Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 040234
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
934 PM EST Tue Mar 3 2015

...Mesoscale Forecast Update...
Issued at 930 PM EST Tue Mar 3 2015

Updated the forecast to add light patchy fog to the area overnight
as it has already formed over southern IN and portions of northern
KY.  Think that mixy winds aloft should prevent vsbys from becoming
widespread dense, but patches of vsbys below 1 mile will be possible
throughout the night.

Flooding looks to become more of a concern over the area tonight
especially over southern Indiana which has already seen around 1
inch of rainfall from a band of steady light to moderate rains this
evening.  Did get a report of the usual flood prone spots in
Washington Co. starting to flood as of 8pm so issued a Flood Advy
from Dubois Co. to Jefferson Co. in Indiana. Precipitation setting
up to our west looks to move over southern Indiana this evening.

Precipitation looks to blossom over the rest of KY tonight.  Overall
QPF amounts have gone down a little in the most recent model runs,
but still expect a general 1-3 inches of QPF over the region and
thus the Flood Watch will continue.

Issued at 615 PM EST Tue Mar 3 2015

A band of light to moderate rainfall has been slowly drifting
southeast over southern Indiana over the last few hours with a few
sites over southern Indiana reporting 1-1.25 inches of rain.  So far
these amounts are under 1 and 3 hour flood guidance and no flooding
issues have been reported, but this rainfall is providing a good
soaking of rain to potentially set the stage for flooding problems
later tonight into tomorrow.  We`ll have to keep an eye on these
areas later tonight.

The latest high-res model data suggests that we`ll continue to see
the band of rainfall near the Ohio River persist through this
evening and continue to drift south.  Rain should blossom over the
entire area around 9-10 pm this evening as better forcing arrives.

.SHORT TERM (Now through Wednesday Morning)...
Issued at 300 PM EST Tue Mar 3 2015

...Heavy Rain and Flood Potential Tonight...

Warm front is blasting north into Indiana and northern Kentucky this
afternoon, with temps climbing well into the 50s in spite of plenty
of low stratus. Precip is blossoming over southern Indiana, but we
have yet to see the heavy precip develop.

Expect to see precip become more focused over southern Indiana early
this evening, and then shift southward into Kentucky as the night
progresses. Rain will be heavy at times through the night, with a
strengthening low-level jet and hitting the approaching cold front.
The front pushes through after midnight, but moderate to
occasionally heavy rain will continue behind the front as deep
moisture remains in place. We have seen a shift in the axis of
heaviest rains, with 2+ inches expected before the changeover to
snow along and north of the WK and Bluegrass Parkways, and into
southern Indiana. In light of this shift, and plenty of water
content in the remaining snowpack, the Flood Watch has been expanded
into the southernmost tier of counties in Indiana. Some uncertainty
in the QPF has been introduced south of the Cumberland Parkway.
However, with the Flood Watch already in place and saturated
antecedent conditions, will leave it in effect in case the heavy
precip does jump south.

As to other threats in the near term, with the slower development of
this precip and still fairly lackluster mid-level lapse rates, will
remove the thunder chances at this time. Also not as concerned with
precip type during the overnight period, as surface temps look to
stay above freezing until just about 12Z Wednesday.

.LONG TERM (Wednesday Afternoon through Monday)...
Issued at 200 PM EST Mon Mar 3 2015

...Major Winter Storm To Impact Kentucky and Southern Indiana...

Overview

A major winter storm is expected to impact Kentucky and southern
Indiana Wednesday afternoon through Thursday morning with a swath of
heavy snow resulting in significant travel and commerce disruptions.

Synoptic/Mesoscale
==================

In short, the synoptic setup through Thursday morning remains
unchanged compared to previous forecasts. Still expecting a very
impressive 300 mb jet of 180 kts to set up across the lower Great
Lakes favoring the right entrance region across the forecast area. A
strong PV anomaly then rides through Wednesday night. There is ample
upper level support with this system.

Cross sections from southern Indiana to northern Tennessee reveal
long periods of strong, sloping frontogenesis. There are pockets of
negative EPV aloft indicating that mesoscale snow banding potential
is very high. The deep saturation coupled with the strong lift as
the thermal profile becomes increasingly in the DGZ indicates that
intense, heavy snow rates of 1 to 2 inch per hour will be possible
across north central Kentucky late Wednesday afternoon through the
early overnight hours Thursday. Thundersnow is not out of the
question.

Model Preference
================

03.12z guidance added confidence by coming in line synoptically and
with its thermal profiles. Yes, there are still subtle differences
with the boundary layer cooling, mainly how fast does the Arctic
spill southward. Earlier runs of the NAM persisted a warm nose aloft
Wednesday evening, but the 12z runs no longer show this idea. As
such, a blend of NAM/GFS/ECMWF was used and agreed well with WPC.

Timing
======

Plan on Wednesday morning precipitation to be all rain across the
forecast area but as the colder air arrives there will be sharp but
steady changeover to brief wintry mix of sleet/rain/snow before
going over to all snow. The changeover will occur initially across
southern Indiana late morning to early afternoon. For north central
Kentucky, this is expected to happen from early to mid afternoon.
Finally, south of the KY parkways, a complete changeover may not
occur until sunset, closer to 6-7 pm.

