surrounding the valleys will likely keep them well
above freezing through dawn. Otherwise, temperatures are mild
ahead of the front range from the low 50s in the far southeast to
the lower 40s in the northwest with most places in the mid to
upper 40s. Dewpoints, meanwhile are running in the upper 20s to
lower 30s across the area. Sfc winds are gusting most places, from
the southwest in the far east to more westerly in the west, at
between 20 and 25 mph.

Not much pcpn is showing up on radar ATTM even behind the front -
though some drizzle is showing up in the obs upstream. Do expect
it to start popping up in our northwest over the next couple of
hours but falling into a relatively warm boundary layer with
limited opportunity to wetbulb cooler. As such, am now looking at
more in the way of liquid drizzle/sprinkles through dawn - owing
to a lack of ice crystals in the column initially. Snow arrives
shortly after dawn most places once the column has had a chance
to cool. There also is a small potential for freezing
rain/sprinkles in the couple to few hours following sunrise as
the colder air moves in, however warm sfc temperatures - road
pavement readings are in the upper 30s to low 40s ATTM - will
limit this concern to elevated sfcs like railings, decks, and
perhaps untreated bridges/overpasses will tailor the ongoing SPS
to address these concerns initially. The snow expectations
are elaborated on further down this discussion.

The models are in excellent agreement aloft as this zero hour as
the next system moves in. Twin pol troughing will swing into the
Ohio Valley today with a lead impulse passing to our northeast by
12z. The core of the trough will then press into the Ohio Valley
through the afternoon with plenty of energy and height falls
working over eastern Kentucky. Just as quick as it arrives today
this trough will lift out to the northeast tonight with heights
rebounding in northwest flow. The flow will swiftly become more
zonal by the end of the day Wednesday with the next impulse moving
in from the west. This latter feature is similarly handled in the
GFS and ECMWF, but lead the lagging CMC. Will favor the blend
here sticking close to the majority consensus. Otherwise, have
used the higher resolution models most heavily through today for
specific forecast details with better than average confidence.

Sensible weather today will see the front and its dynamics bring
showers to the area going over to - from west to east snow during
the morning. Some instability will be available to help drive
this convection into the afternoon for our eastern zones as the
snow patches/bands scatter out. A better Great Lake moisture
fetch looks to set up just northeast of the area later this
afternoon and evening likely sparing the JKL CWA from an extended
snow threat. Have gone above the blended PoPs this morning and
into the afternoon in favor of the scenario depicted by the HRRR
and CAMs. Look for temperatures to fall through the day with some
insolation limiting accumulations compared to the last event.
Additionally, the road sfc temperatures will be much higher than
they were Saturday again minimizing icing impacts. Accordingly,
will continue to handle this event with an SPS focusing on
variable visibility concerns with any squalls given the strength
of the low level wind field. These northwest winds bring in
colder air tonight and have gone a bit lower than the blend in
our more open areas and the ridges for lows tonight. Similarly, 45
on Wednesday, kept those locations as touch cooler with variable
cloud cover between quick hitting systems/clippers keeping the
sunshine from contributing much. Winds were raised today and on
Wednesday as the blend seems to be too influenced by the low
biased URMA for situations such as these.

Look for the SPS to continue through the early evening for the CWA
with less focus on the freezing pcpn potential in favor of
highlighting the more convective snow showers later this morning
and this afternoon with even some squalls possible.

.LONG TERM...(Wednesday night through Monday)
Issued at 245 AM EST TUE DEC 12 2017

The extended will be fairly active and will feature several periods
of precipitation, as multiple weather systems move across the
eastern portion of the CONUS. The first episode of precipitation is
slated to occur Wednesday night through late Thursday morning, as a
weak area of low pressure aloft moves across the Ohio Valley. Rain
and showers will be on tap for that period, with little if any snow
accumulations expected. The second period of precipitation is
expected Thursday night through late Friday night. Temperatures
during this period should be a bit cooler, so more snow is expected
across the area. Very light snow accumulations may occur early
Friday night, as a clipper type system moves quickly through the
region. A third period of precipitation is forecast for Sunday
through Monday. This precip will be associated with a larger scale
and more potent area of low pressure that is progged to move out of
the southern Plains and eventually across the Tennessee and Ohio
Valley regions. Temperatures should be warm enough during the
passage of this system for mostly rain to occur across the area.
Some snow showers may mix with rain Sunday night into early Monday
morning, as some colder air filters into the region. If any snow
accumulations do occur Sunday night, they will be very light and
likely confined to grassy areas and elevated surfaces.

Temperatures during the extended will vary quite a bit, with some
days featuring highs in the 30s and 40s, and other days with highs
in the 40s and 50s. Nightly lows will not be quite as varied, with
min values in the 20s and 30s expected.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)

Scattered snow showers were ongoing at the start of the period,
and were most concentrated over the eastern part of the area.
Showers will show an overall decline as time goes by. Aside from
the more significant snow showers, conditions were mainly VFR.
Localized IFR was likely present in the heavier showers. Once the
last snow showers dissipate over the eastern part of the area this
evening, VFR conditions are expected through the remainder of the
period. Gusty west to northwest winds of this afternoon will
gradually diminish through the evening.




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