A violent storm swept across seven states of the upper Midwest on Wednesday, spawning at least nine tornadoes as well as damaging hail and high winds, and causing transportation chaos in Chicago, America's third largest city.
The storm was described by the National Weather Service as "very dangerous" because of its potential to produce tornadoes and "derechos" - storms in which wind speeds increase as they move.
Menacing dark clouds hung early on Wednesday evening over downtown Chicago, where many people left work early from high-rise buildings in order to beat the storm home. Others were stuck in traffic jams or on trains delayed by the weather.
Numerous tornado warnings were issued for parts of the Midwest and there were preliminary reports of at least nine twisters touching down, all of them in Iowa, the weather service said. A tornado warning tells residents to find shelter immediately.
Benjamin Jeffers, 18, an employee at the Belmond, Iowa Country Club, about 95 miles (150 km) north of Des Moines, said he and coworkers fled to the basement after one of two funnel clouds approached.
"Debris started flying around the golf course and it started to get real close and the other one started to get way big," he said. "It sounded like a big train without the horn. A rumble kind of." The club escaped damage, he said.
Local media also reported a tornado touching down briefly about 30 miles (50 miles) from Chicago in Illinois.
There were no immediate reports of substantial damage, injuries or death from the storms.
Some 12 million people were in the highest risk area, including the densely populated Chicago metropolitan area, according to the weather service. In addition to northern Illinois, the storm was affecting Iowa, Indiana, Minnesota, Wisconsin, Michigan and Ohio.
Four lines of Chicago's commuter rail service were halted during the evening rush hour. Three of them later resumed service. Chicago's O'Hare airport, one of the nation's busiest, was facing flight delays of about an average of 90 minutes, and some 227 flights had been canceled, according to Flightaware.
At Chicago Midway Airport, all inbound flights were being held at their origin until the storm passed.
A Chicago Symphony Orchestra concert at a downtown park at which thousands of music fans were expected, was canceled.
The Chicago White Sox baseball game was postponed, and the National Hockey League said it was monitoring the weather as some 20,000 fans gathered at the downtown United Center for the Stanley Cup finals game between the Chicago Blackhawks and Boston Bruins.
Chicago's office of emergency management urged residents to stay indoors as the storm bore down on the city.
Winds of up to 75 miles per hour were expected from the storms, according to the weather service.
Severe weather also was possible for a portion of the Mid-Atlantic and Southeast, it said.
The U.S. tornado season was relatively quiet until May 20, when a monster EF5 storm, the highest rating, hit the Oklahoma City suburb of Moore, killing 24 people and flattening whole sections of the town. Another wave of storms hit Oklahoma state on May 31, killing about 20 people.
(Additional reporting by Brendan O'Brien and Mary Wisniewski;Editing by Bob Burgdorfer and David Brunnstrom)