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Extreme Weather Toolkit: Severe Weather

CM: Billion Dollar Severe Storms 2024 (EN)

As our climate continues to warm, certain conditions favorable to thunderstorms and tornadoes are occurring more often and severe weather activity is expanding into historically less-active seasons and regions.

Severe storms are thunderstorms that produce tornadoes, damaging winds (58 mph or higher), and/or hail at least one inch in diameter.

Severe storms cause 200 deaths annually in the U.S. and account for half of all billion-dollar weather disasters that have impacted the nation since 1980.

In 2023, the U.S. saw a record-shattering 19 billion-dollar severe storms — nearly 50% more than the previous record year (13 storms in 2020).

Most U.S. regions face severe weather risks — but especially in the South, Plains, and Ohio Valley from April through June. Parts of Oklahoma, Kansas, Nebraska, Arkansas, Missouri, and Mississippi have seen the most severe weather watches over the last 20 years.

People living in mobile homes are particularly vulnerable during high wind events.

Severe storms are localized, short-lived events with limited historical records. And most climate models are too coarse to model the behavior of small-scale and short-lived convective events.

The complexity of these events makes it difficult to directly link their occurrence to global climate change. The relationship between severe storms and climate change is an active area of research.

Here’s what we do know:

More frequent and costly billion-dollar severe storms. The U.S. experienced five times more billion-dollar severe storms during 2004–2023 (153 events) than during the previous two decades (30 events from 1984–2003). The average annual cost (CPI-adjusted) of billon-dollar severe storms rose from $3.7 billion (1984–2003) to $19 billion (2004–2023).

Tornado Trends. When excluding the weakest events, the overall number of U.S. tornadoes each year hasn’t changed since 1970. But tornado activity has become concentrated in more frequent outbreaks (days with multiple tornadoes). Further, the frequency of U.S. tornado outbreaks is increasing faster for the most extreme outbreaks.

There is also evidence that tornadoes are getting more powerful in the U.S. and that fall tornado activity is increasing — especially in the Southeast. And “Tornado Alley” has shifted eastward since 1979, with increased tornadic activity observed in the South, Southeast, and Ohio Valley. There’s no clear connection between these observed trends and climate change, however.

Thunderstorm trends. Severe storms are more likely to form under certain conditions — including high wind shear and convective available potential energy (CAPE). Since 1979, parts of the eastern U.S. have seen up to 15 more days with high CAPE during spring and summer — prime time for thunderstorms.

CAPE is just one of several important factors in severe storm formation, however, and it’s unclear how other factors such as wind shear could respond to future warming. Long-term trends show that the frequency of severe and tornadic thunderstorms hasn’t changed significantly since 1979 over multiple U.S. regions.

Future Potential. Studies suggest that conditions favorable to severe thunderstorms will become more frequent with warming over the 21st century — about 5-20% more frequent per 1.8°F of warming. Whether these changing conditions will ultimately result in the formation of more severe storms remains an active area of research.

A 2023 study projects a 6.6% increase in supercell frequency nationwide — and especially in the densely-populated eastern U.S. — by the end of the century as a result of climate warming.

Updated: April 2024