Snow Amounts
============

The heaviest axis of snow at this time looks to be across central to
north central Kentucky where amounts of 8 to 12 inches are expected.
Locally higher amounts possible. A localized area of 10 to 13 inches
could fall across the northeast forecast area. Either side of this
swath, still expecting significant amounts of 5 to 8 inches.
Finally, the far southeast forecast area, Cumberland region, totals
may be limited by 1) a delay in the changeover and 2) more sleet
than snow. Those areas still could pick up 3 to 5 inches.

Impacts
=======

This system is expected to cause major travel disruptions across the
entire forecast area, significantly impacting the Louisville and
Lexington metro areas. The Wednesday evening commute for the
Louisville metro could particularly be very bad as snow rates
increase toward 1 inch or more per hour. The heaviest snow looks to
fall from around sunset through overnight.

Travel could become nearly impossible late Wednesday evening and
into the overnight hours. North winds will increase during the
evening, becoming sustained 10 to 15 mph. This will greatly reduce
visibilities. The snow will initially a wetter variety before the
true Arctic air helps to make it drier. Still, with these hefty
amounts, scattered power and tree issues could develop.

Confidence
==========

Confidence is very high for 4 to 8 inches of snow across the
forecast area. Our confidence in seeing 8 to 12 inches is still
above average. The impressive and unusually strong synoptic setup
combined with the anomalously high amount of moisture/QPF results in
these high totals. There are still subtle differences where the
heaviest axis sets up but central to north central KY and extreme
southern IN at this time look to bear the brunt of this storm.

Given the warmer ground and road temps, there are still questions
about how quickly the snow accumulates, at least initially. Thinking
that the Arctic air push combined with the intense rates should
overcome any melting at the onset and lead to rapid accumulations.

Finally, how long do we see sleet and does that undercut the snow
totals is still somewhat uncertain. However, given how rapidly the
warm nose aloft erodes with diabatic cooling, think the sleet
transition will be short lived.

As a result, the Winter Storm Watch has been upgraded to a Winter
Storm Warning for the entire area.

Thursday Night through Tuesday
==============================

Record or near record lows for the month of March possible early
Friday morning over an expected fresh snowpack.

Outside of frigid temperatures Friday, the extended forecast looks quite
benign with slowly moderating temperatures.

Upper air pattern forecast for Friday depicts a temporary Rex Block
along the west coast with a 500mb ridge over northern California
north of a nearly closed low over the Baja Peninsula. The Lower Ohio
Valley will lie underneath northwesterly 500mb flow.

Skies will clear late Thursday and near perfect conditions may
develop by Friday morning for good radiational cooling. Light winds,
a fresh snowpack and probable clear skies may produce record lows
for the month of March.

Here are those records for our area:

       Mar 5                   Mar 6
       Record Cold Highs       Record Cold Lows

BWG    27  1960                        1  1960
SDF    26, 1960                       -1, 1960*
FFT    22, 1960                       -3, 1960*
LEX    22, 1960                       -2, 1960*

* indicates the all-time March record low

Despite plenty of sun, highs Friday will struggle to reach the mid
20s over any fresh snow. If snow doesn`t fall near the Tennessee
border, highs there may reach 40.

A couple of very weak troughs will move through the Great Lakes over
the weekend. A slight reinforcing trough move south of the Ohio
River early Sunday. This may bring a few clouds but no
precipitation. Temperatures will slowly warm this weekend with highs
reaching into the lower 40s by Sunday. Overnight lows will depend on
lingering snowpack and whether we have clear skies. Lows in the
teens are likely early Saturday.

An elongated surface ridge axis will essentially remain in place
over the Tennessee Valley through Monday. Moisture associated with
low pressure off the Texas Coast Monday and Tuesday will stay
suppressed well south of Tennessee. Southwesterly flow aloft will
allow heights to build and milder temperatures to develop,
especially by Tuesday. Under generally partly cloudy skies, highs
will likely warm into the 50s by Tuesday.

&&

.AVIATION (00Z TAF Update)...
Issued at 610 PM EST Tue Mar 3 2015

A band of steady rain will slowly move southeast. It will arrive at
SDF before the TAF issuance time and at LEX around 1Z to 2Z.
Scattered showers will affect BWG through around 4z, after which
rain will prevail.

Steady rain is anticipated all night with MVFR visibilities and
borderline IFR/MVFR ceilings. Southwest winds of 12kt with higher
gusts will continue prior to any rain`s arrival. Once rain begins,
southwest winds will average 10 mph. S-SW winds will quickly shift
to NW around 06-09Z as a cold front moves through.

Rain will turn into potentially heavy snow during the day Wednesday.
Snow will begin at SDF and LEX in the 18 to 21z timeframe, and at
BWG around 23z. If snow becomes heavy, visibilities and ceilings
will drop below LIFR.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...WINTER STORM WARNING from 1 PM EST /Noon CST/ Wednesday to 1 PM
     EST /Noon CST/ Thursday FOR KYZ023>043-045>049-053>057-
     061>065-070>072.

     FLOOD WATCH through Wednesday afternoon FOR KYZ023>043-045>049-
     053>057-061>067-070>078-081-082.

     WINTER STORM WARNING from 7 PM EST /6 PM CST/ Wednesday to 1 PM
     EST /Noon CST/ Thursday FOR KYZ066-067-073>078-081-082.

IN...WINTER STORM WARNING from 1 PM EST /Noon CST/ Wednesday to 1 PM
     EST /Noon CST/ Thursday FOR INZ076>079-083-084-089>092.

     FLOOD WATCH through Wednesday afternoon FOR INZ084-089>092.

&&

$$

Mesoscale......AMS
Short Term.....RAS
Long Term......ZBT
Aviation.......JSD




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