Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 040723
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
323 AM EDT WED MAY 4 2016

.SHORT TERM (Now through Thursday)...
Issued at 315 AM EDT Wed May 4 2016

...Showers and Storms with Small Hail This Afternoon...

The bulk of the region remains in the subsident wake of a passing
shortwave thru the pre-dawn hours, however the lull in precipitation
will be short-lived. As we move into the daylight hours, focus will
shift to an anomalous vorticity maximum evident on water vapor
imagery dropping out of the upper Midwest. As the exit region of a
100 knot upper jet moves into the lower Ohio Valley just after
daybreak, the associated low level response in the form of a 30 knot
low level jet will create an isentropic lift component over our
region. This will create a widespread shield of light to moderate
rain showers (and a few rumbles of thunder) sliding southeast
through our region from just after day break through midday. The
evidence of what will occur is already ongoing over northern IL/IN.

Just behind the stratiform rain shield, a sharp cold front will
follow with steady NW surface winds (NW gusts up around 20-25 mph).
With steepening mid level lapse rates underneath the approaching
closed upper low, and perhaps some surface heating in the afternoon,
numerous instability driven showers and storms will develop late
morning into the afternoon. Forecast soundings show freezing levels
in the 6 to 7 K foot range which will easily support small hail in
many showers given modest instability up through 20-25 thousand
feet. Can`t rule out a few larger hail producers if some afternoon
surface heating is realized.

Forecast models are in excellent agreement with today`s evolution
and timing, which puts initial batch of showers along the I-65
corridor around midday, and along the I-75 corridor by early to mid
afternoon. The instability based showers/storms will then follow
from southern IN into central KY through early and mid afternoon.
Precipitaton should then exit our east by early evening. Most spots
should expect around .25" - .33" inches of rain through the day.
Will continue with high pops given confidence in pattern/coverage,
and current radar presentation upstream. Highs today will be in the
low 60s. It`s also worth noting that a cold air funnel can`t be
ruled out in the post frontal regime underneath the upper low this
afternoon/evening.

Tonight - Thursday...

Closed upper low slides right over the region tonight and into the
Carolina Coast by late Thursday. Enough low level moisture should be
around for isolated to widely scattered rain showers mainly across
our east. This will be a cool period with lows tonight in the 40 to
45 degree range, and highs on Thursday in the upper 50s to around 60.

.LONG TERM (Thursday night through Tuesday)...
Issued at 315 AM EDT Wed May 4 2016

Thursday Night - Friday...

Highly amplified upper pattern features expansive closed lows over
the western and eastern coasts, with a blocking upper ridge over the
center part of the CONUS. The Ohio Valley will be in the deep
norhterly flow between eastern closed low and upper ridge to end the
week. With surface high pressure in place and a lack of moisture,
this will be a dry period with lows Thursday night in the mid 40s.
With rising heights/thicknesses on Friday, temps will rise to the
lower 70s.

Friday Night - Saturday...

The upper ridge begins to break down as we head into the weekend in
response to another shortwave diving through Ontario into the Great
Lakes. As this occurs, a frontal boundary will slide toward the Ohio
River by late on Saturday. With surface high pressure still holding
on, this should also be a mostly dry period. However, cannot rule
out a few sprinkles Friday night. By late Saturday afternoon and
evening, scattered showers and an isolated thunderstorm will be
possible along and north of the Ohio River. At this point, it looks
like any precipitation activity should hold off in the Louisville
region until after 8 PM EDT, but do have to introduce a 20% chance
of a shower early evening.

Friday night lows will be milder than previous nights with readings
in the low to mid 50s. Saturday highs should be able to top out in
the low 80s before upper clouds arrive in the late afternoon and
evening.

Saturday Night - Tuesday...

A potentially showery period continues through the weekend and into
early next week as the aforementioned frontal boundary stalls over
the region, nearly parallel to the NW flow aloft. With the frontal
boundary serving as a mechanism for moisture convergence, expect
that isolated to scattered showers and storms will then be possible
mainly each afternoon and evening through Monday. A brief lull will
be possible on Monday night as a progressive upper ridge moves
through. Chances for showers or storms then return for Tuesday as
upper pattern transitions to a deep SW flow ahead of an approaching
trough. The associated low level response will help to lift the
aforementioned frontal boundary back north as a warm front, with
scattered showers and storms possible in the isentropic ascent
regime.

Saturday and Sunday night lows will be in the mid and upper 50s,
with lows by Monday night around 60. Highs in the 70s on Sunday will
warm to near 80 on Monday and Tuesday. These readings will be a bit
tricky to nail down given a waffling frontal boundary in the region.

&&

.AVIATION (06Z TAF Update)...
Updated at 105 AM EDT Wed May 4 2016

VFR through the night with intermittent mid-level ceilings in the
modest warm advection pattern. Front is on schedule diving into
northern Illinois, with a narrow rain shield just ahead of the
boundary.

Expect the rain shield to dive southeast into Kentucky by late
morning, but should be losing coverage as it is left behind by the
stronger upper forcing. Therefore will only carry prevailing SHRA in
SDF, with just VCSH in BWG and LEX. Ceilings will drop into high-end
MVFR, but we can`t rule out briefly going into fuel-alternate
conditions early/mid afternoon. Will reassess in the 12Z TAFs.

SW winds will pick up with the precip, with gusts just shy of 20 kt,
then shift to due west or WNW mid-afternoon. Will continue VCSH
behind the front as cold air aloft will support showery convection.
Can`t rule out isolated thunder or even small hail, but probability
too low to include in TAFs. Ceilings will lift and NW winds will lay
down around sunset.

&&

.LMK Watches/Warnings/Advisories...
KY...NONE.
IN...NONE.
&&

$$

Short Term........BJS
Long Term.........BJS
Aviation..........RAS



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