Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 251709
AFDLMK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOUISVILLE KY
109 PM EDT Sat Apr 25 2015

...Updated Aviation Discussion...

...Forecast Update...
Updated 930 AM EDT Sat Apr 25 2015

Latest guidance including the latest SPC Day 1 Outlook continues to
trend the severe threat slightly to the north. No big changes have
been made to the grids, but have tried to trend the pop forecast a
bit towards the latest hi-res model guidance. Also nudged the area
where severe storms are mentioned slightly further to the north. The
ongoing precipitation has not had any lightning associated with it
so far. However, think we could still see some isolated flashes
through the late morning hours.

Updated 621 AM EDT Sat Apr 25 2015

No major changes needed to the forecast this morning, as it is
largely on track.  Still expecting rain showers to continue to
blossom over the region, especially along and north of I-64 this
morning.  This is all in response to a low-level jet and isentropic
ascent veering into the region.  May see a few rumbles of thunder
especially as we get into the late morning hours, but no severe
weather is expected through noon.

The 06Z NAM has come in further north with the warm frontal
placement late this afternoon, which would bring the I-64 corridor
more into play for severe potential.  However, the latest runs of
the HRRR continue to show the warm front stalling south of I-64 and
very close to the current forecast position.  Given the HRRR has a
decent handle on ongoing convection, and the support of the other
hi-res guidance in regards to expected frontal position this
afternoon, see no need to adjust things further north based on the
06Z NAM quite yet.  Will continue to monitor trends through the
morning hours and especially the 12Z guidance suite.

.SHORT TERM (Now through Saturday Night)...
Issued at 345 AM EDT Sat Apr 25 2015

...Severe Weather Likely This Afternoon and Evening...

Overview
========
The current synoptic setup features a split-flow regime across the
CONUS.  The Ohio Valley will be affected by a PV anomaly currently
across eastern Kansas and western MO, which will push through the
region late this afternoon into tonight.

In response to the ejecting PV anomaly out of the southwest flow
aloft, a surface low has formed across portions of eastern KS.  This
low will slide east today, but will begin to fill as the overall
system encounters confluent flow over the Ohio Valley.  A very sharp
warm front associated with this low pressure system will develop
across the Ohio Valley today.

This Morning
============
With the ejecting wave, isentropic ascent has increased across the
region this morning.  This has led to showers breaking out across
the Ohio Valley, and the radar should continue to blossom through
the morning hours as a low-level jet of 40-50 knots veers into the
region towards sunrise.  Forecast soundings show limited elevated
instability, so severe weather is not expected.  Just some showers
with a few embedded rumbles of thunder appears likely.

This Afternoon/Evening
======================
The placement of the aforementioned sharp warm front will be
absolutely critical to how far north the severe weather threat gets
this afternoon/evening.  The 25/00Z guidance shifted this front
south just a bit from previous runs, which would take much of the
I-64 corridor out of the main severe risk.  Will cautiously trend
the forecast towards this more southern solution given the support
on almost guidance members, but still am a bit weary this front
could end up surging further north, especially if there is less
showers/storm coverage this morning than expected (less convective
enhancement to the front).  This warm front will be the main player
that we will continue to monitor through the day.

At this point, expect the front to align itself roughly along a line
from Owensboro, to north of Elizabethtown, over towards Lebanon
by late this afternoon.  To the south of it, a warm and unstable
airmass is likely to materialize given temperatures rising to near
80 and dewpoints in the middle 60s.  This low-level environment
coupled with very steep lapse rates aloft will support MLCAPEs
anywhere from 1500-3000 J/kg, with much of that CAPE residing within
the hail growth zone (-10 to -30C).

The evolution of the convection could be rather complex.  There
seems to be a decent signal in the hi-res guidance that the strong
warm front may lead to convective initiation, which will likely be
initially discrete in nature.  Given 0-6km shear well over 50 knots,
these will be supercellular, if they are to form. The aforementioned
instability coupled with deep-layer shear will lead to a large hail
threat, with golf ball to even baseball-sized hail not out of the
question in some of these storms.  Bunker`s RM storm motion vectors
place the motion of these potential supercells almost parallel to
the warm front, which is worrisome because any storm that forms
along this front will have access to enhanced helicity, given the
backed surface winds.  Therefore, think there is an enhanced tornado
threat right along this boundary wherever it happens to set up,
perhaps more of a threat than SPC is currently highlighting with
their 5% Tornado risk.

