574
FXUS63 KLMK 071803
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
203 PM EDT Tue May 7 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.

&&

.SHORT TERM /THROUGH TONIGHT/...
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 /WEDNESDAY THROUGH MONDAY/...
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 /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 102 PM EDT Tue May 7 2024

Multiple rounds of showers and storms are anticipated today with
MVFR and brief IFR in strong downpours possible. The latest high
resolution guidance as scattered thunderstorms developing in the
early to mid afternoon hours today and becoming more
widespread/linear after 0z tonight. Any terminal that
experiences a shower or storm today could see brief gusty winds,
hail, reduced cigs/vis, and possible wind shear at 2K feet from
the southwest around 40kts.

&&

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

&&

$$

SHORT TERM...DM/MK
LONG TERM...BTN/MK
AVIATION...MK