947
FXUS63 KLMK 260158
AFDLMK

Area Forecast Discussion
National Weather Service Louisville KY
958 PM EDT Sat May 25 2024

.KEY MESSAGES...

*   Severe storms possible Sunday into Sunday night. Main hazard is
    damaging winds with localized hurricane-force gusts possible.
    Tornadoes, large hail, and flooding cannot be ruled out either.

*   Unsettled pattern will continue into Monday with additional
    shower and thunderstorm chances.

&&

.UPDATE...
Issued at 958 PM EDT Sat May 25 2024

Overall, a fairly quiet evening was in progress across southern
Indiana and central Kentucky.  Only convection was a small storm
over Bourbon/Nicholas counties which will continue to move off to
the east over the next hour.  Appears that we had a boundary that
dropped southward across northern KY this evening.  It was
detectable by radar and winds shifted to the north behind the
boundary.  Dewpoints on either side of the boundary were not all
that different, but many sites saw a small drop in temperatures as
the boundary slid through.

For the overnight period, it looks to be a quiet night as we wait
for convection from the Plains and lower MO valley to move in from
the west.  That looks to occur well after sunrise tomorrow.  We`ll
see some mid-high level cloudiness spread in from the west as that
convection approaches.  Lows tonight will be in the mid-upper 60s.

Remainder of the forecast remains generally unchanged for Sunday. It
appears that we`ll be seeing a couple of lines of storms coming
across the region.  The first looks to come through mid-late morning
tomorrow with a second line coming through Sunday evening.  The
first line will have some instability to work with and will be
capable of producing heavy rainfall, damaging winds, and perhaps
some marginal hail.  Uncertainty increases after that line goes
through as it is not overly clear how much instability will be taken
out by the first line and if the atmosphere will recover.  Model
soundings do show substantial reductions in SBCAPE and MLCAPE.
However, we may have a bit of elevated CAPE to deal with in the mid-
late afternoon that could support elevated convection ahead of the
second line (like the 18Z HRRR suggests).  The second line will then
surge through here in the mid-late evening with a damaging wind
threat.

There will likely be continued shifts in the CAM guidance tonight
and into early Sunday as they start to better initialize on what the
convection across MO does overnight.  We`ll continue to monitor and
evaluate that data overnight.  Folks should continue to stay
vigilant and be prepared for an active weather day across the region
tomorrow.

&&

.SHORT TERM /THROUGH SUNDAY NIGHT/...
Issued at 348 PM EDT Sat May 25 2024

We continue to focus on the severe weather potential for tomorrow
which continues to transition as it comes into better focus. A few
days ago when global models were the only models to have Sunday in
range, hodographs looked ominous. There was a lot of synoptic
forcing, tons of instability, and high levels of shear at all
levels. Tornadoes looked favorable. Then, as the NAM picked up on
Sunday, it showed one or two line segments extending south out of
central Indiana into southern Indiana and north central Kentucky,
but it wasn`t too impressive. Now that the CAMs have had multiple
runs that have captured tomorrow`s events, the forecast continues to
get fine tuned. Every model doesn`t agree on the exact timing, but
the general idea is pretty strong among the models.

Tonight, upper flow is mostly zonal. There is a little bit of a
trough over the Southern Rockies and a little ridging over New
England. Extending south from near Chicago, surface high pressure
extends through Kentucky while a surface low sits over western
Kansas, and as upper flow begins to push the surface low east to
east-northeast, the system is expected to send a line of convection
over the Plains. By Sunrise, the line is expected to be crossing the
Ozarks and Midwest as it surges towards the Lower Ohio Valley.

Around 15-16z Sunday morning, the line is expected to begin
impacting the western edge of the CWA. As it will be late morning,
there could still be some low level stability. Low level lapse rates
are marginal in the 7.5 C/km range, but the main driving force at
this time will be the system`s cold pool. After the line has
traveled all night, the cold pool is expected to be ahead of the
main line. This could help to decay the line as it moves west to
east over the CWA, but as it begins to enter areas on the the
eastern side of the CWA, including the Bluegrass and Lake Cumberland
regions, some afternoon heating could help increase lapse rates to
8.5 C/km or more. This could help to intensify the line into the
later afternoon. The line is expected to be near Louisville around
17-18z and east of the CWA by around 20-21z.

The main thing this line has working for it is instability. MLCAPE
values ahead of the line will begin in the 2,000 J/kg range and
increase to around 2,500 J/kg over southern Indiana and up to 2,700
J/kg over southern Kentucky. MUCAPE values will begin around 2,500
J/kg and increase to near 3,300 J/kg. Low level shear is only
marginal with 0-1km SRH around 90. A tornado can`t be ruled out, but
chances are fairly low. The main threat will be gusty winds from the
storms downdrafts. With the high levels of instability, the
strongest cores could also produce hail. Another concern will be
flash flooding. With all the rain we have had lately, mainly over
southern Kentucky, flash flood guidance is low. Don`t believe the
north to south oriented line will cause too many flooding issues as
the dwell time over any one spot will be limited. There is a chance,
mainly over southern Kentucky. That some storms could develop behind
the line and drop to the south, so if any location gets multiple
rounds or sees training storms over the same area, flash flooding
will likely occur.

