Severe storms have set a fast pace for the first half of 2024. Will the interplay of increasing hazards and climate change lead to another year of high damage from convective storms?

Summer is not even in full swing in the United States yet, and it has already been a year of extremely severe convective storms.¹ So far, 2024 has the second highest number of tornado reports (1,264) of any year in the last 15 years. We also saw a very high number of hailstorms (4,108 – the fifth-most frequent year) and destructive storms (8,978 – the second-most frequent year). Fortunately, this year is still well behind the pace of 2011, which remains the worst year for all three hazards in the United States.

The total number of storms across the country is not a perfect indicator of economic or insured losses. In 2011 and 2023, high numbers of wind and hail storms added up to the highest direct loss years in recent memory (Figure 1). Although 2020 was a typical year in terms of the number of tornadoes and storms (and with a below-average number of hailstorms), economic and insured losses were still high. However, since most destructive storms occur in the summer, our early start to the severe weather season could mean another costly year of high losses from convective storms.

Previous severe storms in 2024

This year’s most powerful storms have been associated with tornado “outbreaks,” which occur when multiple tornadoes emerge from the same large-scale weather pattern within one or more days. From April 26 to 28, severe weather over the Southern Plains and High Plains produced more than 140 tornadoes, including a violent tornado (EF4 intensity) that destroyed a Dollar Tree distribution warehouse in Marietta, Oklahoma. In another major outbreak from May 6 to 10, more than 160 tornadoes struck Oklahoma, Kansas, Tennessee, Alabama, Georgia, and Michigan. This system was so destructive that several National Weather Service offices took the unusual step of declaring tornado “emergencies” (situations in which a serious threat to human life is imminent or ongoing). Finally, the Midwest experienced a prolonged period of tornado-like activity from May 19 to May 27, culminating in a large, violent EF4 tornado that devastated Greenfield, Iowa, killing at least five people.

But wind speed is not the only factor that determines whether extreme weather causes major damage. The derecho that struck the Gulf Coast on May 16 and 17 had peak wind speeds of just over 100 miles per hour (160 kilometers per hour), equivalent to a moderate-strength tornado (EF1). However, because this system passed directly through downtown Houston, it caused widespread and significant damage. The derecho is considered the most destructive wind event to hit the city in a generation, blowing out windows from many high-rise buildings, damaging power lines, and causing the deaths of at least eight people.

The subtle hand of climate change?

Based on what we know about how severe convective storms form and spread, we might expect that a warmer world will also be stormier. Climate model simulations suggest that the United States will experience stronger updrafts under various warming scenarios, which would lead to more favorable conditions for the formation of severe thunderstorms.² But in its latest report, the Intergovernmental Panel on Climate Change finds that past trends in hail, wind, and tornado activity provide little confidence in the evidence. Why is it so hard to say whether severe convective storms have become stronger or more frequent?

One reason we struggle to understand long-term trends in tornadoes, hailstorms, and destructive storms is that the most severe storms are rare. The United States is affected by more tornadoes than any other country in the world. However, since 1950, only 59 tornadoes have been officially classified as catastrophic (EF5)—less than one per year. Moreover, more than a decade has passed since the last EF5 event, the 2013 Moore tornado that destroyed over 1,100 homes in Oklahoma. Fortunately, the most severe storms are rare, but their small number makes it difficult to determine whether or not they are becoming more common using standard statistical approaches.

To tease out subtle but important changes in the risk of severe convective storms, WTW partnered with Columbia University’s Department of Applied Physics and Applied Mathematics to study current and future trends in tornado behavior. In a study recently published in the Monthly Weather Review, our partners Dr. Kelsey Malloy and Prof. Michael Tippett developed a new risk index to calculate the probability of a tornado outbreak based on large-scale weather patterns.³By combining their index with local storm reports, they were able to demonstrate that tornadoes now occur more than twice as frequently in parts of Alabama, Georgia, Missouri and Mississippi than they did in the early 1980s.

Tornado risk solutions from WTW

By leveraging the latest science in our risk models and risk consulting, WTW’s Climate Practice clients benefit from advanced, quantitative insights that support a smarter, forward-thinking approach to managing tornado risk – both for their own assets and across the value chain. Starting with an initial tornado hazard assessment using Global Peril Diagnostic, WTW’s cross-disciplinary solutions – spanning risk engineering, climate science and enterprise risk management – are designed to provide clients with the insights they need to make risk management decisions that increase their resilience.

Footnotes

1 NOAA Storm Prediction Center. Page with severe weather maps, graphics and data. (2024).

2 Brooks, HE Severe thunderstorms and climate change. Atmospheric Research 123, 129-138 (2013).

3 Malloy, K, Tippett, ME A stochastic statistical model for outbreak-level tornado occurrence in the United States based on the large-scale environment. Monthly Weather Review 152 (5), 1141-1161.

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