Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 160534
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1234 AM EST Tue Jan 16 2018

...Updated Aviation Discussion...

.Forecast Update...
Updated 1148 PM EST Mon Jan 15 2018

...Secondary Moderate/Heavy Snow Axis Beginning to Form...

We`re seeing a transition in heavy snow axes at this time as
frontogenetical forcing strengthens with the right entrance region
of the 130 knot upper jet. As the jet continues to pivot around to a
more SW to NE orientation, south central and eastern KY look to sit
under the resulting frontogenesis well into the day on Tuesday. The
lift, combined with saturation through the DGZ, should continue to
pose a threat for bands of moderate to briefly heavy snow. The
biggest question is exactly where to place that band. At this point,
it looks like it will extend somewhere around a Bowling Green to
Lexington line. Plan on upping snow totals along and southeast of
that line, but want to wait a bit longer to see exactly where the
band sets up over the next hour or two. With an increase in totals,
suspect we may have to upgrade the Winter Storm Warning further
east. We`re likely seeing at least 20:1 snow ratios given good
dendritic growth and the arctic boundary undercutting at the
surface. With expected QPF at least another .1 to .2", a good 2 to 4
inches is probably still possible on top of what has already fallen.

Will be making some decisions about what to do across the central,
southern, and eastern portions of the CWA over the next 1 to 2
hours, and will be letting some of the northern counties fall out of
the Winter Weather Advisory shortly.

Updated 1003 PM EST Mon Jan 15 2018

Based on the latest radar/observation/model data, have decided to
upgrade a portion of our western CWA to a winter storm warning.
We`ve received several reports so far of +4" of snow within some of
the narrower bands of moderate to heavy snowfall, and the latest
suite of 00z models indicate that more of our region could see +4"
snowfall.

The NAM/GFS/RAP/HRRR indicate a weakening of 700mb/600mb
frontogenetic forcing over the next 2-3 hours before ramping it up
again after 06z over eastern/southern portions of the CWA.
Theoretically, we should see radar reflectivities come down some
during this timeframe and banding becoming more disorganized, with
intensification of reflectivity after 06z and more organized banding
structure. Right now the winter storm warning area highlights our
best guess for where the potential banding will set up, though it
may have to be expanded if this system continues to over-achieve.

Updated 725 PM EST Mon Jan 15 2018

Went ahead and extended the Winter Wx Advy for all of south central
Indiana through 6Z tonight.  A moderate snow band was still over
portions of southern Indiana/northern Kentucky near the Ohio River
with more light bands downstream over southwest Indiana.  High-res
guidance indicates these bands will persist over southern
Indiana/northern Kentucky through much of the evening hours
providing up to an additional two inches of snow and locally higher
amounts especially closer to the Ohio River.

Elsewhere the Winter Wx Advy looks good and the Wind Chill Advy is
on track for later tonight.  May need to consider a second Winter Wx
Advy extension in time for central KY later tonight based on some of
the evening guidance.

&&

.Short Term...(This evening through Tuesday night)
Issued at 259 AM EST Mon Jan 15 2018

...Snow Will Continue Into Tuesday Morning...

Light to moderate snow will continue to fill in this afternoon and
early evening from west to east. Some portions of southern IN have
already received up to 2 inches of additional snow today. Snow is
beginning to become lighter across portions of southern IN where it
began earliest today. However, some light snow will linger there
through the next few hours.

The Winter Weather Advisory is now in effect for the entire region,
though some areas in east and south central KY may not see snow
starting for another 2-3 hours. Radar returns over these areas
currently are likely not reaching the ground as there is still a
significant temp/dewpoint spread. Snow will begin this afternoon as
the column saturates. Across the rest of the region, there has been
a weakening trend with the snow over the last couple of hours as was
mentioned in the morning discussion. However, as the upper level jet
pivots across the lower Ohio Valley, we should be more in the right
entrance region. This will help to enhance the snow once again this
evening across central KY. Snowfall totals through the event are
still forecast to be generally 1-2 inches across the region with
localized 3 inch amounts. No changes are planned at this time to the
Winter Weather Advisory.

Temperatures will fall quite quickly tonight. Lows will range from
near zero in portions of southern IN to the teens across east
central KY. With winds staying in the 7-10 mph range, this will lead
to very cold wind chills tomorrow morning, ranging from the single
digits above zero to -12. The coldest wind chills will be along and
north of the Ohio River. High temperatures Tuesday will be quite
cold, topping out in the teens across the region.

For tomorrow afternoon into tomorrow evening, lower level moisture
will stream in from the north. Soundings suggest some flurries will
develop in the afternoon and continue into the evening and overnight
hours. Generally, not much accumulation is expected with these
flurries but isolated spots may see a dusting.

.Long Term...(Wednesday through Monday)
Issued at 312 PM EST Mon Jan 15 2018

The upper trough will be shifting east of the area on Wednesday and
as it does so, flurries should come to an end. As we head into the
second half of the work week, high pressure will build in from the
west. Aloft the flow will become more zonal in nature. This will
lead to temperatures warming through the end of the week. Wednesday
and Thursday mornings will again be very cold with low temps in the
single digits. However, winds will be lighter so wind chill will not
be much of a factor. By Friday morning we should be back into the
upper teens to lower 20s for lows. Highs will be back to near normal
on Friday.

As we head into the weekend ridging will build aloft as a low
pressure system develops to the west. Return southerly flow will
usher in more humid and warmer air. Saturday some warm advection
showers will develop across the region. Sunday into Monday the low
pressure system will track northeast across the Midwest into the
Great Lakes region with a cold front approaching the forecast area.
More widespread showers with potentially even a few thunderstorms
are expected Sunday into Monday. Temperatures will be much warmer
into this weekend with highs on Sunday even approaching 60 in some
places.

&&

.Aviation...(06Z TAF Issuance)
Issued at 1230 AM EST Tue Jan 16 2018

Have a couple of bands of snow across the region, one now impacting
a SDF/CVG line and a broader one over/just north of BWG. The first
band should weaken over the next hour, but the second one looks to
pivot somewhere near Etown on its way to LEX later this morning. IFR
conditions are possible within this band, with perhaps near moderate
snow conditions, LIFR conditions in brief periods. Deeper moisture
will leave the region by late morning, so have improving conditions.
One thing to watch for late in the period...some models are showing
a cold deck of clouds moving into HNB and late into SDF where some
light snow could fall again. For now just have the clouds at HNB.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
IN...Winter Weather Advisory until 1 AM EST early this morning for
     INZ076>079-083.

     Wind Chill Advisory until noon EST /11 AM CST/ today for
     INZ076>079-083-084-089>092.

     Winter Weather Advisory until 10 AM EST this morning for INZ091-
     092.

     Winter Storm Warning until 1 PM EST /noon CST/ this afternoon
     for INZ084-089-090.

KY...Wind Chill Advisory until 11 AM CST Tuesday for KYZ023.

     Winter Weather Advisory until 1 PM EST /noon CST/ this afternoon
     for KYZ036-037-040>043-046>049-054>057-064>067-073>078-081-
     082.

     Winter Weather Advisory until 10 AM EST this morning for
     KYZ029>035-038-039-045.

     Winter Storm Warning until 1 PM EST /noon CST/ this afternoon
     for KYZ023>028-053-061>063-070>072.

&&

$$

Update...AMS/BJS/DM
Short Term...EER
Long Term...EER
Aviation...RJS


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