792
FXUS63 KLMK 070756
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
356 AM EDT Tue May 7 2024

.KEY MESSAGES...

*   Potential for multiple rounds of strong to severe storms today
    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 318 AM EDT Tue May 7 2024

The short term forecast discussion is broken down into two sections
that will handle the potential waves/rounds of strong to severe
storms today.

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

Round 1 Timing: 15z (11am EDT) - 20z (4pm EDT)
Confidence on severe potential: Low to Medium

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

The same line of storms that impacted portions of the central and
southern Plains yesterday will come racing toward us this morning,
but it will be nowhere near as potent. Convective allowing models
(CAMs) are in good agreement with a general weakening trend during
the morning hours as the line outraces the better shear and
instability that helped to sustain it. Most models prog what`s left
of the line to arrive on our western CWA border close to ~15z.
Guidance begins to diverge on how it may evolve in the afternoon
hours, with some CAMs suggesting isolated/scattered redevelopment
along the outflow boundary while others keep things fairly quiet.
Environmental parameters by early afternoon would certainly support
severe storms should they develop as stronger shear gradually builds
in from the west while instability increases from surface heating
(low level lapse rates steepening) and moisture advection from 30-
40kt LLJ. Severe storms would be capable of all severe hazards.
Guidance that does lean toward a `quieter` or drier scenario tend to
have a slightly stronger cap around 850-750mb that would impede
convective development, so that will be something worth monitoring
through the morning hours to see which models are `capturing` the
atmosphere better... and ultimately, give us, the forecaster, more
confidence in which scenario may play out.

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

Round 2
Timeframe: 20z (4pm EDT) - 09z (5am EDT)
Confidence on severe potential: Medium

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

The next potential (and much more concerning) 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.

Like this first round, there remains some uncertainty with respect
to convective initiation. Some models/CAMs keep initiation to a
minimum, and within those same models any convection that does
attempt to fire off struggles and remains unimpressive looking.
These models, unsurprisingly, have a slightly stronger capping
inversion in the 850-750mb layer. Other models/CAMs with weaker
capping inversions tend to result in strong convective cores
developing with severe potential. 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 (assuming they develop) 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 this afternoon 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
today, 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 354 AM 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 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 WEDNESDAY/...
Issued at 104 AM EDT Tue May 7 2024

Bit of a complicated, low confidence forecast for the TAF period.
Region is currently observing a wide variety of cigs, with heights
varying from +10kft generally west of I-65 to MVFR/IFR cigs toward
the I-75 corridor. Some clearing has also developed across parts of
the area, and within that clearing fog has developed in locations.
This may be short-lived though as a widespread area of low to mid
level clouds push in from the west.

Forecast is generally on the optimistic side of guidance based on
some of the latest observational and satellite trends. Think most
sites will hover above VFR levels, with the exception of LEX that
will likely bounce below VFR for a time this morning. Conditions
should quickly go to VFR levels at all sites a couple hours after of
sunrise as surface heating scours out any fog and raises cigs. A
broken band of showers/storms will push in from the west late this
morning or early afternoon, which is currently being handled with
VCTS/VCSH for time being given low confidence in coverage. Another
round of showers/storms will be possible again late afternoon into
the overnight hours ahead of a cold front.

&&

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

&&

$$

SHORT TERM...DM
LONG TERM...BTN
AVIATION...DM