IOWA DERECHO 2020

Analysis: Iowa derecho costliest U.S. thunderstorm disaster in history

Damage also outpaces that of most 2020 hurricanes

The roof is missing Aug. 17 from an apartment building at Cedar Terrace in Cedar Rapids, the result of the Aug. 10 derec
The roof is missing Aug. 17 from an apartment building at Cedar Terrace in Cedar Rapids, the result of the Aug. 10 derecho. Some families living in the complex were sleeping in tents outside while waiting for shelter. (Liz Martin/The Gazette)

No single thunderstorm event in modern times — not even a tornado — has wrought as much economic devastation as the Aug. 10 derecho that slammed Eastern Iowa and other parts of the Corn Belt, according to both public and private analyses.

The storm complex, blamed for four deaths, hit Cedar Rapids particularly hard, cutting power to almost the entire city of 133,000 people and damaging a wide array of businesses and homes.

In an October update to its database of billion-dollar weather disasters, the National Oceanic and Atmospheric Administration estimated damages from the derecho, which raced from Iowa to Indiana, at $7.5 billion. This includes agricultural impacts that still are being analyzed, so the total may be revised, said Adam Smith, who manages the database.

The derecho’s financial toll exceeds that of nine of this year’s record 10 landfalling U.S. hurricanes and tropical storms. The exception is Hurricane Laura, which struck Louisiana in late August and caused an estimated $14 billion in damage.

Including the derecho, the United States has been hit by a record-tying 16 billion-dollar weather disasters this year through September.

A derecho is a fast-moving, violent windstorm associated with a thunderstorm complex. One common definition specifies that it must produce “continuous or intermittent” damage along a path at least 60 miles wide and 400 miles long, with frequent gusts of at least 58 mph and several well-separated gusts of at least 75 mph.

The Aug. 10 storm more than qualified. Striking with unanticipated ferocity, the derecho brought winds gusting to more than 70 mph for the better part of an hour over a large swath of Central and Eastern Iowa and northwest Illinois. Numerous locations clocked gusts over 110 mph.

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The winds laid waste to millions of acres of crops, severely damaged or destroyed hundreds of homes and brought down many thousands of trees.

“One could make a strong case that this is the most destructive individual thunderstorm cluster on record in terms of damage cost,” said Steve Bowen, head of catastrophe insight at the insurance broker Aon, in an email. Aon released an initial damage estimate of $5 billion for the derecho, not yet including agricultural impacts.

The derecho’s top winds ripped along the south edge of a mesoscale convective vortex, a low-pressure center embedded within the thunderstorm complex. “The vortex was one of the most distinctive ones of that size that I have ever seen,” said Stephen Corfidi, a meteorologist at the University of Oklahoma and derecho expert, in an email.

The peak wind gust observed in the derecho was 126 mph at Atkins in Benton County. The highest estimated gust, based on the partial destruction of an apartment complex in Cedar Rapids, was 140 mph.

Gusts that strong are comparable to the peak that one would expect in an EF3 tornado or major hurricane. Parts of five Iowa counties were struck by gusts estimated at 110 to 140 mph.

“To have a Midwest city endure (such) wind speeds, and also see such a devastating impact to a large volume of regional crops, is almost unbelievable,” Bowen said. “I don’t think most of the country truly realizes how severe the event ended up being.”

Derecho winds typically last about 10 to 20 minutes at any one spot. In contrast, the 30- to 60-minute duration of severe gusts in the hardest-hit areas Aug. 10 was much more comparable to the passage of a hurricane eyewall than a tornado, whose winds typically last only a few seconds to minutes.

In Cedar Rapids alone, more than 1,000 housing units were deemed unlivable in the week after the storm, according to The Gazette. Hundreds of other homes in the city were damaged.

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Standard home and business insurance policies should cover much of the wind-related structural damage. Some of the hits to agriculture may fall through the cracks of insurance programs, though. For example, many huge grain-storage units were demolished by the derecho, with an estimated $300 million in structural losses alone. Any grain that can’t be salvaged from these destroyed bins wouldn’t be covered by standard crop insurance, according to the U.S. Department of Agriculture.

Iowa state climatologist Justin Glisan highlighted the scope of agricultural impacts in a September webinar. According to initial estimates, more than 3.5 million acres of corn and 2.5 million acres of soybeans were affected in Iowa, or about 20 percent of the state’s total farmland.

Along with the wind itself, crops were thrashed by small hail that was propelled “like machine-gun fire,” Glisan said.

“You’d be hard-pressed to draw a more efficient path across the Corn Belt to create a storm that has such a significant impact on agriculture at a sensitive time of the year,” he said.

The derecho ripped huge holes in the tree canopy above a number of Iowa towns and cities, according to Emma Hanigan, an urban forester with the state’s Department of Natural Resources. At least half of the trees in Cedar Rapids were destroyed or heavily damaged. The toll will only worsen over the coming months, as the wounds left by torn limbs allow pests and pathogens such as oak wilt to infect damaged trees.

“It takes so long to regain that tree cover,” said Hannigan in an interview. “We’re going to see impacts 30 years from now from this storm.”

Unlike the cyclonic winds that wrap around and up into a tornado, a derecho’s winds descend and spread outward, typically pushed by a powerful jet stream that feeds into the back of a thunderstorm complex.

Iowa and neighboring states are especially prone to derechos.

Several derechos strike the United States in a typical year. The first to gain widespread public attention by that label tore from Illinois to the Mid-Atlantic on June 29, 2012. Packing gusts as high as 91 mph near Fort Wayne, Ind., and 79 mph at Reston, Va., the derecho and related severe weather inflicted $3.3 billion in damage (in 2020 dollars) and led to at least 42 direct and indirect deaths, with power knocked out to more than 4 million customers. The Baltimore-Washington area was especially hard hit, with some outages extending for days.

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U.S. derechos typically form along the north edge of a very hot, humid surface air mass with the approach of a strong upper-level impulse. There has been little research on how human-produced climate change might affect derechos, though it’s conceivable that a poleward shift in summer weather patterns might nudge their U.S. distribution northward over time.

The high cost of this year’s Midwest derecho is in line with a trend toward the highest-end global weather disasters becoming even more costly at a disproportionate pace. This phenomenon was analyzed in a 2019 paper in the Proceedings of the National Academy of Sciences.

“We have a whole distribution of damages that we usually average to determine economic impacts ... but it is the extreme events that cause the damages that are most difficult to deal with,” said co-author Francesca Chiaromonte, a professor of statistics at Pennsylvania State University, in a statement.

The authors reported an increasing trend in extreme damages from natural disasters that is generally consistent with a climate change signal.

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