805
FXUS63 KLMK 080459
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
1259 AM EDT Wed May 8 2024

.KEY MESSAGES...

*   Potential for multiple rounds of strong to severe storms
    through early Thursday. All severe hazards will be possible.
    There is some potential that a significant severe weather
    event could develop Wednesday afternoon and night.

*   Localized flooding potential will exist where repeated rounds of
    heavy rainfall occur, especially Wednesday night into early
    Thursday.

*   Below normal temperatures return by Friday and persist into
    the weekend.

&&

.UPDATE...
Issued at 1211 AM EDT Wed May 8 2024

The main activity remains just to our north in southern IN just
outside of our CWA from around Lawrenceburg, IN back towards
Vincennes, IN. Looking at KVWX WSR-88D, you can seen a boundary
extending back towards the southwest connecting the convection
developing across far southern IL. Mesoanalysis shows the bulk of
the convection riding along an instability gradient as well as 1000-
850 frontogenesis. Most of the CAMS as well as the cloud-based warn
on forecasts of an ENE-WSW oriented convective line coming together
over the next couple of hours north and along the Ohio River then
dropping it southward into KY overnight. Convection has struggled
this evening as it has tried to move into central KY due to weak mid-
level lapse rates. Current thought is as this activity slowly works
east and southeast through the overnight hours, we will need to
continue monitor radar as we will have plenty of instability and
could see some slight improvement in mid-level lapse rates. There
also remains bulk shear of 50-60kts. While some of the convection
could become more elevated, a few more severe storms long the
boundary with hail, gusty winds and even a few spin-up tornadoes are
still possible.

Issued at 1022 PM EDT Tue May 7 2024

Small storms developed in southern Indiana over the past couple of
hours and took full advantage of strong low level shear to manage to
produce at least one probable small tornado.

IR satellite showing anvils flowing rapidly downstream in strong
shear, keeping convection in southern Indiana shallow. Weak mid-
level lapse rates are preventing more robust updrafts. With such
strong shear in place, updrafts will need to be strong to maintain
themselves and support deep convection.

However, the cells earlier, which struggled even to produce much
lightning, managed to spin-up a possible tornado! Just goes to show
that large, powerful thunderstorms aren`t always necessary if low
level shear, LCLs, low level lapse rates, etc., support rotation
reaching the ground.

Convection upstream over the southern tip of Illinois and
southwestern most Indiana continues to struggle. Recent HRRR run did
a decent job with the small cells over southern Indiana, and has
been consistent all evening with producing more convection
overnight. So, will not call the all-clear quite yet. Indeed, deeper
convection has been developing into southwesterly flow upstream of
the Indiana/Ohio MCS. Will need to keep an eye on this trend as we
head into the overnight.


Issued at 841 PM EDT Tue May 7 2024

Storms have begun to fire across southern Illinois and southwest
Indiana in a corridor of 1.4" precipitable water, 3500 J/kg MLCAPE,
and 850mb theta-e ridging ahead of a surface boundary roughly from
St Louis to Terre Haute to Dayton. Additional development will be
possible south and southwest of the MCS over central Indiana.

Though mid-level lapse rates are weak, there are several other
factors that support the idea of severe weather over the next
several hours. Storms are entering a region of 1200 J/kg DCAPE with
steep low level lapse rates and ACARS soundings showing dry mid-
level air, supporting wind gusts reaching the surface. 60kt of
effective deep layer shear will support storm organization where
updrafts are strong enough, and with WBZ heights around 9k`, hail
will be a possibility. A TBSS has already been observed with a storm
via KLVX radar.

The above factors plus sfc-500m STP > 6, 200 m2/s2 0-1km SRH, and
700-1000m LCLs support the possibility of tornadoes. SPC has issued
a Tornado Watch for all of southern Indiana and central Kentucky
until 3am EDT/2am CDT.

Residents musn`t let their guard down regarding flooding. CAMS and
training of storms over central Indiana this evening suggest stripes
of heavier rain in locations that receive multiple thunderstorms,
which can lead to localized flooding issues. Flooding can be
particularly dangerous at night.

The most likely areas for severe storms and flooding appear to be
southern Indiana and north central Kentucky.


Issued at 612 PM EDT Tue May 7 2024

Quiet weather persists at this hour with warm, dry air aloft over
the region and strong forcing remaining to our north. Over the next
couple of hours the best chances for storms will continue to be off
to our north, especially over central Indiana.