Further south (across far southern KY, near TN border), strong
mixing coupled with the fact that the surface low will be filling
will lead to more veered surface winds, which should cut down
somewhere on the effective SRH.  However, with westerly low-level
jet expected to strengthen towards 26/00Z, hodographs will be
long/curved enough to support an isolated tornado threat early this
evening in areas south of the warm front.  Coverage of these storms
not tied to the warm front will likely be rather isolated, given a
lack of convergence and the presence of a weak cap in the warm
sector.

By a bit later this evening, the actual surface low will push into
west-central KY.  Near the triple point of this low, enhanced
convergence will likely lead to a more concentrated area of
convection, perhaps a mix of bowing segments and supercells which
will push into the I-65 corridor toward 02-04Z.  By this time,
daytime instability should be beginning to wane, but still think
this second round will carry a severe threat of large hail, damaging
winds, and isolated tornadoes as it pushes southeast across
south-central KY.

Bust Potential
==============
There remains some potential bust factors with this setup.  The
first and most obvious, is the morning convection.  We will need
this to clear out and some sunshine to break out south of the warm
front this afternoon for us to destabilize.  Given the general
guidance agreement, do have pretty high confidence this will occur
and we will become quite unstable across southern KY late this
afternoon.

The other potential bust factor will if discrete development will
occur late this afternoon/early evening.  Warm fronts can be tricky
in initiating surface-based convection, so we could go through the
day not seeing much becoming sustained along the front.  In
addition, the weak cap could hold through the rest of the warm
sector giving very little coverage of discrete convection until the
more concentrated line of storms approaches with the surface low
well after peak heating.

Summary
=======
While there is bust potential, have fairly high confidence in severe
weather this afternoon and evening.  Think the main threats will be
hail and damaging winds, but an enhanced tornado threat will be
found right along the warm front.  This is something we will
continue to monitor closely this afternoon.  Even north of the warm
front in Lexington and Louisville, forecast soundings show quite a
bit of elevated instability above the stable boundary layer, so
severe hail will certainly be a possibility even in areas north of
the warm front.

.LONG TERM (Sunday through Friday)...
Issued at 300 PM EDT Sat Apr 25 2015

Overall tranquil weather pattern expected for much of the extended
forecast period. Average flow aloft pattern will be somewhat blocky
and amplified with troughs over the northeastern and western U.S.
and ridging in the central U.S. However, an mid-level low will
meander east over the southern Plains to the south of the ridge.
Models suggest that this system will move east into the Gulf Coast
states and weaken mid to late next week, and not phase with a
northern stream shortwave rotating southeast over the upper Ohio
Valley. As a result, dry weather should prevail through the period.

With cool surface high pressure to the north of our area early in
the week, temperatures will be below normal with highs in the upper
50s to mid 60s Monday afternoon, low-mid 60s Tuesday, and mid-upper
60s Wednesday. Expect lows in the lower 40s in many areas Tuesday
morning (possibly upper 30s in valleys), and 40s Wednesday morning.

For late week into next weekend, the ridge aloft should slide east
resulting in continued dry weather and a moderating trend, with
afternoon highs expected in the 70s Friday and Saturday.

&&

.AVIATION (18Z TAF Update)...
Updated at 107 PM EDT Sat Apr 25 2015

Ceilings lowered to IFR at SDF and LEX this morning as they
remain north of the warm front. This front should stay to the south
of both these terminals. Therefore do not see much improvement in
ceilings, except for perhaps a few hours late this afternoon when
storms are expected to develop. These storms will be hit and miss,
but any that do develop could produce hail. The storms will weaken
after sunset with some showers lingering into the overnight hours in
the wake of a low pressure system crossing the area. IFR ceilings
are expected to persist through much of the night, beginning to lift
around dawn. Winds will shift to northerly to northeasterly as the
low moves through.

BWG is south of the warm front. Ceilings through the afternoon will
likely bounce back and forth between MVFR and VFR as clouds scatter
out from time to time. Strong to possibly severe storms will develop
late this afternoon in the vicinity of BWG. These will move out by
mid to late evening as a cold front moves through. IFR ceilings are
expected to develop in the wake of the front and persist into the
morning hours tomorrow.

&&

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

$$

Update.........EER/KJD
Short Term.....KJD
Long Term......RAS
Aviation.......EER




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