767
FXUS63 KLMK 062357
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
757 PM EDT Mon May 6 2024

...Updated Aviation Discussion...

.KEY MESSAGES...

*   Shower and thunderstorm activity have started to decrease
    this evening. Still, isolated coverage remains possible
    across south-central Kentucky.

*   Potential for multiple rounds of strong to severe storms Tuesday
    through Thursday. All severe hazards will be possible Tuesday
    through Wednesday night. 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 TUESDAY NIGHT/...
Issued at 327 PM EDT Mon May 6 2024

Now through this Evening...

The shortwave trough and CVA are moving through the region supplying
ample lift to support convection. Showers and storms are developing
and growing in coverage currently and will continue until just after
sunset when instability dissipates. Around 1500 J/kg SBCAPE, ample
moisture advection into the region, and weak wind shear is
supporting some strong storms. Slow cloud level winds and storm
motions may lead to precip loading downpours, and as a result, gusty
winds and some hail. A limiting factor for these storms are weak mid-
level lapse rates.


Overnight through Tuesday Morning...

Convection will dissipate with fleeting diurnal conditions. The
shortwave trough will move off to the east of the region and weak
ridging will replace it, promoting drier conditions and thinner
skies. Southerly winds will allow for weak WAA into the region. Low
temperatures are expected to be similar to the previous night in the
low-mid 60s. Recent precipitation, light winds, and thinner skies
may lead to some thin patchy fog that will only last for a few hours
in the early morning before winds begin to pick back up.

A strong, stacked low pressure system over the northern Plains will
extend an occluded front to the triple point over northern Iowa. A
cold front will extend from this point into the mid-Mississippi
Valley and a warm front lifting north through the Ohio Valley. A
strong line of storms will surge ahead of the cold front. Guidance
trends have depicted the line of storms outpacing the cold front and
the greatest instability and dynamics, therefore, likely weakening
as it approaches the region. Strong moisture advection will steepen
lapse rates and provide modest instability for the remaining
dynamics to benefit from. With the weakening line, damaging winds
and quick spin-up tornadoes are the main threats. Most likely area
and timing are counties along the Ohio River and north around 12-
16Z.


Tuesday Afternoon through Tuesday Night...

Continued moisture advection into the region and an axis of
instability over the Mississippi River will allow for convective
initiation along this instability gradient. Single cells may develop
into supercells with an elevated supercell composite parameter and
well curved hodograph. All severe weather threats are possible for
Tuesday afternoon and into the overnight hours. Localized flooding
threat is also possible for storms tracking over the same area,
mostly due to the previous days rain. With a mature, stronger
supercell, significant hail and a strong tornado are possible.

In the late evening hours, the cold front will draw closer to the
Ohio Valley. Stronger forcing along the front may allow for upscale
growth into a squall line. As this line moves through the region,
instability will diminish and the cold front will weaken and slow,
resulting in a weakening QLCS as it moves through the region. This
line will likely exit the region before sunrise on Wednesday
morning.

&&

.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 /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 756 PM EDT Mon May 6 2024

Impacts/Confidence:
- Medium to high confidence in MVFR/IFR ceilings tonight
- High confidence in breezy southerly winds tomorrow
- Medium to high confidence in MVFR ceilings Tuesday morning
- Low to medium confidence in showers/storms Tuesday morning

Discussion...Current radar and satellite show decreasing cloud cover
and shower activity as most of the weather moves to the east. For
tonight, expect dry and mostly clear conditions to combine with
enough low-level moisture from recent heavy rainfall so that patchy
fog/low stratus can form after midnight. As a result, models
continue indicating MVFR/IFR ceilings with MVFR visibilities.
Ceilings will eventually improve close to sunrise as decaying line
storms approaches from the west and surface winds increase from the
south. Chances of morning showers, and even storms, are greatest for
the eastern half of the TAF area. VFR conditions will return during
the afternoon hours before another wave of storms move through late
Tuesday afternoon into the evening.

&&

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

&&

$$

SHORT TERM...SRM
LONG TERM...13
AVIATION...ALL