Are severe thunderstorms and tornadoes in the spring out of the norm in Iowa?

An active spring may have some Iowans wondering how the 2024 storm season compares to others.

Jerry Schnoor, a professor of civil engineering at the University of Iowa and the co-director of the Center for Global & Regional Environmental Research, who focuses on researching environmental change, says the recent onslaught of severe weather is not that unusual.

"In Iowa, we get the most storms during May and June, but the number of storms hasn't statistically changed a lot," Schnoor said. "In the past decades; however, [Iowa] has received more precipitation, 10 to 20% more than in the 1900s; this is a result of changing weather patterns."

The professor admitted that it's difficult to predict if the severity of typical weather patterns like thunderstorms and tornadoes is increasing.

"There's some thought that climate change may have made the severity worse, but I would say that is still uncertain," Schnoor said.

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Storms need a combination of factors

Tornadoes and thunderstorms often spring up in various conditions, such as mix of warm, moist air, sufficient wind shear, and weather fronts.

Sixty-three tornadoes ripped through Iowa on Dec. 15, 2021 − the single biggest day of confirmed tornadoes in state history. That was an unusual weather pattern, Schoor said.

Johnson County experienced at least two tornadoes early Friday morning, with one confirmed near Frytown. A tornado was unofficially spotted near Lone Tree as well.

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"[Iowa] generally gets about 40 tornadoes per year in Iowa that touchdown and that it has been fairly constant going way back to 1950," Schoor said. "Whenever we get the EF4 or EF5 tornadoes, that is unusual. I would say the severity has increased more than the number of tornadoes."

The tornado that swept across Greenfield, Iowa, on Tuesday afternoon was at least an EF4 tornado, the National Weather Service said Thursday afternoon. That's an upgrade from its initial assessment of an EF3 tornado.

At least two dozen tornadoes swept Iowa and devastated Minden in late April.

More: How do tornadoes form? Explaining the severe weather after dozens of recent Iowa tornadoes

What conditions are needed for a tornado to form? Here’s how one expert explains it.

First, let’s understand some terminology.

An updraft is a “rising column of air” and a downdraft is a “column of air pushing downward,” according to the National Weather Service.

For tornadoes to form, we need thunderstorms, which have very strong updrafts and downdrafts, said Bill Gallus, a professor in the department of Geological and atmospheric sciences at Iowa State University.

But there’s something else needed for a tornado to form: Quickly increasing wind speeds at increasing heights, and an increase in the different directions of those winds, he said.

These changes in wind can be described as wind shear, which creates a “rolling motion in the air.”

“If we have these changes in wind as you go up, we have rolling motion in the atmosphere,” Gallus said. “And then if we get a thunderstorm, we have both really rapidly rising air and sinking air and what that does is it tilts the rolling motion, so that it basically becomes the kind of rotation that we see in a tornado.”

Des Moines Register reporter Paris Barraza contributed to this story.

Jessica Rish is an entertainment, dining and business reporter for the Iowa City Press-Citizen. She can be reached at JRish@press-citizen.com or on X, formerly known as Twitter, @rishjessica_

This article originally appeared on Iowa City Press-Citizen: Are Iowa's severe thunderstorms and tornadoes out of the norm?