Iowa derecho in August was most costly thunderstorm disaster in US history
By BOB HENSON | Special To The Washington Post. | Published: October 17, 2020
No single thunderstorm event in modern times - not even a tornado - has wrought as much economic devastation as the derecho that slammed the nation's Corn Belt on Aug. 10, based on analyses from the public and private sectors.
The storm complex, blamed for four deaths, hit Cedar Rapids, Iowa, particularly hard, cutting power to almost the entire city of 133,000 people, and damaging most of its 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 August derecho, which raced from Iowa to Indiana, at $7.5 billion. This includes agricultural impacts that are still 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 U.S. has been hit by a record-tying 16 billion-dollar weather disasters this year through September.
A derecho is a fast-moving, violent wind event 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 event 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.
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, Iowa. 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 wind 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 a minute or two.
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 were damaged.
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 - pea to penny size - 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," Glisan added.
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. The term derecho (deh-REY-cho), which can mean "straight ahead" in Spanish, was first applied to these events in 1888 by Gustavus Hinrichs, a professor at the University of Iowa and the state's first climatologist.
The term languished until it was revived in the 1980s by Robert Johns, now retired from the National Weather Service Storm Prediction Center.
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 amid intense summer heat.
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, especially in midlatitudes. 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 news release.
The authors reported an increasing trend in extreme damages from natural disasters that is generally consistent with a climate change signal.
"Increases in aggregated or mean damages have been modest," they said, "but evidence for a rightward skewing and tail fattening of the distributions is statistically significant and robust - with most pronounced increases in the largest percentiles (e.g., 95% and 99%), i.e., the catastrophic events."