Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 120527

Area Forecast Discussion
National Weather Service Louisville KY
1227 AM EST Tue Dec 12 2017

...Updated Aviation Discussion...

.Forecast Update...
Issued at 849 PM EST Mon Dec 11 2017

...Wintry Precipitation Possible Tuesday Morning During Rush Hour...

Lots to consider over the next 12 hours...

Made some tweaks to the current T/Td/Wind grids this evening to
bring values in line with the latest obs. We`ve stayed pretty mixy
ahead of the strong cold front that is now entering the CWA, and
therefore temperatures were running warmer than anticipated. Expect
these to start falling off steadily from here on out as the front
pushes through.

Of bigger concern, wind gusts have been stronger than expected to
this point, both upstream and already at a few ob sites in our CWA.
So, beefed up the wind gusts to the 25 to 35 mph range. A little
concerned that gusts could approach advisory criteria at times, but
should fall short. Will keep an eye on things, and likely add some
wind gust wording to the ongoing Special Weather Statement and HWO.

As far as the overnight and Tuesday morning precipitation goes, not
much has changed from the previous thinking. Still think the best
chance for a coating to a half an inch of snow will be in our
Bluegrass region where moisture is slightly deeper and saturated up
to around -12 to -15 C for a period. In conjunction, steep low level
lapse rates will give each snow shower a convective and gusty
component which could cause some minor impacts on visibility/
roadways. Further west, coverage and intensity of any snow shower is
not expected to be as great as moisture is not as deep and only
saturated to around -10 C at the peak. Still, there could be some
minor impacts to the morning commute with any snow shower.

Still a bit concerned about an initial freezing drizzle component as
saturation only occurs to about -5 or -6 C at onset. The lack of
available ice crystals in this regime could lead to patches of -fzdz
before transition to snow in the deeper moisture. At this point,
don`t expect much if any impacts from any freezing drizzle as most
road surfaces warmed well above freezing based on observed road

Wanted to conclude by mentioning that overall confidence in any
impacts in the morning is lower than normal. We have some pretty dry
air in place ahead of the moisture, and there is some question as to
how much precip potential will be lost to saturating the column.
Plus, very gusty winds sometimes make it hard for light snow to
accumulate on flat surfaces, so any snow that does fall could mainly
collect on roadway edges. Too many factors working against impacts
in the morning to have higher confidence. Will continue with the SPS
in our eastern counties where the best potential for impactful
weather is located, because if precipitation is reaching the ground
at least a few slick spots are possible.


.Short Term...(This evening through Tuesday night)
Issued at 300 PM EST Mon Dec 11 2017

...Wintry Precipitation Possible Tuesday Morning During Rush Hour...

Current surface analysis shows an area of low pressure near the
WI/IA/IL border with a strong trailing cold front. A warm front
extending ahead of the low has pushed past the lower Ohio Valley,
and allowed temperatures to warm up into the mid 50s in south
central KY to upper 40s in southern Indiana. Winds have been a
little breezy out of the southwest at times due to the boundary
layer mixing that has occurred.

The low pressure system and cold front will approach the region this
evening, with the cold front clearing the CWA shortly after
midnight. Northwesterly winds will pick up behind the cold front due
to strong CAA and pressure rises. Mid/low level moisture will also
increase behind the front, though the best moisture will probably be
east of the CWA due to the orientation of N/NW flow off the Great
Lakes. Most model guidance, including ensembles, paint a broad swath
0.01" to 0.05" QPF in southern Indiana and northern/eastern Kentucky
between 3am - 1pm (EST) Tuesday. South-central Kentucky may see some
precipitation develop, but moisture profiles are less favorable and
accumulating precipitation seems unlikely. Best chances for
accumulating precipitation still looks to be in the Kentucky
Bluegrass region, but again, overall amounts will be very light.

