Over 430,000 people were without power in the wake of the storms in eastern Nebraska, Iowa and northern Illinois. Doppler radar showed winds over 120 mph at about 1,000 feet above the ground as the storms moved through Des Moines. A personal weather station in Des Moines clocked a surface wind gust of 85 mph, while several locations clocked gusts exceeding 100 mph.
As of 3 p.m. local time, Doppler radar indicated the most intense winds in the “progressive derecho” stretched from Rockford, Ill. to Ottawa, near Interstate 80. That core of wind, with embedded gusts exceeding 80 mph, will arrive in Chicago between 3:30 and 4:30 p.m..
“The potential for widespread and destructive damaging wind gusts of 70-100+ mph and perhaps a tornado or two will continue as a line of storms moves quickly eastward across northern Illinois,” the Weather Service wrote in a special bulletin. However, as the storms moved into the city, its maximum winds weakened slightly, to the still damaging range of 60 to 80 mph.
The NWS office in Chicago warned area residents to treat warnings “like a tornado warning,” stating: “This is an extremely dangerous line of storms… Head for safe shelter indoors well away from windows (interior windowless room or basement if you have one.)”
The NWS also told the media and emergency managers that some areas would see winds gusting to greater than 58 mph for more than 15 minutes. This could increase the odds of some structural damage, in addition to extensive destruction of trees and power lines.
AT 2:54 p.m., the National Weather Service in Chicago issued a severe thunderstorm warning for the city and areas east to the Indiana State Line, covering 7.8 million people.
They noted that the derecho “has a history of producing widespread wind damage across western and north central Illinois,” and warned residents to expect “tornado-like” wind speeds. Radar also indicated the potential for isolated tornado activity north and northwest of Chicago, though a brief spin-up can’t be ruled out anywhere along the line as it plows through the city. Tornado warnings were issued near Rockford, Ill. for a time Monday afternoon, as the NWS observed areas of enhanced winds on the leading edge of the line.
One tornado likely did touch down near Lisbon, about 60 miles southwest of Chicago. Winds gusted to 62 mph in Aurora, Ill., west of downtown Chicago, as the storms barreled through.
Shortly before the storms moved into Chicago, the local NWS office warned of “widespread and possibly long duration power outages.”
A derecho is an event that power companies typically cannot prepare for several days in advance, like they would do for a hurricane. Instead, power companies may be caught off guard by these storms, which grew in ferocity just this morning.
Numerous reports of significant, and at times extreme winds, have been received from across the Corn Belt:
- 112 mph near Midway, Iowa
- 106 mph near La Grand, Iowa measured by personal weathers station
- 100 mph near Hiawatha, Iowa
- 99 mph at Marshalltown Municipal Airport
- 99 mph near Albion, Iowa
- 95 mph estimated near Marshalltown, Iowa
- 91 mph near Marshalltown, Iowa
- 90 mph in Atkins, Iowa
- 90 mph in Blairstown, Iowa
- 78 mph at Ankeny Airport
- 75 mph at Des Moines airport
- 85 mph in Moline, Ill.
- 86 mph in Davenport, Iowa
Gusts above 80 mph were ubiquitous with the line of destructive storms.
In Cedar Rapids, Iowa, an eyewitness on social media described “utter destruction.” The Iowa Department of Transportation reported Interstate 35 and other roads were blocked due to overturned vehicles and storm damage between Cedar Rapids and Iowa City.
The Weather Service is advising residents to consider changing travel plans to avoid the time when storms are expected, secure loose objects or bring them inside — these would include trash bins, outdoor furniture, lawn ornaments, signs, outdoor plants, and other items that could become dangerous projectiles during a period of high winds.
The strongest winds may precede any rain, lightning, or even thunder, making it imperative that residents heed all warnings.
“HEADS UP to outdoor dining facilities & any facilities with umbrellas & tents, such as pools, & including backyards,” tweeted the National Weather Service in Chicago. “Make preparations NOW, don’t wait for the storms to arrive.”
Widespread winds between 70 and 80 mph, with a few gusts topping 90 mph, are likely.
“A derecho will rapidly progress across eastern Iowa and northern Illinois this afternoon,” the NWS stated in issuing the severe thunderstorm watch message. “Widespread severe wind gusts, some of which should reach 80-100 mph are anticipated along the track of the bow. Brief tornadoes are also possible.”
Interstate 80 will also be affected from northwestern Indiana westward, with powerful wind gusts posing a significant danger to motorists, who should seek shelter or avoid traveling until the storm passes.
Residents of mobile homes in the greater Chicago area may also consider relocating to a structure with a solid foundation during the afternoon as storms roll through.
On radar, a narrow arc of thunderstorms could be seen ahead of the main derecho as it approached Cedar Rapids, Iowa. That is a sign of “warm air advection,” or a surge of extra warm air screaming north in advance of the storms. That may allow the derecho to intensify further.
Derechos feed off warm, humid air. Chicago was 85 degrees at noontime, with a dew point of 72. The dew point measures how much moisture is in the air. When the dew point exceeds 70 degrees, the air is downright tropical. That will allow for explosive thunderstorm growth to continue.
The line of thunderstorms was producing upward of 70 cloud-to-ground lightning strikes per minute.
Satellite imagery of the impending derecho was striking Monday afternoon. Overshooting tops can be seen as bubbles in the overcast along the eastern limb of the cloud mass where intense thunderstorm updrafts lurk. Rippling outwards from them are gravity waves, akin to wavelets in a pond, indicating extreme turbulence nearby.
Particularly impressive were the tendril-like high clouds and transverse banding within it — appearing as strips of shading radially outwards from the center — illustrating healthy outflow, or storm exhaust, at the upper levels. That’s a common feature on satellite associated with strong hurricanes.
The same upper air pattern powering the derecho also sparked severe thunderstorms across the Northern Plains over the weekend. Grapefruit-sized hail fell in the Black Hills of South Dakota west of Rapid City on Saturday, while nasty storms affected the Twin Cities on Sunday.
Derechos have proven problematic for many from the Plains to the Northeast this year. A derecho barreled through Philadelphia on June 4, with winds topping 80 mph downtown and 90 mph east of the city.
Another derecho knocked out power to 650,000 across the Ohio Valley and Midwest on June 10.
Below find some damage photos from the derecho along its path through Iowa: