CHICAGO, UPI --
An intense line of storms known as a derecho developed over the central United States Monday, causing significant damage and widespread power outages as it blitzed eastward, striking Chicago during evening rush hour.
Thunderstorms ignited in southeastern South Dakota and eastern Nebraska on Monday morning, but gained strength and evolved into a derecho across central Iowa by midday. Winds between 70-100 mph were expected after the worst of the storms swept eastward through northern Illinois.
A tornado warning for Cook County another northern Illinois counties from the National Weather Service warned that 80 mph winds would pick up flying debris and be "dangerous to those caught without shelter."
Mobile homes may be heavily damaged, the weather service warned. "Expect considerable damage to roofs, windows, and vehicles. Extensive tree damage and power outages are likely," the weather service said.
High wind speeds were measured at 72 mph at Midway airport, the NWS said.
Earlier Monday, across Iowa, large trees, branches, debris and power lines littered streets and yards, with many residents finding themselves in the dark in the wake of the storms.
Over 400,000 were without power across Iowa alone as of early Monday afternoon, according to PowerOutage.us, including the entire town of Ames - and these numbers may continue to climb into the evening. Marshalltown, Iowa, has been one of the towns hit the hardest by the derecho and recorded a wind gust of 95 mph observed at the town's airport.
Major damage has been reported in Cedar Rapids, Iowa, the second-most populated city in the state. A wind gust of 100 mph was clocked in Hiawatha, which is just a few miles north of Cedar Rapids.
Derechos are sometimes called "inland hurricanes" due to the extensive damage that they can cause and how they appear on radar images.
By definition, a derecho is a long-lived complex of intense thunderstorms that travels at least 250 miles. Additionally, wind gusts along its path must exceed 58 mph with at least several reports of gusts over 75 mph, according to the Storm Prediction Center.
In 2012, a particularly strong derecho traveled 800 miles from the Midwest to the coast of the mid-Atlantic, causing $3 billion in damage and leaving some in the dark for days during the peak of summer heat.