A destructive derecho, or fast-moving violent windstorm, slammed the Chicago area Monday afternoon, unleashing damaging gusts that brought down trees and knocked out power. In Iowa, where gusts topped 100 mph, the damage was even more severe.

Early Monday afternoon, the National Weather Service Storm Prediction Center issued a rare “particularly dangerous situation” severe thunderstorm watch for eastern Iowa, northern Illinois, extreme northwest Indiana and southern Wisconsin. It cautioned destructive wind “gusts to 100 mph” were possible after violent storms raked eastern Nebraska and western Iowa earlier in the day.

By Monday evening, nearly 1.5 million people were without power in the wake of the storms from eastern Nebraska through Indiana, where the storms had also caused wind damage. By 8 p.m. central time, the storms had reached western Ohio but were weakening.

Iowa was hardest hit by the bowing storm complex. Several locations clocked wind gusts exceeding 100 mph, while gusts between 75 and 85 mph were common. The storm’s intensity marginally waned as it headed east, but Chicago’s Midway Airport clocked a gust to 73 mph.

As of 8:45 p.m., Doppler radar indicated that storm complex stretched from Toledo to Dayton in Ohio to just west of Louisville. Compared to several hours earlier, the storms had lost a great deal of intensity, with most wind gusts below levels considered severe. However, severe thunderstorms watches remained in effect from western Ohio curling back through western Kentucky into extreme southeast Missouri through midnight.AD

Chicago experienced its most severe winds around 4 p.m. when, in addition to the 72 mph gust at Midway, O’Hare International Airport saw a 62 mph gust, and a wind gust to 85 mph was recorded by a Weatherbug station in Lincoln Square, Chicago.

The Weather Service tweeted that “much of northern Illinois has pockets of damage with downed trees, debris, and power lines blocking roadways” after the worst had passed.

Even in the storm’s wake, the Weather Service warned of the potential for additional damaging gusts lasting another hour or two around Chicago, issuing a high wind warning until 7 p.m. for lingering gusts to 55 to 65 mph.

After passing Chicago, the storms bolted through much of northern Indiana and southern Michigan. On the eastern shores of Lake Michigan in southwest Michigan, the derecho was predicted to generate large waves and push high water ashore, prompting a lakeshore flood warning.AD

Before the storm’s arrival, the Weather Service office in Chicago advised 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 Weather Service warned via Twitter of winds to 80 to 90 mph headed for downtown Chicago, which is a city full of skyscrapers: “Stay away from windows and head indoors immediately if walking near high rise buildings.” Skies quickly darkened in the city as the storms moved in.

The Weather Service 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.AD

The severe thunderstorm warning for the city and areas east to the Indiana State Line, issued at 2:54 p.m. by the Weather Service, affected 7.8 million people.

Several tornado warnings were issued in northern Illinois on Monday afternoon from near Rockford, Ill., to Chicago’s north and west suburbs, including Evanston, Ill., as the Weather Service observed areas of enhanced winds on the leading edge of the line.

One tornado probably 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.

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.AD

Numerous reports of significant winds, 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 weather 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
• 86 mph in Davenport, Iowa
• 85 mph in Moline, Ill.
• 78 mph at Ankeny Airport
• 75 mph at Des Moines airport
• 72 mph at Midway Airport in Chicago

In 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 that Interstate 35 and other roads were blocked due to overturned vehicles and storm damage between Cedar Rapids and Iowa City.

The Weather Service advised 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.

“HEADS UP to outdoor dining facilities & any facilities with umbrellas & tents, such as pools, & including backyards,” tweeted the Weather Service in Chicago. “Make preparations NOW, don’t wait for the storms to arrive.”AD

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 revealing on Monday afternoon. Overshooting tops could be seen as bubbles in the overcast along the eastern limb of the cloud mass where intense thunderstorm updrafts lurk. Rippling outward from them were 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 outward 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-size 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.

Derecho blasts Philadelphia region with 80 mph winds, damaging supercell storm follows

Two days later, a derecho slammed the western Dakotas with serious wind and a few tornadoes.

Another derecho knocked out power to 650,000 across the Ohio Valley and Midwest on June 10.

Below find some damage photos and video from the derecho along its path through Iowa and northern Illinois: