Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 161123
AFDLMK

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

...Updated Aviation Discussion...

.Forecast Update...
Updated 521 AM EST Tue Jan 16 2018

...Winter Storm Warning Expanded Thru Louisville Metro...

Fleeting enhancements of moderate snow have continued over the
Bullitt, Jefferson, Oldham, Nelson, Spencer, Shelby, and Henry
county region for some time, and with snow totals approaching 4
inches we decided to expand the Winter Storm Warning into that area
at 4 AM. Recently received a report of 4.5 inches of new snow in
eastern Jefferson county, and we have measured 4.3 inches over the
last 12 hours here at the office. Snow should begin to taper off
around or just after daybreak with totals ranging between 3 and 5
inches in the new warning area.

Also wanted to mention that we re-issued all of the headlines to
simplify/consolidate the groups so that the overall WSW is much
easier to read.

The rest of the headlines and forecast are on track. Refer to the
discussion below for more details.

&&

.Short Term...(Today through Wednesday)
Issued at 316 AM EST Tue Jan 16 2018

...Snow Accumulations Continue Mainly Across Central KY Today...

The main focus for frontogenetical forcing has now shifted solidly
to southern and eastern portions of the CWA as the right entrance
region of a 130 knot upper level jet churns over the area. The
associated SW to NE oriented moderate band of snowfall will very
slowly make eastward progress through the rest of the morning,
diminishing and pushing east of the area by mid to late afternoon.
At the surface, the Arctic boundary has undercut the upper level jet
dynamics, helping to crash temperatures, introduce dangerous wind
chills, and increase snow ratios. Will try to summarize each
geographic region below, but keep in mind the lack of reports this
time of night is making it a little difficult to determine how much
has already fallen in some spots. So, storm totals are a bit tough
to gauge.

...South and East of a Bowling Green to Lexington Line...

We had a 20:1 ratio here at the office, so another .1 to .15 of QPF
should yield an additional 2 to 3 inches roughly along and east of a
Bowling Green to Lexington, KY line. Can`t rule out isolated 4 inch
amounts somewhere just south of the line mentioned above. Overall,
this should yield storm totals of 2 to 4 inches in areas outside of
the current Winter Storm Warning so plan to keep the advisory for
now. If an area does see persistent snow this morning, we may still
have to upgrade. Still expecting another 1 to 2 inches in the
current Winter Storm Warning which should put totals well above
warning criteria, so need no changes there either.

...Southern Indiana...

Snow has either ended or is very light at this point, so expect
little to no additional accumulation. Will consider trimming a few
counties off of the current Winter Storm Warning and Winter Weather
Advisory in the near future.

...Advisory Counties in north central and the Kentucky Bluegrass...

Light snow will persist through daybreak before tapering around
midday. Additional amounts of 1 to 3 inches are possible which
should put most storm totals in the 2 to 4 inch range. No plans to
change the headlines at this time.

...Wind Chill Advisory...

The Wind Chill Advisory still looks good for much of southern
Indiana and far west central KY. Still coming up with values around
-10 C this morning as single digit temps combine with steady WNW
winds.

...The Rest of Today, Tonight, and Wednesday...

The low level NW flow behind the Arctic boundary will also help to
bring in another round of low level moisture from the NW later this
afternoon, tonight, and through part of Wednesday. Given that this
moisture will be associated with -12 to -20 C temps (the DGZ),
expect persistent flurries and/or light measurable snow showers
starting later this afternoon over our NW and spreading over the
area through the evening, finally tapering from W to E on Wednesday.
Given very high snow ratios, have a dusting to a half an inch of
additional snow accumulations associated with this low level
moisture. We`ve seen this a couple times already this winter, and
honestly that may be a bit conservative...We shall see. Not sure how
many additional road problems the snow will cause given it`s fluffy
nature.

Highs today only reach the low and mid teens. Low tonight will
mostly be in the 0 to 5 above range. Winds will only be around 5
mph, so not as concerned about a Wind Chill Advisory. Nevertheless,
values will still be dangerous in the -10 to 0 range in most spots.
Highs Wednesday should recover to around 20 degrees.

.Long Term...(Wednesday night through Monday)
Issued at 316 AM EST Tue Jan 16 2018

Wednesday Night - Sunday...

...Warming Trend On The Way...

We`ll go from well below normal to well above normal temps by the
weekend! Dry NW flow aloft and surface high pressure will control
the pattern through the end of the work week. Then, the upper
pattern shifts to a more zonal flow, with high pressure shifting off
to the east for the weekend. We`ll see dry conditions for much of
this time with moderating temperatures rising around 10 degrees from
each day`s previous high. So, highs on Thursday right around
freezing will jump to the low 40s Friday, the low 50s Saturday, and
to the mid and upper 50s by Sunday (possibly warmer). Lows will also
moderate during this time. Low temps on Thursday morning around 10
above will rise to around 20 on Thursday night, just above freezing
Friday night, and in the lower 40s by Saturday night.

By Saturday, lee cyclogenesis will occur over the central and
southern Plains, with a resulting tightening pressure gradient over
the Mississippi and Ohio Valleys. The associated warm/moist
advection will be responsible for our rapid warm up, but will also
be responsible for increasing rain chance by the end of the weekend.
In fact, a few thunderstorms may not be out of the question by
Sunday night or Monday morning. We could begin to see some small
chances for a rain shower associated with isentropic lift by
Saturday morning as the warm sector gets established over the area.
However, prefer a mostly dry forecast through Sunday afternoon,
before the real rain chances arrive ahead of a potent storm system.

&&

.Aviation...(12Z TAF Issuance)
Issued at 620 AM EST Tue Jan 16 2018

Since last main update, still have two snow bands over the area, one
running from Leitchfield to just about east of SDF and another just
south of BWG to Berea. These slowly are making their way eastward,
and expect conditions to improve soon at SDF...though BWG/LEX will
take longer. MVFR to occasionnaly IFR snows are forecast through the
morning hours before deeper moisture leaves the region. Another deck
of cold clouds will move into HNB laster this afternoon and then get
to the other sites this evening and overnight, possibly bringing a
few snow showers as well as MVFR cigs.

&&

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

KY...Wind Chill Advisory until 11 AM CST this morning for KYZ023.

     Winter Weather Advisory until 1 PM EST /noon CST/ this afternoon
     for KYZ035>037-039>043-046>049-054>057-064>067-073>078-081-
     082.

     Winter Storm Warning until 1 PM EST /noon CST/ this afternoon
     for KYZ023>031-033-034-038-045-053-061>063-070>072.

&&

$$

Update...BJS
Short Term...BJS
Long Term...BJS
Aviation...RJS



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