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Chicago in crosshairs of severe weather threat in Midwest

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(CNN) — A powerful storm rolled into the Windy City on Wednesday evening, part of a powerful system that threatened cities and towns around the Midwest, quickly and with fury.

“Severe weather now impacting Chicago area,” the National Weather Service tweeted around 6 p.m. (7 p.m. ET). “Stay alert!”

Wind gusts neared 50 mph around that time, as dime-sized hail fell, the service’s Chicago branch said.

By then, residents in Kendall County, Illinois, about 50 miles southwest of Chicago were urged to hunker down after storm spotters “reported wall clouds” that suggest a possible tornado, according to weather service.

Meanwhile in DeKalb County, some 60 miles west of Chicago, the storm downed trees and power lines, said the county sheriff’s department said.

To the east of the Windy City in Lake County, Indiana, a severe thunderstorm warning advised residents to brace for 80 mph wind gusts and pingpong ball-sized hail.

It was all thanks to a fast-moving and, at times, powerful storm system that moved across the Upper Midwest and into the Ohio Valley on Wednesday evening.

One of the first indications that it would be tumultuous day came around 4:30 p.m., when a “confirmed tornado” touched down about 8 miles east of Belmond in Wright County, Iowa.

There are reports of structural damage and power outages in that north central Iowa county due to suspected tornado touchdowns there, though the extent was not immediately known, said state homeland security spokeswoman Lucinda Robertson.

The storm’s impact wasn’t unexpected: The Storm Prediction Center had warned that Indiana, Ohio and much of Illinois, including the city of Chicago, faced a “high risk” — the most perilous category — of severe weather through Wednesday night.

The portion of the United States under a moderate risk for severe weather extended farther, and includes the cities of Cedar Rapids, Iowa; Indianapolis, Indiana; and Columbus, Ohio.

The threat of tornadoes was particularly high in southwest Wisconsin and northwest Illinois, thanks to severe thunderstorms capable of producing a twister.

There was a possibility they could strike well beyond that, though, with the weather service issuing tornado watches through midnight Wednesday for much of Ohio, southwestern Pennsylvania and northern West Virginia.

Russ Schneider, director of the Storm Prediction Center, explained that the type of severe weather that hit the Upper Midwest was a derecho.

Derived by the Spanish word for “straight ahead,” derechos are a weather phenomenon that traditionally happens only a few times a year. It is defined as a line of storms that produces a swath of damage more than 240 miles long with gusts of at least 58 mph.

A derecho forms thanks in large part to warm, humid air, instability in the atmosphere and jetstream winds, which can organize the storms into isolated storms called super cells.

Those super cells rotate among themselves, then cluster into powerful wind systems that can become derechos, Schneider said.

A line of such storms travels quickly — often at around 50 to 60 mph, which is much faster than most other types of storms.

“(So) what looks like a very dark cloud on the horizon very rapidly becomes an imminent threat,” explained Schneider. “(People should) make sure they know where they go to seek shelter, and what actions they need to take as warnings are issued.”