Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 212245

Area Forecast Discussion
National Weather Service Louisville KY
645 PM EDT Wed Mar 21 2018

.Forecast Update...
Issued at 616 PM EDT Wed Mar 21 2018

Have canceled the western portion of the Winter Weather Advisory,
including for southern Indiana and the Louisville metro. Sun is
quickly making an appearance as low-level moisture is scoured out
from west to east. Have left the advisory up east of I-65, but the
lingering snow bands are relatively light. Most notable area at this
point may be Casey Co. Nearby observations at Somerset and
Monticello both featured 4 mile visibility.

Of bigger concern now is the formation of black ice tonight.
Temperatures have risen into the 40s under sunshine across the
western half of the area. As of this writing, SDF is up to 39
degrees. As mentioned in the short term discussion, clearing skies
and light winds will allow temperatures to plummet tonight. Ongoing
snow melt on any road surfaces will refreeze overnight. Expect roads
to be slick in some areas late tonight and through the Thursday
morning commute. Plan on replacing the Winter Weather Advisory with
a Special Weather Statement to highlight this risk.


.Short Term...(This evening through Thursday night)
Issued at 308 PM EDT Wed Mar 21 2018

...Black Ice and Slick Roads Possible Tonight and Thursday AM...

As of mid afternoon, surface analysis showed the storm system
responsible for the snow in the past 24 hours now has moved to the
mid-Atlantic. High pressure was centered over western Missouri.
Visible satellite imagery showed mostly cloudy to overcast skies
along/east of I-65, and mid afternoon readings were in the low/mid
30s under clouds to around 40 west of I-65.

Main forecast concern in the near-term is temperatures tonight, sky
trends, and impacts to travel due to refreezing and black ice.

Good consensus in the hi-res models that the low level moisture
still pivoting across southern Indiana and central Kentucky will
erode and drift south/east through this evening. As the low/mid
level ridge axis works east tonight, this should result in mostly
clear to clear skies. With light/variable winds expected, the
radiational cooling setup will allow temperatures to drop quickly
after sunset into the 20s, coldest where the snow pack is deepest.
Area storm reports and analysis shows southern Indiana and north
central Kentucky, around the Louisville metro, with the highest
amounts. Some rural and sheltered colder spots may dip into the
upper teens.

Any melting that occurs this afternoon due to solar insolation or
temperatures above freezing will put moisture back on the roads.
Once temperatures fall into the 20s, this could refreeze and create
problems on roadways through the Thursday morning commute. Any
secondary, elevated, or untreated roads will be slick tomorrow
morning. Motorists should use caution. Will address this with a
Special Weather Statement and through our other messaging outlets.

Temperatures are expected to climb above freezing mid to late
morning and should rise well into the 40s across the entire area.
Some 50 degree readings possible.

A weak weather system will also approach the area Thursday night
into Friday, bringing a slight chance of rain showers to west
central Kentucky.

.Long Term...(Friday through Wednesday)
Issued at 310 PM EDT Wed Mar 21 2018

...Active Weather This Weekend...

The main focus in the long term period is on this weekend`s strong
storm system that could bring heavy rain and thunderstorms, but also
the potential for winter weather.

21.12z guidance in reasonable agreement showing an area of low
pressure racing across the central Plains through Ohio Valley. The
eventual track of the low will dictate the temperature gradient that
will likely setup across Indiana and Kentucky. A large shield of
precipitation is expected to move in Friday night and Saturday,
especially across southern Indiana and north central Kentucky. A
very tight temperature gradient will exist across the area, where
portions of southeast Indiana and far northern Kentucky could start
off as rain/snow/wintry mix before changing over to all rain in the
afternoon. The colder GFS soundings suggest a snow / wintry mix at
the onset while the NAM, is much warmer and would be mostly rain.
Across south central Kentucky, there may be enough instability for a
few thunderstorms. Will have to watch this system closely to see if
colder air is in place for more of a winter weather threat.

Regardless, this system comes very moisture-rich with PWATs climbing
over 1 inch. Rain could be heavy at times Saturday and total
rainfall forecast Friday night - Saturday night is 1 to 2 inches.

Consensus is that Sunday will be a dry, in-between kind of day
before southwest flow aloft sets up across the central Plains. This
set up would open up the Gulf of Mexico moisture plume and with
multiple weather systems lifting through, could spell a very active
period of weather next week across the central Plains, mid-MS, TN,
and OH Valleys. Given the wide variety of potential timing issues
and location of weather systems, kept precipitation chances 50-80
percent until confidence increases.


.Aviation...(00Z TAF Issuance)
Issued at 645 PM EDT Wed Mar 21 2018

A long north-south ridge of high pressure will keep skies mostly
clear (once the low clouds leave LEX in the first hour of the TAF)
through tomorrow. Winds will drop off quickly this evening and be
light overnight. They will then pick back up a bit with the daylight
tomorrow but remain only in the 5 to 10 knot range out of the
northwest as that surface high continues to move in.


KY...Winter Weather Advisory until 8 PM EDT /7 PM CDT/ this evening
     for KYZ032>043-045>049-054>057-064>067-076>078-081-082.



Short Term...ZT
Long Term...ZT
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