The violent windstorm killed one person in Indiana, and more than 1 million residents remained without power the morning after.
Along its path, fierce winds overturned trucks, flattened corn crops, and knocked trees onto homes and vehicles. Numerous structures were damaged in Iowa and northern Illinois. Social media images showed two-by-fours turned into projectiles that punctured the sides of buildings. In Wheaton, Ill., the winds toppled a church steeple.
100-mph winds rake the Corn Belt
The National Weather Service logged more than 700 reports of severe weather, including a wind gust to 112 mph in Midway, Iowa.
Marshalltown, a city 50 miles northeast of Des Moines in Marshall County, measured a wind gust to 99 mph. It had been struck by a violent tornado on July 19, 2018. The mayor described it as “deja vu all over again.” Elsewhere in the county, gusts ranged between 91 and 106 mph, akin to an enormous EF1 tornado.
The core of the most severe wind was at times 30 to 50 miles wide, with the swath of gusts locally exceeding 90 mph and lasting longer than 15 minutes in duration. In Marshalltown, the neighboring towns that trucked in aid in 2018 were unable to chip in after Monday’s blow, amid dealing with their own recovery.
The Marshalltown Times-Republican reports that city police received more than 50 calls about possible gas leaks in the wake of the storms.
“This particular storm touched every corner of our community,” Mike Tupper, chief of police in Marshalltown, said in an interview. He said the town remains without power as crews grapple with the long task of cleanup.
"There’s a lot of significant rooftop damage to homes and businesses,” Tupper said. “Trees being taken down, a lot of poles taken down. The biggest challenge right now is with electricity.”
Tupper was struck by how long the winds lasted, which contributed to the damage. “These were sustained wind speeds well over 80 mph for 20-plus minutes,” he said.
In Ankeny, Iowa, a suburb on the northern side of Des Moines, city officials anticipate that the cleanup “could take up to four to six weeks to pick up the entire city,” according to a news release.
Severe damage also occurred in Cedar Rapids. An eyewitness on social media described “utter destruction.” The Iowa Department of Transportation reported that Interstate 35 and other roads were blocked due to overturned vehicles and storm damage between Cedar Rapids and Iowa City.
Iowa will probably sustain enormous agricultural losses, including to the corn crop, a staple of the state’s economy. About 33 percent of the state’s crop was impacted according to Gov. Kim Reynolds (R). Corn is typically harvested between late September and mid-October, and at this time of year, it’s too late to replant.
Storms strike the Chicago metro area
In Chicago, winds gusted up to 73 mph as tornado sirens wailed throughout the city. A wind gust to 85 mph was recorded by a WeatherBug station in Lincoln Square.
The storms cleared Monday evening while the setting sun cast delicate pastel hues across the evening sky, an odd contradiction of the fury that had ensued a few hours earlier.
ABC7 News Chicago reported on the severity of damage to electrical lines in the area.
“It’s not just a repair job,” said Rich Negrin, a spokesman for Commonwealth Edison, the largest electric utility in Illinois. “In some of the areas that are impacted, it’s actually a rebuild.”
Video emerged on social media of the winds peeling the roof off a building downtown.
Ahead of the storm, the National Weather Service warned Chicagoans of a “particularly dangerous situation” and advised them to prepare for “tornado-like” winds.
Six injuries have been reported in the area served by the NWS Chicago, five being in Forreston, Ill., and one in Peru, Ill.
The Weather Service credited accurate forecasts and coordination with local officials for reducing the storm’s toll on the region.
“Despite the massive coverage and impressive intensity of the wind damage, there were a very limited number of serious injuries,” the Weather Service office serving Chicago wrote on its website. “This speaks to the effort of our partners including fellow meteorologists in the weather enterprise effectively communicating the threats and action needed to be taken, as well as emergency management and law enforcement for their preparedness efforts beforehand and assistance efforts immediately after.”
Although there were no reports of significant winds greater than 80 mph in Indiana, one fatality did occur near Fort Wayne when a mobile home overturned in the high winds and trapped a pair of occupants. A woman was pronounced dead at a nearby hospital, while a boy escaped with minor injuries and was in good condition.
What is a derecho?
Derechos are arcing bands of thunderstorms that produce wind damage, some significant, along a path exceeding 400 miles. Monday’s was a “progressive” derecho, composed of one main bow echo — or curved squall line — containing a concentrated swath of wind.
The wind was most extreme over Iowa and northern Illinois before storms fanned outward and brought slightly lesser winds to a much broader area.
The secret of derechos is their proclivity for tapping into jet-stream energy. They form along the boundary of warm air to the south and cooler air to the north, feeding off the instability that results from the temperature contrast. Then warm, sinking air on the back side of the storms dries up and helps drag down strong jet stream momentum. This “rear inflow jet” is what nudges the center of the squall line forward and causes the storms to “bow out.”
A 700-mile path of fury
Monday’s derecho developed from the same weather pattern that produced storms with softball-size hail over the Black Hills west of Rapid City, S.D., on Saturday. More storms affected Minneapolis on Sunday.
By early Monday morning, fledgling storms over South Dakota had dipped into Nebraska, causing power failures to start the day in the greater Omaha area.
But as they moved into Iowa, the storms began taking on the classic “bow echo” shape — aptly named for their archer’s bow-like appearance. Shortly thereafter, Doppler radar detected winds of 120 mph at 1,800 feet elevation — a sign that wind gusts over 90 mph were possible in the northern Des Moines suburbs.
Wind velocities that strong are more typically spotted only on radar associated with intense tornadoes or landfalling hurricanes.
At their peak, the storms were producing upward of 70 to 100 cloud to ground lightning flashes per minute.
The storms became outflow dominant as they approached Chicago, meaning they were exhaling more air than they were ingesting as the rapidly-moving squall started to outrun its upper-level support. In the process, several kinks in the line developed, where surges of air exiting the storm contributed to localized zones of spin. That prompted several tornado warnings for parts of Chicago.
The force of the winds, coupled with the change in air pressure accompanying them, even raised concerns of a “meteotsunami” affecting eastern Lake Michigan. The National Weather Service in Northern Indiana issued a lakeshore flood warning to account for oscillations in water levels resulting from the storms’ passage. Data suggests a one- to two-foot swing occurred.
“Fluctuating water levels will continue in the wake of Monday evening storms,” the National Weather Service wrote. “This will cause an increased risk for dangerous RIP currents and structural currents as well as lake shore flooding through [Tuesday] morning.”
Winds remained strong in Chicago even after the leading edge of storms passed, with some locations reporting 20 to 30 minutes of winds gusting above 50 mph. That was the result of a “wake low,” or a low pressure system formed by subsiding warm, dry air on the back side of the storms.
It has been a busy season for derechos, which have caused problems from the High Plains to the East Coast.
Philadelphia was hit by a progressive derecho with 80 mph winds on June 4. Gusts topped 90 mph east of the city in New Jersey.
Two days later, western parts of the Dakotas were blasted by strong derecho winds and a few tornadoes.
Earlier this year, a derecho brought 70 mph wind gusts to Nashville in May.
Jason Samenow and Andrew Freedman contributed to this report.