The Palm Sunday Tornado Outbreak: Sixty years later

A quick web search for historical events that occurred on April 11 reveals that there are many reasons why this is a historic day:

• In 1968, U.S. President Lyndon B. Johnson signed the Civil Rights Act into law.
• In 1970, the Apollo 13 mission was launched from Kennedy Space Center.
• In 1976, the Apple I computer, created by Steve Wozniak, was released. 
• In 2012, The Avengers movie premiered.

However, what your search might not have listed is that in the U.S., April 11 is in the history books for an event of atmospheric violence. The Palm Sunday tornado outbreak on April 11, 1965, was a devastating series of severe thunderstorms that spawned numerous tornadoes striking the U.S. Midwestern states of Illinois, Indiana, Iowa, Michigan, Ohio, and Wisconsin.

The outbreak produced 55 confirmed tornadoes, 18 of which were of F4 intensity (with estimated winds of 207 to 260 miles per hour) resulting in approximately 270 fatalities and over 3,660 injuries. It is one of the deadliest tornado outbreaks in U.S. history, with the state of Indiana suffering the highest death toll, with 141 fatalities.

The financial damage in 1965 was estimated at US$250 million. Taking into account population changes and exposure growth, the loss today could approach or even surpass US$10 billion.

Figure 1: Location in Branch County, MI, impacted by the Palm Sunday tornadoes in 1965: Image credit: National Weather Service - Northern Indiana. https://www.weather.gov/iwx/1965_palmsunday_50

How does this event rank alongside other similar events? Well-known tornado expert Thomas P. Grazulis who has written some of the definitive historical catalog books on U.S. tornadoes, created the Outbreak Intensity Score (OIS) in 2023 to rank various tornado outbreaks. The 1965 Palm Sunday tornado outbreak ranked as the fourth–worst in recorded history.

In my blog, I provide a deeper dive into the atmospheric conditions that led to an explosion of thunderstorms and tornadoes in 1965, how the outbreak helped advance society’s preparation for these types of events, and what would happen if a similar outbreak were to occur today.
 

A unique atmospheric setup led to numerous violent tornadoes

The Palm Sunday tornado outbreak was driven by a combination of meteorological factors, creating an environment that was highly conducive to severe thunderstorms and tornado formation. Setting the stage was a vigorous low-pressure system over the northeastern High Plains that coupled with a strong jet stream to bring widespread atmospheric instability into the Midwest.

This strong jet stream and its accompanying winds were strong enough to carry topsoil from Illinois and Missouri eastward, producing hazy skies before the storms formed. The accompanying map shows the tornadoes that occurred on April 11, 1965, and their tracks (red lines).

Figure 2: Palm Sunday 1965 tornado tracks. Courtesy: Dr. Russell Schneider, Storm Prediction Center

On April 10, 1965, this combination of factors produced a localized tornado outbreak from the Kansas-Missouri border into central Arkansas, spawning an F4 tornado that devastated the town of Conway, Arkansas, killing six people and injuring 200 others.

A warm front surged northeastward from Iowa into Wisconsin and Michigan, interacting with an upper-atmospheric disturbance to produce isolated supercell thunderstorms across the warm sector of the storm system in eastern Illinois, southeast Wisconsin, southern Lower Michigan, Indiana, and Ohio.

The combination of very warm and humid conditions moving northward via a strong low-level jet stream, powerful vertical wind shear, and the strong upper-level disturbance all contributed to these supercells becoming long-lived and producing numerous violent tornadoes during their life cycle. 

Figure 3: Multi-vortex tornado near Dunlap, IN, on April 11, 1965. Image credit: Paul Huffman, Elkhart Truth.

Several tornadoes developed near the warm front, with 18 becoming powerful F4 tornadoes. The first F4 passed dangerously close to Dubuque, Iowa, and traveled 40 miles into southwest Wisconsin, killing one person. Another F4 tornado ravaged McHenry and Lake County in northern Illinois, killing six people. An additional F4 tornado formed as the warm front passed through southern Lower Michigan near Grand Rapids, killing five and injuring 142 others.

Ohio and Indiana bore the brunt of the outbreak, with the majority of the strong-to-violent tornadoes occurring across these two states. The most famous tornado of the outbreak, shown in the picture above, was the Indiana twin vortex tornado affecting the neighboring towns of Dunlap, Midway, and Northern Goshen.

This iconic tornado struck a trailer park early evening near Midway, Indiana. It became powerful enough that it split into two very large vortexes rotating around a central point, causing one solid damage track. The tornado was responsible for 31 deaths and 252 injuries. No more than an hour later, another twin-funnel tornado then ravaged sections of Dunlap and Goshen, claiming an additional 36 lives and injuring over 320 others. This tornado may have been the strongest of the day, with some experts assigning an F5 level with estimated winds over 260 miles per hour. 
 

