Tornadoes and Flash Flood in Indiana, April 2 to 4, 2025

Area Forecast Discussion
National Weather Service Louisville KY
1005 AM EDT Wed Apr 2 2025

.KEY MESSAGES...

* SIGNIFICANT SEVERE STORM THREAT THIS EVENING INTO TONIGHT

* LIFE-THREATENING FLOODING POSSIBLE FROM TONIGHT INTO THE WEEKEND,
  WITH ADDITIONAL CHANCES FOR STRONG TO SEVERE STORMS LATER THIS WEEK

&&

.UPDATE...
Issued at 1005 AM EDT Wed Apr 2 2025

Latest surface analysis places a 990 mb low over KS/MO border with a
warm front extending E-SE through western KY and a cold front
stretching SSW into TX. As the warm front lifts northward throughout
the day, expect temperatures to increase into the low 80s and dew
points to surge into the low 60s. As the low tracks NE toward the
Great Lakes today, the cold front will bring severe weather to our
area this evening. Depending on how upstream convection evolves
throughout the day, a line of severe thunderstorms or a string of
supercells will move into the OH Valley, entering western KY and our
southern IN counties as early as 7PM EDT. The line of storms will
slow down as it enters the OH Valley, complicating the time frame
for severe weather, but expect severe potential to extend into the
early morning hours of Thursday, roughly ending around 4 AM EDT. The
cold front will stall over the region, but strong SW flow marked by
a 55-60kt LLJ, will result in continued moderate to heavy rain
overnight and into the day. All hazards are possible with this storm
system, including damaging straight line winds, large hail, and
tornadoes. Flooding will also be a major concern as storms
potentially train along the aforementioned stalled boundary; flood
hazards will include flash flooding, areal flooding, and river
flooding.

Have updated the forecast to include stronger wording of hazards in
WX grids. Products have been updated.

Please take preparations now as this has potential to be an historic
flooding event.

&&

.SHORT TERM /THROUGH TONIGHT/...
Issued at 355 AM EDT Wed Apr 2 2025

****SIGNIFICANT SEVERE STORM THREAT THIS EVENING****

Now Through This Afternoon...

High pressure continues to move off to the northeast and the low
pressure is strengthening over the central Plains at this hour.
These features will allow winds to veer to the south through the
morning hours. A strong 60-65kt LLJ will move over the region ahead
of the low pressure system. This LLJ coupled with scattered to
broken skies will allow for mixing to bring these LLJ winds down to
the surface via momentum transfer. By mid-late morning and through
sunset, winds will be sustained 20-30 mph and gusting 40-50mph.

Through the day, the warm front will lift north over the region.
This will allow for temperatures to increase into the low-mid 80s.
Strong moisture transport along the strong LLJ will increase dew
points from the mid 30s into the low 60s.

With stout capping and better forcing to the west, thunderstorm
activity should be limited through day today.

This Evening Through the Overnight...

The low pressure system will continue to move northeastward into the
Great Lakes region. The associated cold front will move east over
the Mid-MS Valley and into the lower OH Valley. As the front
approaches the region, the forward motion will slow overnight.

A strong mid-level trough will move through the upper mid west,
shedding plenty of vorticity over the Ohio Valley this evening and
tonight. A strong mid level jet with strengthen and will place the
mid-MS valley and western OH Valley under the right entrance region.
These features will allow for ample large scale lift over a region
of plentiful moisture. PWATs will be around 1.75 inches, which is
the max of climatology.

With the region remaining dry through the day today and undisturbed,
instability will build. MLCAPE is expected to be between 1200 and
1800 J/kg.

All of these ingredients will be conducive for discrete supercells to
develop over the mid-MS Valley. These storms will then push east
along the cold front. As these storms move east, they are expected
to slow and grow upscale into a QLCS with embedded supercells, with
the potential for bowing segments. As these storms move east, they
will move into the strong LLJ which will increase low-level shear
and increase the tornado potential. All hazards will be expected,
with strong and potentially long-track tornadoes possible. The hail
threat is greatest over southern Indiana and west of I-65.

Additionally, there is a concern for flash flooding, as the line
will slow and strong storms will train over the same areas.

