Hundreds of thousands of customers are without power across the Midwest on Monday after deadly, severe weather battered the region on Sunday.

Nearly 300,000 customers are without power in Michigan Monday afternoon. Another 50,000 are without power in Wisconsin and 40,000 are in the dark in Indiana.

The National Weather Service said it recorded more than 200 wind damage reports and at least four tornadoes were reported across Michigan, Missouri, Tennessee and Kentucky.

Five fatalities have been attributed to the storm.

Three children -- a 2-year-old girl, her 4-year-old brother and their 11-year-old cousin -- were killed when the minivan they were in was hit by a large tree in Michigan, the Kalamazoo County Sheriff’s Office said.

Weather appeared to be the main contributing factor, the sheriff's office said.

The "high winds ... came down fast," Sheriff Richard Fuller told reporters.

"There's no vehicle that would stand up to this," he said, adding that there was "nothing anybody could've done."

In Valparaiso, Indiana, one person was killed when "severe crosswinds" blew a tractor and a trailer onto their sides, according to local authorities.

The National Weather Service said a second person was killed north of Millersburg, Indiana, when wind from a thunderstorm blew over an Amish buggy.

The severe weather threat continues Monday, with both tornado and severe thunderstorm watches in effect across multiple states in the South.

Some storms could bring hailstones the size of tennis balls and damaging winds of up to 60 mph.

The storms hit Louisiana and Georgia in the morning and will reach Jacksonville, Florida; Charleston, South Carolina; and Charlotte, North Carolina, by the afternoon.

The Southeast region is where the strongest of the storms are expected, with damaging wind, large hail and tornadoes possible.

Washington, D.C., Philadelphia and New York could see storms by the evening rush hour.

ABC News' Darren Reynolds and Jessica Gorman contributed to this report.