161
FXUS63 KLMK 070721
AFDLMK

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

...Updated Short Term Discussion...

.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.

*   Breezy Thursday with gusts of 30 to 40 mph possible.

*   Cooler 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 327 PM EDT Mon May 6 2024

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

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

Confidence continues to increase for a significant regional severe
weather event Wednesday afternoon into Wednesday night, with the
Ohio Valley in the bullseye.

A surface low will proceed from MO/IA mid day Wednesday to northern
IN/OH by sunrise Thursday, with a cold front dropping southwestward
to Texas. A strengthening low level jet from the western Gulf Coast
to the upper Ohio Valley will be in place ahead of the main 5H
trough over the Midwest and south of a couple of 120kt jet streaks
crossing the Great Lakes. The atmosphere is expected to destabilize
in the afternoon as temperatures rise slightly above convective
temperatures into the low and mid 80s with dew points in the mid
60s. ECMWF EFI is showing climatologically very unusual CAPE/Shear
values (for early May!) and LREF shows a 50-80% joint probability
of CAPE > 1000 J/kg and deep bulk shear > 40 kt.

Scattered storms are expected to develop Wednesday afternoon as the
atmosphere destabilizes with only weak capping currently showing up
on sounding progs. Old outflow from Tuesday evening storms and
differential heating boundaries may serve as focusing mechanisms.

Tornado threat: Increasing low-level curvature on hodographs showing
the potential for plenty of streamwise vorticity ingestion by
storms, and eSTP values are suggesting potential for tornadoes >=EF2
Wednesday afternoon. HREF showing a greater than 90% chance of LCL
heights below 1km.

Hail threat: Forecast soundings are showing wide CAPE in the -10 to
-30C hail growth zone with wet bulb zero heights of 7-10k`. Strong
deep layer shear of 60-70kt will be possible with steep mid level
lapse rates. Strong storm top venting will assist in supporting
robust updrafts.

Wind threat: In addition to the above, low level lapse rates will be
steep and strong precipitation loading in torrential downpours will
aid downdrafts.

Heavy rain: Model data are showing near record precipitable water
values of 1.5-1.9" and deep warm cloud depths getting pushed
northward between ridging over Florida and the cold front
approaching from the west. The best chance for flooding issues will
likely be Wednesday night, after several rounds of showers and
storms Monday-Wednesday and as the final round of intense convection
moves through ahead of the front. WPC has the entire region in a
Slight Risk on the Day 3 ERO.

Storms that erupt to the west Wednesday afternoon and evening closer
to the cold front may gel into a line as they approach the
MIssissippi River and move through southern Indiana and central
Kentucky overnight. Though these storms may become slightly elevated
and may start a weakening trend as they get east of I-65 after
midnight, all severe modes still appear possible. Confidence is
greater in this overnight line of storms than in the supercellular
afternoon activity. Nighttime storms and flooding are particularly
dangerous, so this should be taken seriously.

**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:

Occasional shortwaves will bring scattered shower/storm chances,
especially Thursday and Saturday, as disturbances dive down from
Canada around a gyre above northern Ontario and Hudson Bay. At this
time no severe weather is expected during this period, however. The
big story on Thursday will be windy conditions behind the departing
spring storm system, with gusts possibly reaching advisory criteria.
Friday will be breezy and cool with afternoon highs only in the 60s.
Temperatures will then moderate as we head through the weekend to
Monday.

&&

.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...13
AVIATION...DM