(CNN) -- Swift, severe thunderstorms killed a man in Michigan and wiped out power for hundreds of thousands in Illinois on Monday.
The storms, which developed in Nebraska and western Iowa starting late Sunday, did extensive wind damage across much of Iowa overnight.
The storms are expected to hit southwestern Pennsylvania and West Virginia Monday night. The National Weather Service placed a severe thunderstorm watch for a total of 15 counties in the states through 10 p.m. ET.
In the Chicago area, ComEd spokesperson Arlana Johnson said there were 561,000 residents without power late Monday afternoon, and full restoration could take several days. More than 817,000 have been impacted by the storm, the largest number affected in a decade.
In Michigan, at least one person died due to the storm, CNN has confirmed.
No name has been released yet for the 38-year-old man, said Jon Hess, under sheriff of the Kent County, Michigan, Sheriff's Department.
A tree in the backyard fell onto a garage, where the man, his 37-year-old wife and their six children were sheltering when the storm hit Cutlerville, Michigan, south of Grand Rapids, at about 11 a.m.
Everyone but the man was able to safely escape into the house.
Tim Pietryga, spokesman for Consumers Energy, said 96,000 customers were without power across Michigan.
The hardest-hit areas were Kent, Muskegon and Kalamazoo counties, he said. Primarily the southwestern area of the state was impacted.
It will take most of Monday and Tuesday to restore power to residents, Pietryga said.
The Michigan storms started at 7 a.m. and continued throughout the morning. More than 100,000 customers were impacted at the height of the storms.
Hope College, in Holland, Michigan, sustained much tree damage at the school's core campus area, where many trees were either toppled or snapped, Tom Renner, director of public relations, said. There were no injuries on the campus or structural damage, he said.
A spokesman for DTE Energy said 66,000 customers were without electric services and they were still assessing damage.
Wayne County, which encompasses Detroit, is the hardest-hit area, DTE Energy spokesman Scott Simons said.
In Chicago, more than 100 flights were canceled at O'Hare International Airport Monday morning, according to Gregg Cunningham, a spokesman for the city of Chicago's Department of Aviation. There was one cancellation at Midway International Airport, Cunningham said.
CNN's Joe Sutton contributed to this report.