A derecho swept through the Midwest a year ago. Now there are efforts to better define the storms
It was an unusually warm day in mid-December.
“Everybody commented that it was just kind of spooky,” said Matt Thompson, who co-owns Lost Grove Ag Services, a seed and fertilizer application business near Harcourt, Iowa. “You don't see that on December 15.”
Thompson heard reports that it was going to storm, so he made sure all machinery at the business was put away. By 6 p.m., he said, tornado sirens were going off and wind was coming through.
By the time the sun came up the next day, five of the businesses’ six buildings were gone.
“We didn’t know what we were going to do,” Thompson said. “It was pretty devastating to see. It was unbelievable. I’ll never forget that.”
The Dec. 15 derecho was unique. It was the first recorded in December anywhere in the U.S. Wind gusts exceeded 80 miles per hour. The straight-line winds and accompanying tornadoes left five people dead and nearly $2 billion dollars in damage stretching from Kansas to Iowa to Michigan.
It wasn’t the only destructive derecho in recent years. In August 2020 a derecho with winds in excess of 100 miles per hour caused an estimated $12.5 billion in damage in Iowa, Illinois and several other states and killed four people. Now, scientists are grappling with how to better define derechos and working to predict whether climate change will lead to more of the extreme wind events.
What are derechos?
Derechos are widespread, long-lived windstorms caused by thunderstorms.
The term “derecho” — which means “straight ahead” or “straight line” in Spanish — was coined in the late 1800s by University of Iowa scientist Gustavus Hinrichs, who used it to describe a straight-line windstorm caused by thunderstorms that moved across Iowa. But the term wasn’t really used by meteorologists until nearly 100 years later, in the 1980s.
In the last couple of years, Iowa in particular has been caught in the crosshairs of derechos.
Bill Gallus, a meteorology professor at Iowa State University, said derechos thrive on warm humid air in the atmosphere’s lower levels that creates strong thunderstorms, something the Midwest often has.
“Those thunderstorms are able to tap into very strong winds happening higher up in the atmosphere,” Gallus said, “even up toward the jet stream so that they can bring those strong winds down to the ground.”
The December derecho was what is known as a “serial derecho,” which is more common in the cooler season and often has a wide path.
A “progressive derecho” typically takes place in late spring or the summer and can occur when strong thunderstorms form, where there may also be a layer of dry air present. Gallus said the rain and hail that is created in the thunderstorm then melts and evaporates, cooling the atmosphere.
“Cool air is heavier than warm air and that cool, heavy air is able to plunge down to the ground, and so gravity is accelerating it. And when it hits the ground, it spreads out as violent winds,” Gallus said. “And that is what happened more in Iowa in the August 2020 derecho.”
Both derechos had huge impacts, causing billions of dollars in damage, several deaths and widespread power outages.
There’s no official database for derechos like there is for hurricanes or tornadoes that can show historic trends. That means it’s hard for scientists to say how many derechos have taken place in Iowa and other states over the last 20 years.
Part of the reason there isn’t a robust derecho database is because they have been “somewhat loosely defined in the past,” according to Matthew Elliott, a warning coordination meteorologist with the National Weather Service’s Storm Prediction Center in Norman, Oklahoma. Without having a formal definition it can be hard to declare some severe weather events as derechos.
“The [derechos] that are the big ones, those are the easy ones to determine,” Elliott said. “It's the ones that are more on the margins where it's, ‘Well maybe it was, maybe it wasn't.’ The definitions at the moment are so much more subjective when it comes to determining some of the finer scale details of a derecho.”
The National Weather Service is working on coming up with an official definition and creating a body of scientists who will determine whether or not to categorize a storm as a derecho. Once they have those in place, that will help the weather service create an official database and better communicate derechos and their impacts to the public.
Elliott said it’s unclear if derechos would be classified retroactively and how far back in time those classifications would go. Though, once they have a label and better data, he said, it will make forecasting derechos easier and will give people more warning to get to safety.
“When you hear the word ‘derecho,' it’s got to trigger something,” Elliott said. “It’s got to trigger that ‘This is the worst windstorm that I’m going to see.’”
The National Weather Service improved its alert system after the highly destructive derecho of August 2020. Now when a severe thunderstorm warning is issued with strong winds of at least 80 miles per hour, people get an alert on their phones.
Utility companies say they’ve also improved their communications with towns and cities to restore power as quickly and safely as possible. The August 2020 derecho caused power outages for at least a million customers across the Midwest. A spokesman for Alliant Energy said after that derecho, Alliant inspected every circuit that was impacted.
Walker Ashley, a Northern Illinois University atmospheric scientist and disaster geographer, said more should be done with urban planning and building codes.
“We build at the bare minimum standards in this country,” Ashley said, “and that has all sorts of consequences from heating costs to damage within extreme damaging wind events.”
Ashley says as cities grow and sprawl out, they’re putting more people in harm’s way of extreme weather, like derechos.
Derechos and climate change
The lack of data on derechos makes it difficult for scientists to predict how a warming Earth could impact the frequency and location of the extreme wind events in the future.
Climate change is causing drier conditions, heavier rains and hotter temperatures across the Midwest and the High Plains. Iowa State University’s Gallus said there is more energy in the atmosphere as it warms and that could pave the way for more powerful and more frequent derechos.
“Since derechos like warm, humid air, you’re probably going to be seeing them in places or at times of the year where you didn’t see them before,” he said. “So they may be starting to happen more to the north.”
Elliott, the National Weather Service warning coordination meteorologist, said there are some clues that the corridor over Iowa where summer thunderstorms tend to happen will shift north over time.
“I can't say specifically how soon or how fast that will move,” Elliott said. “But it won't be something that tomorrow, you wake up and it's suddenly in Canada.”
That doesn’t mean the chance of derechos will go away for the Midwest. Elliott said there is another corridor in the Southern Plains, where derechos tend to happen. That corridor and its thunderstorm activity could shift northward over time toward the Midwest.
As conditions make way for what could be more frequent or severe derechos, more people are expected to be impacted. Northern Illinois University’s Ashley said that’s in part because of greater “exposure,” a term he uses to describe people spreading out across the landscape.
“The problem, as we move into the future, is that we expect exposure to increase. But we also expect the climatological risk of these severe storms to increase as well,” Ashley said. “So those two things going up at the same time means we're going to have bigger disasters in the future.”
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest. It reports on food systems, agriculture and rural issues. Follow Harvest on Twitter: @HarvestPM