Timing: The line will most likely enter southern Indiana around or
after 0Z, reaching the Louisville Metro around 4Z, and stalling
after 7Z over the Bluegrass and southeastern areas.

Confidence: Confidence is high that southern Indiana, northern
Kentucky, and west-central Kentucky will see severe conditions.
Confidence significantly decreases east of these areas, giving
waning instability and the stalling cold front.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Issued at 355 AM EDT Wed Apr 2 2025

**** MAJOR FLOODING (POTENTIALLY LIFE THREATENING) EXPECTED****

Thursday and Friday...

As the long term forecast period begins Thursday morning, convection
should be ongoing as scattered showers and thunderstorms align from
southwest to northeast across the region. During the morning hours,
there should be a relative minimum in the severe threat given the
diurnal minimum in instability within the warm sector. However,
during the day on Thursday, and as subsequent rounds of showers and
storms move across the region through Sunday morning, the flash
flooding threat will build as "training" of storms occurs.

The environment will be primed for heavy rainfall with precipitable
water values between 1.50-1.75" along the quasi-stationary surface
front. While some of the lift for showers and thunderstorms Thursday
and Friday will be due to instability and convective processes, as
the sfc front and upper-level flow pattern stretches out from west-
southwest to east-northeast, isentropic uplift will become
increasingly favored as southerly low-level flow is forced up and
over a near-sfc cool, stable air mass where rain has fallen. An
enhancement in the shower/storm coverage is likely Thursday evening
into Friday morning as a mid-level shortwave slides to the NW of the
area. This may help to trigger a more organized MCS, or just greater
coverage of clusters of cells. At this time, global ensembles paint
the heaviest swath of rainfall during this period between the
Kentucky Parkways and the Ohio River, with the ECMWF Extreme
Forecast Index of >0.9 and a shift of tails of 2 in this zone,
indicating high confidence in very heavy rainfall. Ensemble mean 24-
hour precipitation amounts are generally between 1.5-3" in this
heaviest corridor, though reasonable high end amounts (which would
be likely where training of storms occurs) is between 4-5".
Significant flash flooding would be very likely wherever this
heaviest corridor sets up.

As far as the severe potential for Thursday and Friday is concerned,
confidence is a bit lower than for heavy rainfall, and will be
dependent on how storms evolve in preceding waves. To the south of
the front, persistent southerly flow and rich moisture advection
will supply plenty of instability, with 1000-1500 J/kg of SBCAPE in
medium-range progs. However, if you get too far away from the front,
you will lose the better forcing from the low-level boundary and
stronger flow aloft. Farther to the north within the core of
precipitation, cool air near the surface will eliminate sfc-based
instability and substantially curtail the overall severe threat.
There should be a 50-100 mile wide corridor on the southern edge of
the precipitation shield which would have the highest potential for
severe weather given the combination of instability and forcing. All
severe hazards would be possible, although a more unidirectional
shear profile may limit the tornado potential slightly.

Saturday and Sunday...

A positive trend in guidance in the last 24-36 hours shows the upper
ridge over the southeast U.S. building slightly back to the
northwest Friday afternoon into Saturday morning. This could help to
shift the "train tracks" of heavy rain into western KY and central
IN as the sfc front lifts northward as a warm front, giving us a 12-
24 hour window of relatively drier conditions, especially south of
the Ohio River. This trend can be seen in ensemble indices, as the
strongest PW anomalies and QPF amounts lift toward the I-70 corridor
Saturday morning.

Unfortunately, this trend will reverse during the day on Saturday as
the upper trough to our west finally gets picked up by a northern
stream disturbance and begins to eject off to the northeast. This
should lead to one final push of strong storms and heavy rainfall
Saturday afternoon into Sunday morning, with the sfc front finally
crossing through the area on Sunday. Ahead of the front, moisture
indices like PW and IVT once again surge to near the maximum of
climatology, with several more inches of rain likely with this final
wave. Rain should end from west to east during the day on Sunday,
with cooler air rushing in for the end of the weekend and the start
of next week.

Early Next Week...