With the first line of convection outrunning the surface low, a
second line is expected to develop early tomorrow night just ahead
of the low. It could begin to enter our Indiana counties by around
4z Monday, and Kentucky counties an hour or two later. This second
line won`t be as mature as the first line, so the convection should
stay on the leading edge of the convection which could help overcome
the near surface inversion given the time of day. It also looks like
instability could be more limited, but low level shear will be much
higher with this second wave. 0-1km SRH will approach 400 under a 50
knot low level jet. The high LCLs above 1km and stable low levels
are reasons against tornado development, but strong low level shear
in a convective environment could help to produce a spin-up. Believe
the chances for a tornado will be greater in this second line
compared to the first line, but gusty winds will be the main threat.
The threat of hail will be less in the second line given the much
lower instability. The line will likely begin to dissipate before
the line exits the CWA by at least 9z.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
Issued at 348 PM EDT Sat May 25 2024

Monday through Tuesday Night...

Surface cold front is still forecast to push through the region
during the day on Monday which will bring an end to the severe
threat locally.  Now the upper level trough axis will remain across
the region, so some convective redevelopment is likely during the
afternoon across the region.  Some stronger storms can`t be ruled
out across our east during the afternoon.  Highs on the day will
range from the upper 70s to the lower 80s.  Breezy conditions will
be seen as well on Monday with sustained winds of 10-15 mph with
occasional gusts to 20-25 mph at times. Drier air will filter into
the region by Monday night and dry/tranquil weather is expected
through the remainder of this forecast period.  Lows Monday night
will be upper 50s to around 60.  Highs on Tuesday will be in the
upper 70s to the lower 80s with overnight lows in the mid-upper 50s.

Wednesday through Saturday...

Upper trough axis will push east of the region by Wednesday and
heights will gradually build into the day on Thursday.  Toward the
end of the week, the pattern looks to become a bit more blocked as
the east coast trough closes off and becomes a closed low just south
of Maine.  Another shortwave trough axis will be pushing through the
Plains, while the Ohio Valley remains in a sharp but narrow ridge
axis.  Overall, the pattern for Wednesday through Friday supports
dry weather conditions.  Within the southwest flow by Saturday, we
could see some isolated/scattered storms fire, though the best
potential for rainfall looks to hold off until late Saturday night
and early Sunday.

Highs Wednesday and Thursday will be in the upper 70s to near 80,
with overnight lows in the low-mid 50s.  A few of our valley
locations could see temps dip into the upper 40s each night.
Temperatures will warm a bit by Friday as readings rise into the
upper 70s to the lower 80s.  Mainly lower 80s are expected by
Saturday.

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 747 PM EDT Sat May 25 2024

Impacts/Confidence:
- Medium to high confidence in MVFR VIS at LEX/RGA tonight
- Medium confidence in timing and impacts of heavy rain and storms
  late tomorrow morning into the afternoon
- Medium to high confidence in gusty southerly winds tomorrow

Discussion...Surface cold front has stalled across northern Kentucky
this evening with surface high pressure moving eastward along the
Great Lakes. Radar shows isolated, weak showers moving towards LEX
and RGA in the coming hours, but the loss of daytime heating and
sporadic coverage results in very low confidence to mention any
impacts. For tonight, winds will change to the south as the a
surface low pressure ejects from the Central Plains to the Midwest
and the stalled front lifts to the north. In addition, model
guidance still indicates MVFR VIS and few/sct low ceilings for the
Bluegrass region around 8Z which goes in line with the lifting
frontal boundary pattern.

For tomorrow, two waves of heavy rain and potentially strong storms
are anticipated related related to the aforementioned surface low
and attendant cold front. The first wave will arrive from west to
east late in the morning and continue through the early to mid
afternoon hours. Although confidence in CAM models have increased,
there is still some timing uncertainties, so expect the onset of
strong storms to suffer slight modifications in future updates.
Nonetheless, MVFR to IFR visibilities will be common as well as
strong wind gusts (30+ kts) and a non-zero chance of severe
weather. Gusty southerly winds around 15 to 20 knots will continue
after the first round of unsettled weather.

Extended Outlook...The second wave of heavy rain and strong storms
will arrive probably after 27/0Z as the cold front races thorugh the
region.

&&

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

&&

$$

UPDATE.......MJ
SHORT TERM...KDW
LONG TERM....MJ
AVIATION.....ALL