Boundary showing up in visible satellite data from east central
Illinois to southeast Missouri is expected by CAMs to support
additional thunderstorm development this evening. Forcing
mechanisms, however, are subtle, with the more obvious triggers of a
surface cold front well to our west from northern Illinois to
northeast Texas and the aforementioned 5H impulse weakening slightly
as it lifts northeast. There may be a slight increase in the low
level jet from around St Louis into the Ohio Valley as the sun sets
that could assist in convective development.

While confidence in storms is not as high as earlier, patience is
often a virtue in these set-ups. It is not yet time to let our guard
down. If storms do fire, all severe hazards will be possible,
including tornadoes in a strongly sheared environment.


Issued at 422 PM EDT Tue May 7 2024

Isolated showers have developed across central Kentucky over the
past couple of hours but have struggled with weak forcing and mid-
level capping/weak lapse rates. Much stronger storms have erupted to
our northwest over Illinois along a CAPE gradient and closer to a 5H
speed max advancing from Missouri into Illinois.

In central Kentucky and southern Indiana, warm, moist air is in
place with temperatures in the mid and upper 70s and dew points into
the lower 70s...resulting in increasing SBCAPE. CAMs suggest
additional storm development after about 5pm EDT, though remaining
widely scattered as the upper impulse over Illinois lifts to the
northeast towards the southern tip of Lake Michigan, remaining to
our northwest.

If storms do develop here this evening, they will be in an
environment of dramatically increasing 0-500m SRH/STP, especially
after 7pm EDT, and thus will have to be watched closely for signs of
rotation.

Going deeper into the evening, additional storm development to our
west will begin to move into the region for a continuing chance of
severe storms, especially over southern Indiana and central
Kentucky. The most likely timing for the most widespread convective
activity appears to be between 8pm and midnight EDT. A gradual
diminishing trend in intensity is expected after midnight, though
showers and thunderstorms will continue to push southeastward
through central and southern Kentucky.

&&

.SHORT TERM /THROUGH THIS AFTERNOON/...
Issued at 130 PM EDT Tue May 7 2024

Latest 500 MB analysis and water vapor imagery shows a strong upper
level low pressure system over the Dakotas with plenty of jet stream
energy and associated dynamics and moisture through the mid MS river
valley and into the Ohio River Valley.  Dew point temperatures
across the region were in the upper 60s to lower 70s with a very
humid feel ahead of the cold front draped across the Mid MS river
valley. Ongoing convection from the first round of showers and
thunderstorms was pushing into the I-75 corridor.  The environment
outside of ongoing convection is uncapped based on mesoanalysis
data, which increases concern for points west of I-65 where shear is
steadily increasing.

====================================================

The next round of Thunderstorms
Timeframe: 20z (4pm EDT) - 09z (5am EDT)
Confidence on severe potential: Medium

====================================================

The next potential round of strong to severe storms will come ahead
of a cold front to our west. Storms could start firing off by mid
afternoon and push eastward into the western CWA border starting
around 20-21z. HREF guidance progs an axis of ~70 degree dewpoints
to nudge in from the southwest ahead of the front, and with low
level lapse rates at their steepest levels due to peak heating, HREF
MUCAPE generally range from 2,000-3,000 J/KG. Strong deep layer
shear of 45-55kts will be co-located with this higher instability
axis, which will support organized convection that includes
supercells. Should severe storms develop, all severe hazards would
be possible.

Additional re-development along the front overnight as it slides
into the region remains possible. Confidence is low on the severe
potential with this activity, though, as the earlier afternoon and
evening storms could leave cold pools or stabilize the atmosphere in
their wake. Assuming the storms have access to an untapped or
`recovered` warm sector, all severe hazards would be possible with
severe activity.

It`s important to note that the severe weather environmental
parameters we`ll observe into the overnight hours are very unusual
for this region and something we typically see only a handful of
times per year. If we get severe supercellular storms, some will be
capable of producing very large hail in excess of 2 inches. Modest
low level shear and helicity in this moderately unstable environment
could result in a strong (+EF2) tornado as well.

&&

.LONG TERM /TONIGHT THROUGH TUESDAY/...
Issued at 130 PM EDT Tue May 7 2024

==== Significant Severe Weather Outbreak Possible Wednesday
afternoon-Wednesday Night ====

==== Local Flooding Possible Wednesday Night ====

Moderate to high confidence remains for a significant regional
severe weather event Wednesday afternoon into Wednesday night for
parts of the Ohio and Tennessee Valleys.