Precipitation type with this setup continues to be tricky. Model
soundings are not showing much in the way of saturation in the DGZ
to start the event (3am - 8am). The coolest temperatures aloft
where good saturation occurs are about -8 C or warmer, and based
on past research, formation of ice crystals would be limited in
the clouds. This would mean the clouds would be composed mostly of
supercooled liquid droplets, and drizzle and perhaps some snow
grains would be the predominant p-type. Surface temperatures will
be hovering near the freezing mark for many locations, so if any
drizzle were to fall from the clouds, it could potentially freeze
on contact (assuming its surface is at or below freezing). Given
how warm it has been today, it may take some time before the
ground, roads and other surfaces drop to a freezing temperature. A
quick look at some of the road temperature sensors in the region
display values in the upper 40s to low 50s, and these will take
some time to cool. Elevated surfaces and bridges would be the
first to see any effects since they`ll respond quickest to
temperature changes.

Cooler air will continue to advect in aloft through sunrise, at
which point temperatures aloft where the best moisture resides will
begin to support the transition over to snow showers as more ice
crystals develop in the clouds. These snow showers will continue
through about noon, with the greatest chance of accumulating
snowfall occurring in the Kentucky Bluegrass region. Overall amounts
will remain light, but a few areas may be able to pick up a half
inch before all is said and done.

After coordinating with the Jackson (KY) and Wilmington (OH) office,
have decided to highlight the threat in an SPS rather than an
advisory. Overall amounts will remain below advisory criteria, and
while the "bulk" of it will be occurring during the morning rush-
hour, think travel impacts should be limited to untreated
bridges/overpasses given warm road/ground temperatures. Subsequent
forecast updates will continue to monitor model trends and surface
temperatures in the event things come in cooler and wetter.

The other story Tuesday will be the cold and wind. Temperatures
during the day will struggle to rise, and high temperatures may
occur early in the morning. Winds will be blustery out of the
northwest, with gusts nearing 20kts. Wind chills during the morning
will be in the low 20s, but transition into the teens for many
locations by the afternoon. The colder air will continue to filter
in tomorrow evening, and overnight lows will be very chilly, with
widespread teens and low 20s expected.

.Long Term...(Wednesday through Monday)
Issued at 310 PM EST Mon Dec 11 2017

The main theme for the long term period is for a couple more clipper
systems through the end of the week followed by a warming trend into
the weekend.

The next clipper looks to arrive Wednesday night into Thursday.
General consensus is that most of the moisture associated with the
clipper will pass north of the region. This has been fairly
consistent with previous forecasts, so no major changes have been

A shortwave trough will drop out of the Saskatchewan Thursday and
dig into the northern Plains/Midwest. Energy from this system may
begin to phase with a northern stream wave on Friday, though it
appears that by the time it does so, it will be well east of the
region. Still, we may see some light precipitation develop Friday,
but temperature profiles could be a close call between it being rain
or snow.

We`ll transition to zonal flow by the weekend. Warmer air will
advect in from the southwest, providing a break from the cold we`ve
had recently. A cold front may approach or pass the region late
Sunday, but timing in long range models is a bit inconsistent at
this time.


.Aviation...(06Z TAF Issuance)
Issued at 1225 AM EST Tue Dec 12 2017

The cold front will continue to move through the region tonight.
Winds currently out of the W to WNW will shift to more northwesterly
over the next few hours. Gusts are expected to continue through the
overnight hours and into the day. Gusts of 20 to 30 knots can be
expected today, diminishing around sunset this evening.

Ceilings will lower to MVFR as the low levels saturate. Guidance
does show ceilings lowering to fuel alternate levels tonight.
Upstream obs also show a large swath of ceilings below 2 kft, so
have gone a bit more pessimistic with ceilings early this morning
for this forecast. Clouds should start to lift around or in the few
hours after sunrise and break up by late afternoon.

As far as precipitation goes, LEX still looks to stand the best
chance for seeing any light snow this morning with less confidence
at SDF. Will continue to carry -SHSN for LEX and VCSH for SDF. There
still could be a brief chance for freezing drizzle before the snow,
but with confidence rather low in any precipitation actually moving
over a terminal at this time, will keep it out of the TAF.




Short Term...DM
Long Term...DM
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