The spring of 2024 was unsettling for residents across large parts of the United States as tornado warnings and sirens urged them to seek safety.
More than 1,100 tornadoes have been reported through May — a preliminary figure but nearly double the 30-year average at that time and behind only 2011, when deadly twisters ripped through the southeastern United States.
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The United States experienced multiple outbreaks in multiple states in 2024. Tornadoes damaged homes from Texas to Minnesota and east to West Virginia and Georgia. They caused widespread destruction in several cities, including Greenfield, Iowa; Westmoreland, Kansas; and Bartlesville, Oklahoma. Barnsdall, Oklahoma, was hit twice in two months.
In May, at least one tornado occurred somewhere in the country almost every day.
Greenfield, Iowa, after a powerful EF4 tornado tore through town on May 21, 2024, amid an outbreak of deadly tornadoes.
What makes some years have so many tornadoes? I’m a meteorologist who studies tornadoes and thunderstorms. Here’s what created the perfect conditions for these violent storms.
2 Key Ingredients for Tornadoes, on Steroids
The hyperactive season is due to the abundance of two key ingredients for tornadoes: wind shear and instability.
The jet stream, a band of strong, high-altitude winds that blows primarily from west to east, circulating between warm air to the south and cold air to the north, plays an important role in how weather systems evolve and in wind shear.
In April and May 2024, the jet stream often dipped southward across the western United States before turning northeastward across the Plains. This is a favorable pattern for tornado production in the central United States.
The area is historically considered tornado alley and some of the influences that can fuel tornado-prone weather conditions. The curved red line indicates a warm front east of the jet stream. Image courtesy of NOAA
In the area east of the jet stream’s southern drop-off point, air rises, creating a powerful low pressure system that causes winds close to the ground to blow in a different direction than those higher up, contributing to wind shear.
This year was even more active, with record heatwaves persisting in Mexico and Texas, while the Rockies and far northern United States remained cool. The large temperature difference created a stronger-than-normal jet stream, causing wind speeds to vary greatly with altitude. As a result, wind shear was enhanced.
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Changing wind speeds with altitude can cause the air to roll. Rapidly rising air in a thunderstorm can then tilt the rolling motion to create a swirling thunderstorm that can concentrate the rotation into a tornado.
The Gulf of Mexico has also been much warmer than normal, producing abundant heat and moisture that could be carried north and fuel thunderstorms. That creates atmospheric instability, the other key ingredient in tornadoes.
Image courtesy of National Weather Service
Weakening El Niño was a warning
This perfect combination of ingredients for tornadoes was not a complete surprise.
El Niño and La Niña, two opposing weather patterns centered in the Pacific Ocean, can affect winds and weather around the world. A 2016 study found that when El Niño turns into La Niña, the number of tornadoes in the central Plains and upper Midwest is often higher than normal.
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That’s exactly what happened in the spring of 2024. Tornadoes occurred primarily in the traditional “Tornado Alley” from northern Texas to South Dakota, with an extension across the Corn Belt into Iowa and into Ohio, consistent with the findings of this study.
How El Niño and La Niña influence tornado behavior.
How is tornado activity evolving?
The active spring in the Great Plains, however, has been somewhat unusual. Studies show a long-term trend of decreasing tornadoes in this region and an increase in tornadoes farther east, near or just east of the Mississippi River.
This change is consistent with what climate models suggest is likely to happen over the rest of the century as global temperatures rise.
A map showing the average number of days per year with an EF1 or greater tornado within 25 miles of each point shows the eastward shift of tornado alley. The time period covered is from 1986 to 2015. Image courtesy of NOAA Storm Prediction Center
The expected decline in tornadoes in the Plains is likely related to increased heat over the highlands of the desert southwest and Mexico. This heat flows down onto the Great Plains a few thousand feet above the ground, creating a hood or lid. This hood allows heat and moisture to accumulate until it forms and forms a thunderstorm. This warm, moist air is why the central United States is home to the most violent tornadoes on the planet.
One theory is that climate change will make the ice cap harder to break through, reducing the number of tornadoes in the Plains. Meanwhile, increased heat and moisture elsewhere will fuel more tornadoes in the East.
Long-term trends and climate model predictions also suggest that more tornadoes are occurring during the colder months, particularly in the Southeast. Tornadoes are also occurring less frequently each year, but on the days they do form, they are more likely to occur in multiple bursts.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
