Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 281337
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
937 AM EDT Fri Apr 28 2017

.Forecast Update...
Issued at 935 AM EDT Fri Apr 28 2017

Still a difficult forecast to pin down for this afternoon, but it
now looks like the warm front well to our south will push/re-form
northward into our area by this afternoon. Tried trending the
forecast to some of the models that have done better with dewpoints
and winds of late, and that places the warm front somewhere along
the Ohio River by late this afternoon. This southward shift in the
forecast location for that front brings an increased threat for
tornadic supercells to the I-64 corridor, and SPC has responded with
an increased percentage in that region.

NAM finally has come into agreement with this thinking, and it shows
the stout cap it had in earlier runs breaking down by rush hour.
CAPEs will be sufficiently high for large hail to form with these
storms, with shear aloft also supportive of that tornado threat,
again especially along exactly where that warm frontal boundary ends
up. We will have to watch available observations to place this
boundary through the evening hours. Have updated the forecast to
include the threat for various severe storm aspects and otherwise
blended with current observational data.

&&

.Short Term...(Today through Saturday)
Issued at 400 AM EDT Fri Apr 28 2017

...Severe Storms Possible Late this Afternoon through Tonight...

==========
Overview
==========

A warm front will develop eastward from a strengthening low pressure
system over the south central U.S. today.  The front will become
quasi-stationary over the Ohio Valley this afternoon and into the
evening hours.  The frontal zone will be the main area of convective
activity especially later today over our region.  A warm moist,
conditionally unstable environment will exist south of the boundary.

Models are still struggling with the placement and evolution of the
warm front today and so confidence is on the low side as far as best
placement of showers/storms.

============================
Severe Wx Threats & Timing
============================

A good number of models develop isld-sct showers/storms in the warm
sector over TN and into central KY by mid to late afternoon.  These
cells will likely need a trigger to materialize given a decent cap
in place in the warm sector.  If they can develop, they have
potential to become strong to severe discreet cells posing several
threats including damaging winds, large hail, and perhaps an isld
tornado.

This evening numerous storms should fire along the warm front and
potentially move east over our region.  There are some models that
lift the warm front northward faster resulting in less evening
convection in our region.  Our forecast most resembles a model
consensus with the best chance for evening storms over southern
Indiana and portions of north central KY along the Ohio River. Wind
profiles will be supportive of strong to severe discreet rotating
cells upon initiation with all hazards on the table including
damaging winds, large to very large hail, and perhaps an isld
tornado.  Precipitable water values will increase to 1.5-1.7 inches
which is well above normal and could result in intense rainfall
rates or even flash flooding issues tonight.  Vigorous convection
should continue through much of the evening and then begin to wane
after midnight as instability decreases and the warm front begins to
lift north. Depending on how far north the front lifts, a portion of
southwest Indiana may still be under the gun for strong to severe
storms by early Sat morning.  By mid to late morning Sat, most
convection should be north of the area with strong capping building
again.

=============
Saturday...
=============

After morning convection lifts north with the warm front, the rest
of Sat is expected to be mainly dry although a few cells may develop
Sat afternoon/evening.  Strong capping over the region should
prevent widespread convection however.  If any isld cells can get
going Sat afternoon/evening they could pose a severe threat given
the environment.

Saturday will be unseasonably warm as well with highs in the mid
to upper 80s. See the long term discussion for records.

.Long Term...(Saturday night through Thursday)
Issued at 250 AM EDT Fri Apr 28 2017

...Strong to Severe Storms and Periods of Heavy Rain Late this
Weekend...

Saturday Night - Sunday Night...

Saturday night and the first half of Sunday most models agree we`ll
be mainly dry in the warm sector of the approaching low pressure
system.  Sunday afternoon precipitation chances will increase as the
cold front approaches from the west.  Scattered storms will be
possible west of I-65 Sun afternoon/early evening.  Numerous storms
will be possible Sun night along and head of the front.  Wind
profiles and CAPE values will be supportive of organized clusters or
line segments capable of strong to damaging winds and hail.
Instability will wane later into the overnight hours and so with the
current timing, the risk for stronger storms may be less east of I-
65.  Heavy rain will be a concern Sun night as well with
precipitable water values running well above normal.  Any areas that
saw good rainfall Fri night/early Sat may be at greater risk to see
some flooding develop with Sun night`s convection.

Temperatures will continue to be unseasonably warm challenging
records.  Here are record warm lows and highs for April 29-30.

            4/29 Warm L   4/29 Record H   4/30 Warm L   4/30 Record H
Louisville   67 (1951)      89 (1899)      70 (1899)     91 (1894)
Lexington    67 (1899)      86 (1899)      70 (1899)     91 (1942)
Bowling G.   67 (1899)      91 (1894)      68 (1899)     92 (1942*)
Frankfort    63 (1899)      89 (1914)      66 (1910)     91 (1942)

* = and previous years

Monday - Tuesday...

Monday precipitation will quickly move east of the area early in the
day.  Some very light rain showers are possible over southern IN Mon
afternoon/evening, but overall the area should be mainly dry.  Winds
behind the cold front will turn to the southwest and crank up quite
a bit Monday with a very strong low pressure system over the upper
Midwest.  Expect southwest winds of 15-20 mph with gusts up to 35-40
mph on Monday.  Dry and breezy conditions will remain for Tues
although gusts shouldn`t be as strong as Monday.

Expect high temps both Mon/Tue in the 60s to around 70.  Lows
Mon/Tue nights will drop back into the upper 40s/lower 50s.

Wednesday - Thursday...

Precipitation chances will return by mid week as an upper level
trough drops into the southern Plains and progresses eastward
through the latter half of the week.  We`ll be on the cooler, more
stable side of the this weather system resulting in below normal
temps and on and off chances for mainly rain showers through the end
of the work week.

&&

.Aviation...(12Z TAF Issuance)
Issued at 656 AM EDT Fri Apr 28 2017

The challenge for the TAF forecast remains when/if thunderstorms
will affect any of the TAF sites. Scattered storms are expected to
develop this afternoon and look to affect mainly BWG and SDF. There
is more question as to whether cells will make it far enough
northeast to affect LEX.

For tonight, more storms will form along a warm front, initially
across southern IN. Where this front sets up will have a big impact
on where storms develop tonight. BWG looks to stay mainly dry
overnight. LEX and SDF could see showers and storms later into the
overnight hours.

Other than precipitation, ceilings will lower this afternoon and
evening, but will keep them VFR for now. Winds will pick up out of
the south and become gusty by mid day to early afternoon. Winds of
10-15 knots out of the south will continue overnight tonight.


&&

.Climate...
Issued at 830 AM EST Fri Apr 28 2017

Temperature and rainfall records for the weekend:

            4/29 Warm L   4/29 Record H   4/30 Warm L  4/30 Record H
Louisville   67 (1951)      89 (1899)      70 (1899)    91 (1894)
Lexington    67 (1899)      86 (1899)      70 (1899)    91 (1942)
Bowling G.   67 (1899)      91 (1894)      68 (1899)    92 (1942*)
Frankfort    63 (1899)      89 (1914)      66 (1910)    91 (1942)

            4/29 Rainfall 4/30 Rainfall
Louisville   2.02" (1927)   2.37" (1983)
Lexington    1.31" (2014)   3.21" (1909)
Bowling G.   2.40" (1912)   3.00" (1911)
Frankfort    2.20" (2002)   2.62" (1909)

&&

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

$$

Update...RJS
Short Term...AMS
Long Term...AMS
Aviation...EER
Climate...RJS


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