Twister trends: the past, present and future of tornadoes in Iowa

When the skies cleared on March 31, and residents emerged from their shelters, some were met with destruction.

The National Weather Service confirmed more than 20 tornadoes touched down in Iowa, with homes, farms and trees damaged in Johnson, Keokuk and Washington counties.

The University of Iowa College of Engineering’s James Street Laboratory in Coralville — home to a model of California’s Santa Clara River used to study sustainable fish passage — was in ruins.

Portions of the structure crumpled, transforming the research hub into piles of debris. Vehicles were crushed or tossed against walls by the winds. It’s a “total loss,” the university said in a news release.

“We are devastated to see the damage to these core engineering research facilities,” said Harriet Nembhard, dean of the College of Engineering, in the release. “Engineers are resilient. We will rebuild, and we will continue our critically important research.”

Tornado outbreak

That day, at least 122 tornadoes descended from the skies to plunder Iowa and other Midwestern states — marking the fourth most tornadoes in a day since the 1950s. At least 30 twisters were recorded in the National Weather Service Quad Cities bureau’s service area, which covers Iowa and Illinois, according to a Thursday update.

The contributed to a new U.S. record: Tornado counts for this January, February and March reached at least 410 — the highest on record for the first three months of the year. This year’s tornadoes also doubled the number of people killed, from 23 to more than 50.

Iowa’s history is riddled with tornadic activity, but records are punctured by inconsistent record-keeping. Researchers are trying to fill these gaps to uncover meteorological trends. They’re also investigating how those trends may shift in the future with a changing climate — and finding that dangers may grow for certain areas.

“Tornadoes are always gonna occur in Iowa. They've always occurred in Iowa,” said Walker Ashley, an atmospheric scientist and disaster geographer at Northern Illinois University in DeKalb. “We're only just examining changes in risk.”

A twister is coming — but how?

Tornado formation requires a recipe of environmental ingredients, including instability — the energy that fuels thunderstorms. Instability is created as hot air rises and forms a layer, creating a cap that traps warm air beneath it.

“Eventually, it gets so hot and steamy that it’s like you're sending up a hot-air balloon on steroids. It breaks through the cap,” creating a very strong thunderstorm, said William Gallus, an Iowa State University professor of meteorology. “We basically can accumulate huge amounts of energy in the United States. That doesn't happen hardly anywhere else in the world.”

That instability needs another element to create twisters: wind shear, when wind direction or speed changes as air rises higher.

“Those are the two big ingredients,” Gallus said. “If you have those things, you can get thunderstorms that spin, and then those types of storms are more likely to produce tornadoes.”

History can be a mystery

Meteorologists now estimate that around 1,200 tornadoes touch down in the U.S. each year. But the nation’s tornado history — including Iowa’s track record — isn’t fully known.

Before the advent of satellite tracking, tornadoes could only be recorded if they were physically spotted. Severe tornadoes, with their trails of destruction, were more likely to be reported than weaker ones. Even then, data-keeping wasn’t consistent across locations, resulting in variability between NWS offices.

Those historical inconsistencies now make it harder to calculate past tornado trends. Researchers are trying to fill in gaps in the reporting database by mapping historical environmental conditions that could have produced tornadoes.

“It's really hard to estimate the non-event, or the event that we missed,” said Harold Brooks, a senior scientist with the NOAA National Severe Storms Laboratory in Norman, Okla. “We've looked at more variables and tried to build better models, but they're still not perfect.”

Iowa’s observational record of tornadoes may stretch back to the 1800s, when small towns and farms began to spread across the state. That record improved by the 1950s and strengthened significantly when the state’s satellite record ramped up in 1980 — giving a better look at unobserved tornadoes and their path lengths.

Since 1980, more than 1,900 twisters have touched Iowa ground.

The large majority were ranked EF0 or EF1 — the two lowest ratings on the Enhanced Fujita scale, reserved for tornadoes with wind gusts between 65 and 110 miles per hour.

There has only been one EF5 tornado in Iowa since 1980: the one that blasted Parkersburg in 2008, destroying more than 300 buildings, killing nine people and sending at least 70 more to the hospital

“We don't get a lot of strong tornadoes across the state,” state climatologist Justin Glisan said.

Bigger outbreaks, bigger variability

Since the 1950s, days that see just one or two tornadoes are diminishing. Instead, when tornadoes drop, they drop in hordes.

That’s because higher temperatures, humidity and pressures are priming the atmosphere to produce more supercells — the bombastic rotating thunderstorms that can produce damaging winds, large hail and tornado outbreaks.