Much quieter weather is expected during at least the first half of
next week, with a much-needed reprieve from heavy rainfall and
severe storm chances. The upper flow pattern over North America will
be rather amplified, but will take on an inverse phase with deep
ridging out west and troughing across the east. This should place
our region within a NW flow regime aloft, and sfc high pressure will
gradually sink into the region by the middle of next week. This flow
regime will be supportive of cold and dry air advection, with
ensemble mean PW and temperatures below climatological normals.
Light precipitation from transient clipper systems can't be ruled
out, and we'll have to watch for potential frost/freeze conditions;
nevertheless, this pattern shift will support a much calmer period
of weather.

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 712 AM EDT Wed Apr 2 2025

VFR conditions expected through the morning hours. LLWS will
continue for a few hours after sunrise.

Warm front lifts northward through the area early this afternoon,
with MVFR stratus for a few hours over SDF and HNB. Others remain
VFR as winds pick up out of the south. Gusts in the 37-42kt range
will pick up in the afternoon and last through sunset.

Strong to severe storms move in later in the evening and into the
overnight. Brief IFR vis in the heaviest rain, and gusty winds.

&&

.LMK WATCHES/WARNINGS/ADVISORIES...
KY...Wind Advisory until 1 AM EDT /midnight CDT/ Thursday for
     KYZ023>043-045>049-053>057-061>067-070>078-081-082.
     Flood Watch from 8 PM EDT /7 PM CDT/ this evening through
     Sunday morning for KYZ023>043-045>049-053>057-061>067-
     070>078-081-082.
IN...Wind Advisory until 1 AM EDT /midnight CDT/ Thursday for
     INZ076>079-083-084-089>092.
     Flood Watch from 8 PM EDT /7 PM CDT/ this evening through
     Sunday morning for INZ076>079-083-084-089>092.

&&

UPDATE...CG
SHORT TERM...SRW
LONG TERM...CSG
AVIATION...SRW

Hazardous Weather Outlook
National Weather Service Louisville KY
409 AM EDT Wed Apr 2 2025

INZ076>079-083-084-089>092-KYZ023>043-045>049-053>057-061>067-
070>078-081-082-030815-
Orange-Washington IN-Scott IN-Jefferson IN-Dubois-Crawford-Perry-
Harrison IN-Floyd-Clark IN-Hancock-Breckinridge-Meade-Ohio-Grayson-
Hardin-Bullitt-Jefferson-Oldham-Trimble-Henry-Shelby-Franklin-
Scott KY-Harrison KY-Spencer-Anderson-Woodford-Fayette-Bourbon-
Nicholas-Nelson-Washington KY-Mercer-Jessamine-Clark KY-Larue-Marion-
Boyle-Garrard-Madison-Butler-Edmonson-Hart-Green-Taylor-Casey-
Lincoln-Logan-Warren-Simpson-Allen-Barren-Monroe-Metcalfe-Adair-
Russell-Cumberland-Clinton-
409 AM EDT Wed Apr 2 2025 /309 AM CDT Wed Apr 2 2025/

...WIND ADVISORY REMAINS IN EFFECT FROM 8 AM EDT  /7 AM CDT/ THIS
MORNING TO 1 AM EDT  /MIDNIGHT CDT/ THURSDAY...
...FLOOD WATCH REMAINS IN EFFECT FROM THIS EVENING THROUGH SUNDAY
MORNING...

This Hazardous Weather Outlook is for portions of southern Indiana
and central Kentucky.

.DAY ONE...Today and tonight.

Strong non-thunderstorm winds are expected today, with gusts of 40
to 50 mph expected.

Severe storms are expected this evening through most of the overnight.
Damaging winds, tornadoes, and hail are expected. Strong tornadoes
are possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Additional showers and strong storms are expected to continue from Thursday
into the weekend. There is considerable concern for a high-impact,
major rainfall and flooding event across the lower Ohio and Tennessee
Valleys from Wednesday night through the weekend. Widespread minor
to moderate river flooding is expected, and major river flooding is
possible.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will likely be needed this evening and tonight for
severe storms and again through the weekend for flash flooding
reports.

&&

More information, along with other weather, hydrological and
climate information can be found, at weather.gov/louisville.

SRW/CSG