Some residents across the region may be without power for several days as crews work to restore electricity to more than 440,000 customers, ComEd officials said.
Monday's storm knocked out power to 868,000 customers, the largest total since at least 1998, the utility said.
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"We didn't keep records the same way before 1998," spokeswoman Judy Rader said. "This is the most since we started using this method to measure."
ComEd officials said "several days" could pass before power is completely restored to all the approximately 463,000 still without power. A more precise timeline was not available.
"This is the worst storm we have ever seen (in terms of power outages)," spokesman Fidel Marquez said earlier Monday.
Winds reached 75 mph and created a seiche on Lake Michigan as the storm raced through the area in roughly 30 minutes, according to the National Weather Service. Although it dropped only about a quarter-inch of rain, the storm caused lightning strikes, uprooted trees and damaged buildings.
Marquez said it did even more damage to ComEd's system than an August 2007 storm that knocked out power to 680,000 customers.
Northern Lake County was hit particularly hard, he said, and ComEd crews set up command centers in Crystal Lake and Libertyville to supervise restoration efforts. About 234,000 customers remained without power in the utility's northern region as of 11 p.m., Rader said.
Another command center was opened in Bedford Park near the CSX Railroad yard in Cook County.
More than 480 crews worked to fix downed power lines and return electricity to the region, starting with essential customers such as hospitals and fire and police stations. Because of severe weather reports, ComEd called crews in advance from Iowa, Pennsylvania and Missouri to help.
As of 11 p.m., approximately 95,000 customers in ComEd's West suburban region remained without power, 69,000 in the Chicago region and 42,000 in the southern region.
Metra trains experienced delays of up to two hours after operators shut down three Union Pacific lines for up to 50 minutes due to weather warnings. Officials said some trains in areas such as Lake Bluff, Zion, Cary and Waukegan experienced even further delays once they got running due to downed trees and power lines on the tracks.
Metra spokesman Michael Gillis said passengers were alerted to scheduled changes by more than 80 service advisories throughout the day.
Delays and cancellations also affected O'Hare and Midway airports throughout the day, with departing and arriving flights running up to 90 minutes late at both locations. O'Hare saw more than 200 flight cancellations by 4 p.m., while Midway reported only minor cancellations.
As storm cleanup continues, Marquez advised residents to report downed power lines at www.comed.com and to keep their distance.
"Do not approach these power lines, because it could be dangerous," he said.
He said any residents without power who have health issues should move to "a more comfortable area," while others who might lose food due to lack of refrigeration should check www.usda.gov to learn what is safe to eat.