Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 112022
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
322 PM EST Mon Dec 11 2017

.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
made.

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...(18Z TAF Issuance)
Issued at 1210 PM EST Mon Dec 11 2017

Main forecast concern in the aviation forecasts will be wind and
some precipitation Tuesday morning.

A low pressure system will move through the Great Lakes and swing a
cold front through the Ohio Valley this evening.  Winds out of the
southwest will increase this afternoon and evening.  The front will
cross the region tonight and switch winds to the northwest.  Winds
will remain quite gusty with sustained winds of 13-15kt and gusts to
25kts at times.

Late tonight, probably after 12/08Z, some light snow showers will
push into the region from the northwest.  Overall chance of
measurable snow looks rather low, but have maintained a period of
VCSH at KSDF and KLEX.  Precipitation type remains a question.
Initially light snow should be the main precipitation type at onset.
However, significant drying in the DGZ may result in freezing
drizzle becoming a predominant precip type after 12/12Z.  So this
will be something that we`ll watch closely this afternoon and
evening.

Ceilings early in the period will remain VFR along with
visibilities.  Ceilings are likely to drop to MVFR at all the TAF
sites after 12/08Z.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...None.
KY...None.
&&

$$

Short Term...DM
Long Term...DM
Aviation...MJ


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