Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 250521

121 AM EDT Sat Apr 25 2015

...Updated Aviation Discussion...

...Forecast Update...
Updated 907 PM EDT Fri Apr 24 2015

In the near term, some patchy light rain and/or sprinkles will
continue to move across southern KY this evening associated with a
weak mid-level wave pushing eastward.  With dry air in place, a lot
of this precipitation on the radar is not likely hitting the ground,
but is going towards saturating up the column.  Speaking of dry air,
we still have a ton of it across southern Indiana and over portions
of north-central/east-central KY.  Did have to lower dewpoint
forecasts a bit this evening across our northeastern half as the
model blends are still a bit too fast moistening things up.  Further
to the southwest, moisture is beginning to pool and dewpoints in the
lower 40s have already moved into southwest KY.  The column is
expected to moisten considerably overnight.

No real changes on the overnight forecast.  Short term guidance
continues to show the warm front pushing into the region overnight.
Forcing along the warm front is expected to produce widespread
shower activity late tonight into Saturday morning.  No severe
weather is expected, but model soundings do show a little bit of
elevated instability late tonight, so some embedded thunderstorms
are not out of the question.

As for the severe weather threat for Saturday...

Late afternoon guidance continues to show that the warm front will
stall out somewhere along the I-64 corridor in southern Indiana and
northern Kentucky.  The northward placement of this feature
continues to be one of the critical cruxes of the forecast as the
short term and high resolution runs continue to oscillate with
their position of this feature.

What we do know is that in areas south of this boundary, the
multi-model consensus continues to show that some drier mid-level
air will punch into the region tomorrow afternoon resulting in skies
scouring out.  With good insolation, temperatures will likely rise
into the mid-upper 70s with dewpoints pushing into the mid-upper
60s.  The NAM solutions continue to be a little more aggressive with
the moisture allowing dewpoints to rise into the low 70s across
southern KY.  So with temps in the upper 70s and dewpoints in the
upper 60s, we`re looking at CAPE values of 1500-2700 J/Kg across the
region.  Model proximity soundings continue to show strongly veered
winds and long/sweeping low-level hodographs suggestive of deep
layer shear adequate for supporting rotating updrafts.  Thus, it
still appears that cluster of storms (most likely supercells) will
be possible across much of central KY tomorrow afternoon.

One notable concern is that LCL heights are not all that low across
southern KY tomorrow.  The LCL heights do come downward as one heads
northward toward the warm front.  Thus, the combination of shear and
high instability across southern KY tomorrow may result in more of a
very large hail threat with a slightly lower tornado threat.  We did
see a similar LCL height stratification similar to this back in the
March 2012 outbreak (with tornadic supercells in the vicinity of the
warm front, with large hailers down across south-central KY).

Along and just south of the warm front, is where the main concern
still resides.  This area will have the the shear, instability, and
low LCL heights that may result in more of a tornadic threat, along
with the very large hail and damaging winds. Again, placement of the
actual warm frontal boundary will be the forecast challenge here.

The high resolution model simulations suggest that convection would
likely redevelop in the warm sector along and south of the warm
front tomorrow afternoon.  Model soundings suggest convective
temperatures of 74-77 degrees should get things going which would
probably be in the 3-4PM time frame, highly dependent on the amount
of insolation during the afternoon.

Areas north of the warm frontal boundary (north parts of southern IN
and far northern KY) will likely remain much more stable along with
being much cooler (highs in the upper 50s/lower 60s vs. upper 70s)
with more stratiform rains possible.  However, the northern areas
are not out of the severe weather threat as the warm frontal
placement could end up being further north placing more of the area
in a severe weather risk.

While there is still some uncertainty in the overall evolution of
this system, we want to stress its a good idea to remain vigilant of
the weather tomorrow afternoon and evening.  Residents of southern
IN and central KY should review their severe weather plans and be
ready to put those plans into action before severe weather strikes.

.SHORT TERM (Now through Saturday Night)...
Issued at 325 PM EDT Fri Apr 24 2015

...Severe Weather Possible Saturday Afternoon and Evening...

Severe weather still looks to be a good bet for Saturday afternoon
as a mid level wave and its attendant surface low move through the
lower Ohio Valley. For tonight moisture will be on the increase as a
warm front moves in from the south. Some light showers will be
possible before midnight in association with a weak wave moving
through. However, the better chance for showers and possibly a few
thunderstorms will be after midnight through mid morning as the warm
front pushes north. These thunderstorms are not expected to be
severe at this time but could produce brief heavy rainfall.

