By Greg McCune
CHICAGO (Reuters) - A violent storm swept across the upper Midwest on Wednesday and headed toward the U.S. East Coast, spawning several tornadoes as well as damaging hail and high winds, but skirted Chicago without doing major damage.
The storm had been 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.
The threat caused transportation chaos in Chicago, America's third largest city. Many people left work early from high-rise buildings in order to beat the storm home, and others were stuck in traffic jams or on trains delayed by the weather.
The worst of the storm and the high winds passed just south of Chicago.
"So far we have lucked out and have not had the intense corridor of damaging winds," said John Hart, a meteorologist with the weather service's Storm Prediction Center in Norman, Oklahoma.
Numerous tornado warnings were issued for parts of the Midwest, and the weather service website showed 16 reports of tornadoes, including twisters in Iowa, Illinois and Ohio. A tornado warning tells residents to find shelter immediately.
Benjamin Jeffers, 18, an employee at the Belmond Country Club in Iowa, about 95 miles 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.
Stefanie Bond, spokeswoman for the Iowa Emergency Management department, said four tornadoes were reported in Wright County, Iowa, where a couple of businesses and a home were destroyed. No injuries or fatalities have been reported in the state, she said. Another tornado was reported in Franklin County, she said.
In Carroll County, Illinois west of Chicago, a dispatcher said one person was injured but details were not immediately available.
The storm was headed east to the Pittsburgh and Cleveland areas later on Wednesday and to the East Coast by Thursday morning, Hart said.
The area facing the greatest threat of storms on Thursday, including high winds and possible tornadoes, stretches from eastern Virginia and the Washington, D.C., area north to Philadelphia and southern New Jersey, Hart said.
While the storm did not produce high winds and tornadoes in the Chicago area, it did dump 3 to 5 inches of rain which could cause flash flooding in southern suburbs, the weather service said.
Four lines of Chicago's commuter rail service were halted during the rush hour, but later resumed service. More than 360 flights were canceled at Chicago's O'Hare airport, one of the world's busiest, and other flights were delayed, the Chicago Department of Aviation said.
An additional 55 flights were canceled at Chicago's Midway Airport.
A Chicago Symphony Orchestra concert at a downtown park, expected to be attended by thousands of music fans, was canceled, and a Chicago White Sox baseball game was postponed.
The Stanley Cup finals hockey game at the downtown United Center went ahead as scheduled between the Chicago Blackhawks and Boston Bruins, with some 20,000 fans in attendance.
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 on May 31, killing about 20 people.
(Additional reporting by Brendan O'Brien, Mary Wisniewski and Renita Young; Editing by Bob Burgdorfer, David Brunnstrom and Mohammad Zargham)