Iowa’s peak month for tornadoes is typically June — the wettest month of the year for the state’s upper two-thirds — followed by May, July and April. But, more and more, twisters are making appearances during other times of the year.

The derecho that passed through Iowa on Dec. 15, 2021, for instance, birthed 63 tornadoes — setting a record for the most tornadoes in Iowa at the time.

“We've actually seen a decrease in the amount of tornadoes that we've seen across Iowa over the last few decades,” Glisan said. “We're seeing more outbreak days versus these one- or two-off tornadoes.”

Another trend gleaned from historical estimates shows that tornado variability is increasing, meaning the frequency of twisters fluctuates from year to year.

Iowa’s 2011 tornado season, for example, set records for tornado count and fatalities. Meteorologists expected the trend to continue — but the next three years were relatively quiet for such storm activity, ISU’s Gallus said.

That variability has been observed in the state ever since, along with variability in precipitation trends.

“What we've noticed is there's much bigger swings in how many tornadoes you get per year,” Gallus said. “Even day to day, it looks like we go longer periods that are quiet, and then when we do get tornadoes on a given day, we tend to have more of them.”

Better lead times, better protection

Tornado trends evolve alongside technology advancements, leading to better forecasting in advance of outbreaks. That gives increased lead times to warn people to take shelter.

“Your lead times for warnings are anywhere from 10 to 15 minutes, if not a little more, whereas it was half of that just a decade or two ago,” state climatologist Justin Glisan said.

Take advantage of that lead time by prepping for worst-case scenarios. Be prepared to shift locations following storms that may weaken your shelter.

Future outlook a ‘mixed bag’ for Iowa

Historical records for severe weather may be hazy — but researchers are working to clear up future projections.

Ashley, of Northern Illinois University, and his collaborators studied how environmental ingredients for supercell formation and their related perils, like tornadoes and hail, will change by the next century. The team referenced two climate models — an intermediate scenario and a pessimistic scenario — to simulate future weather conditions.

It’s the first attempt at tracking supercells into the future in light of climate change, Ashley said.

“I don't want to suggest that suddenly climate change is the boogeyman, that tornadoes are caused by climate change. That is not the case,” he said. “What we're trying to figure out is, is climate change affecting the overall — overall, that's a keyword — distribution and intensity of supercells and thus resulting in tornadoes and hail?”

The team’s results, released this month in a peer-reviewed paper, found that supercells are becoming more frequent in the eastern U.S. and slightly declining in the iconic “tornado alley” of the Great Plains. The simulations suggest that this trend will continue and perhaps even amplify by the late 21st century.

Supercell formation should increase significantly in the Ozarks, mid-South and the Tennessee Valley. The Midwest, on the other hand, shows more uncertainty.

“In terms of Iowa, I would say it's a mixed bag when it comes to supercells,” Ashley said. “I don't have any confidence one way or the other.”

The common time frame for supercell formation is shifting as well, from July through August to February through April.

Projections also suggest that when intense supercells do form, they will get even more intense in the future. Those perils are heightened for impoverished communities growing more vulnerable to disasters.

“That's scary because the ones that produce the killers and the ones that produce the big hail are oftentimes our most intense,” Ashley said. “If those particular strong ones are going to get more intense in the future because the rotation is more intense in the storm, that suggests that potentially the perils could be worse.”

Next steps include derecho research

Ashley’s recent paper only scratches the surface of the 700 terabytes of weather simulation data that supercomputers took two years to calculate. He’ll be diving further into the treasure trove by looking at more types of severe weather and their impacts.

In preliminary research, which hasn’t been peer-reviewed yet, he found that derechos — defined by their strong, straight-line winds — may nearly double in the Midwest by the end of the 21st century.

“To me, it was one of the more striking results that we got out of these simulations,” Ashley said. “I would argue that derechos are probably far more of a concern than supercells or even tornadoes in terms of their potential impact.”

“I would argue that derechos are probably far more of a concern than supercells or even tornadoes in terms of their potential impact.” — Walker Ashley, atmospheric scientist and disaster geographer at Northern Illinois University

He plans to build on his results by researching midcentury projections, too.

“We want to know what's going to happen within our lifetime,” he said. “We need to be more policy oriented. And that requires us to think (about) the next couple of decades.”

Brittney J. Miller is the Energy & Environment Reporter for The Gazette and a corps member with Report for America, a national service program that places journalists in local newsrooms to report on under-covered issues.

Comments: (319) 398-8370; brittney.miller@thegazette.com