The warm front will stall out across central Kentucky tomorrow
as the surface low drops southeast towards the area. Models continue
to indicate we should have a dry slot punching in south of the
front. This could lead to some clearing with temperatures warming
into the mid to upper 70s. In addition, moisture will continue to
pool south of the front with dewpoints rising into the lower 60s.
Through the afternoon the atmosphere will become unstable south of
the front with CAPE values rising to 1000-1500+ J/kg. Strong mid
level winds will result in effective shear of 50-60 knots. In
addition, helicity values will rise to 250+ m2/s2, with higher
values near the stalled front.

All in all, supercells will be the preferred storm mode with this
system. Very large hail and damaging winds could be present with any
supercell. Some of the storms could produce tornadoes as well,
particularly in the vicinity of the front. There is still some
uncertainty as to where exactly this front will end up. Currently
the NAM is the outlier and furthest to the south, so am leaning away
from its solution slightly. Currently it looks like the highest
threat for strong severe storms will be along and south of the I-64
corridor. However, a few strong to severe storms cannot be ruled out
north of there. These storms look to begin to develop around 3-4 pm
EDT and continue through around 10-11 pm.

In addition to the severe threat, there is some concern with heavy
rainfall potentially leading to some flash flooding. This would be
particularly a concern along the front and just to the north of it
where we could have some training of cells. The best chance for this
would be across southern IN and north central KY. This area has been
the wettest so far this month, so this will bear watching carefully
tomorrow as well.

.LONG TERM (Sunday through Friday)...
Issued at 315 PM EDT Fri Apr 24 2015

High impact weather is expected to be well south and east of the
region by Sunday morning.  However, a trailing mid-level wave and
cold front will swing through the region and bring an end to the
rainy weather.  It appears that the previous forecast remains on
track with precipitation ending by midday Sunday with high pressure
building into the region.  High pressure at the surface combined
with a northwesterly flow aloft will provide cool and dry weather
for the early part of next week.  Rather deep troughing is expected
to remain over the far eastern US as a series of upper level waves
rotate within a larger gyre aloft.  This closed upper low will
eventually move off to the east by midweek or so.  Temperatures
early next week will remain below seasonal normals with highs in the
upper 50s to the lower 60s.  Overnight lows will cool into the upper
30s to the lower 40s.

Around midweek, closed low coming out of the southern stream will
weaken and become an open wave as it gets into the southern Plains.
As this southern wave move eastward, a stronger northern stream wave
will continue to dive south rather sharply and may phase with that
southern stream system.  Should this occur, we would see rather
cloudy and showery conditions from Wednesday into early Thursday
before ridging builds back in from the west.  As we get into the
latter half of the week, the global ensemble guidance suggest that
ridging will build in from the west as a deep closed low moves off
the US east coast.  This will keep the Ohio Valley in a dry NW flow
with temperatures starting off cool but slowly moderating through
the end of the week.  Unseasonably cool temperatures will be the
rule on Wednesday with highs in the low-mid 60s, but we should see
near normal temps (upper 60s to lower 70s) return by Thursday and
continue into Friday.  Overnight lows look to be generally in the
mid-upper 40s.


.AVIATION (06Z TAF Update)...
Updated at 118 AM EDT Sat Apr 25 2015

Deteriorating conditions will push in through the TAF period, as an
area of low pressure approaches from the west.  Showers will
continue to expand in coverage early this morning, bringing MVFR
vsbys at cigs at times to all terminals.  Will leave in VCTS wording
given some marginal elevated instability, but think any thunderstorm
activity will be rather isolated.  Winds will be out of east at 5-10
knots through the remainder of the overnight period.

A warm front, now located over MO/AR will lift northeast through the
morning hours.  Just north of this front, a period of IFR cigs can
be expected at all sites.  In fact, the latest guidance suite
suggests this front will likely stall to the south of KSDF and KLEX,
meaning low cigs may persist in these locations through much of the
day.  Will trend the forecast toward the pessimistic side to account
for this trend.  KBWG will likely scour out this afternoon and mix
down SSW wind gusts around 25 knots.  Late this afternoon into this
evening, severe thunderstorms are expected to break out over central
KY, mainly affecting KBWG.  However, there is enough elevated
instability to warrant VCTS wording at KSDF and KLEX as well with
this activity.  Winds will turn more northerly late in the TAF
period as the area of low pressure pushes to the southeast.

06Z TAF Forecaster Confidence
Ceilings    : Medium
Visibilities: Medium
Winds       : Medium




Short Term.....EER
Long Term......MJ
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