Event impact: Early warning system improvements

While the outbreak produced a high number of injuries and casualties and destroyed several towns in the Midwest, the outbreak also led to several significant changes in the way the population at large is warned about impending severe weather, many of which are still being used today.

Figure 4: SKYWARN is a volunteer program with between 350,000 and 400,000 trained severe weather spotters who help keep their local communities safe by providing timely and accurate reports of severe weather to the U.S. National Weather Service.

Because of the high death toll from the event, the U.S. National Weather Service (NWS) was motivated to reassess the overall severe convective storm warning system. While the post-event assessment determined that the Severe Local Storms Unit (SELS) provided adequate warnings, the public’s knowledge of the warnings was lacking.

The assessment results led to the NWS creating the Natural Disaster Warning System (NADWARN) division to coordinate disaster and emergency management duties among various agencies. This division eventually became known as SKYWARN, a network of storm-spotting trained meteorologists to assist the NWS with ground truth during severe weather events. 

With a lack of public awareness and clarity about severe weather threats, one of the most notable developments stemming from the Palm Sunday outbreak was the establishment in 1966 of the terms 'tornado watch' and 'tornado warning'. Due to the belief that panic would ensue if tornado warnings were broadcast over radio and television, from the 1950s to the time of the outbreak, this type of wording was shunned, though its usage would prove pivotal in improving communication and reducing confusion during tornado events.

The outbreak also pioneered the activation of civil defense sirens during tornado warnings. The sirens would help the public take action on the impending threatening weather if they were not in a location where they could receive tornado warning information via radio or television.  

Dr. Tetsuya Theodore 'Ted' Fujita, who discovered suction vortexes — small, intense mini-tornadoes within a main tornado — was also the first to study the Palm Sunday outbreak using aerial imagery, with his work going on to revolutionize the understanding of tornado dynamics.
 

What if the Palm Sunday outbreak happened today?

Over the last 60 years since the 1965 tornado outbreak, the region ravaged by the tornadoes has seen dramatic growth in terms of population and wealth. In many cases, urban sprawl has merged some previously disconnected towns, and housing and contents prices have increased significantly. 

Comparing the six states affected by the Palm Sunday outbreak since 1965, population growth and the number of residential structures have increased across the states. The table below compares the population and housing counts for each state in 1960 and 2020, and the overall growth for each. 

The total population of the six states climbed by nearly 30 percent within sixty years, with the housing count also increasing by approximately 78 percent.

Taking these and inflationary factors into account, and applying them to the US$250 million estimated 1965 loss, factors such as exposure concentration changes, social inflation, exact location of tornado swaths, and replacement costs driven by material and labor expenditure volatility, could see losses in 2025 for a comparable 1965 Palm Sunday tornado outbreak approach or potentially exceed US$10 billion.

Today's loss level would place it in the upper echelon of loss-producing events, alongside the Lower Mississippi Valley super outbreak in April 2011 and the August 2020 Midwest derecho. There is, however, much uncertainty in this estimate due to the parameters listed above.

Moody’s set to change the severe convective storm modeling landscape in 2025

Severe convective storm events like the 1965 Palm Sunday outbreak have a significant impact on society, the insurance industry, and financial markets, with large-scale uncertainty in terms of losses. Large-scale tornado events present challenges to recovering local economies, due to potential labor shortages, raw material shortages, short-term material price fluctuations, and business interruption from damaged manufacturing facilities.

Insurers must plan carefully for such events to ensure they have their claims-handling practices in place including an appropriate number of adjusters to deal with the volume of requests. They also need to plan for the size of claim payment and have claims-paying abilities so policyholders can access funds to repair and replace damaged facilities, provide housing as quickly as possible, and build back to make properties more resilient to hail, wind, severe thunderstorms, and tornadoes.

This summer, we will launch our new Moody’s RMS™ U.S. Severe Convective Storm high-definition (HD) model for insurers, reinsurers, and brokers to help build a more resilient and sustainable marketplace. This model will feature a dramatically expanded event set, including large and long-tracked tornadoes and outbreaks as experienced on April 11, 1965.

Our model will not only provide a more comprehensive understanding of tornado loss severity but also improve modeling of the other two convective storm sub-perils—namely hail and straight-line winds—to reflect today’s loss occurrences and allow for more informed decisions on capital allocation and exposure management for this frequent peril. 

For more information about the upcoming model, please email me here. We look forward to continuing to support your risk management needs with our market-leading solutions.