Synopsis: Closed upper low centered over the northern Plains
Wednesday morning will gradually work eastward towards the Upper
Mississippi Valley during the day as a weak shortwave ridge builds
over the Ohio Valley. At the surface, a quasi-stationary boundary
will be located just along the Ohio River as an elongated area of
sfc low pressure over the central plains slowly works towards
central IN by Thursday morning. As the sfc low works towards the
Ohio Valley, the stalled boundary will become a warm front with dew
points south of the boundary over southern IN and central KY in the
mid/upper 60s to even a few areas to the south near 70. As
temperatures warm into the low/mid 80s during the afternoon, the
atmosphere will quickly destabilize with SCAPE values ranging
between 1500-3000 J/kg. HREF ensemble soundings indicate 0-6km bulk
shear > 50 kt.

The main challenge will be timing and placement of initial
convection during the day Wednesday. While the setup for a
significant severe weather threat continues over the Ohio Valley as
indicated by the SPC enhanced outlook (3/5) over our entire county
warning area, there are a few elements that could potentially impact
the overall severe threat during the day Wednesday.

Hi-res models are in agreement of remnant boundaries associated with
Tuesday nights storms to be located across central TN Wednesday
morning. This boundary looks to be the focal point for convection
initiation during the day Wednesday. Timing and location of
convective activity Wednesday morning into the early afternoon could
play a large role in just how strong storms get later in the
afternoon and evening over central KY and southern IN. Some of the
hi-res convective allowing models want to develop convection over
far western KY while others start to develop activity over MO
Wednesday morning. Any cloud cover and potential outflow from this
activity arriving early in the day before maximum heating would
likely limit storm severity towards the afternoon. Another limiting
factor with early convection to our south and southwest early
Wednesday would be to limit the LLJ which is indicated by the 00z
HRRR and to some extent the NAM3K. Finally, placement of the stalled
boundary and its northern progression allowing for the CWA to be in
the deep warm moist sector behind increasing instability over the
area. A lot of the above mentioned factors will be determined by how
the storms Tuesday into Tuesday night unfold over the area. With all
that being said, all threats are still possible Wednesday afternoon
with initial cellular development before becoming more of a QLCS
ahead of an approaching cold front Wednesday night into Thursday
morning.

Tornado threat: Model sounding continue to indicate low-level
curvature on hodographs showing the potential for plenty of
streamwise vorticity ingestion by storms, with some eSTP values for
tornadoes >=EF2 Wednesday afternoon.

Hail threat: With forecast soundings showing large CAPE in the -10
to -30C hail growth zone. Strong deep layer shear and steep mid
level lapse rates keep large hail a possibility.

Wind threat: While we may not have a strong LLJ to work with, high
PWAT values leading to precipitation loading along with steep low-
level lapse rates increase the threat of strong damaging wind gusts.

Flash Flood threat: With PWAT values above 1.50" and numerous
clusters of storms Wednesday afternoon along with a line of storms
Wednesday night into Thursday moving through the area. Add to that
already wet ground from rains over the last couple of days will
increase the threat and concerned for localized flash flooding.
Especially in areas south of the Kentucky parkways that will see
heavy rain on Tuesday then again on Wednesday.

With the above mentioned limiting factors, confidence is higher for
linear severe storms associated with the approaching cold front than
the development of supercells during the afternoon and early evening
on Wednesday.

**Like any forecast, there are sources for potential bust. Any
convection that might get going Wednesday morning could decrease
instability for afternoon redevelopment, as would cloudiness being
more widespread, or stronger capping in our position well within the
warm sector. While a significant severe weather event is still a
possibility, this isn`t a done deal. Stay tuned to the latest
forecast information between now and Wednesday afternoon to keep
abreast of changes.**

Thursday - Monday:

Amplified pattern develops Thursday through the remainder of the
forecast period with a large ridge building to the west and deep
trough across the eastern US. This will keep occasional
shower/storms chances in the forecast thanks to a series of
shortwave disturbances moving through the northwest flow aloft.
Temperatures during this period will be below normal for early May
with highs in the 70s and even 60s on Friday.

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1258 AM EDT Wed May 8 2024

A broken line of showers and storms continues across southern
Indiana, southern Illinois, and western Kentucky this morning ahead
of a cold front. This activity should gradually push southward as
the cold front slowly sags into Kentucky, but eventually stall out
across southern Kentucky later this morning. How this boundary
evolves later in the day will be crucial to storm chances at TAF
sites, and at this current time, the BWG terminal will likely
experience several rounds of showers/storms leading to poor flying
conditions today.

Late this afternoon into the overnight hours, one or multiple lines
of storms are expected to push through the region ahead of a cold
front and will impact all TAF sites.

Outside of storms, VFR conditions are expected for most of the
forecast period.

&&

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

&&

$$

UPDATE...13/BTN
SHORT TERM...DM/MCK
LONG TERM...BTN
AVIATION...DM