Powerful thunderstorms including a tornado unleashed heavy rain and strong winds across the Upper Midwest region on Monday, knocking out power to thousands of customers, downing trees, damaging buildings and making some roads impassable.

More storms were expected throughout the afternoon and into the evening.

At least one tornado was confirmed near Northern Lake Winnebago in Wisconsin.

Tom Nelson, the county executive for Outagamie County, which includes Appleton, Wis., told The Associated Press that there was “extensive and severe damage” in Menasha and Fox Crossing, and reports of collapsed buildings. “All indications are that this was a devastating system that has caused extensive damage,” he said.

Photos and videos on social media appeared to show buildings damaged in areas around Appleton. The Appleton Police Department said there were several large tree limbs down throughout the city and that it was working with other nearby agencies to “respond to many storm related calls” on Monday afternoon.

By early afternoon, the National Weather Service had already issued multiple tornado warnings as the storms began sweeping through, including at least one described as a “particularly dangerous situation,” the most severe warning.

The utility We Energies said it had about 35,000 customers without power in and around Appleton on Monday afternoon. “We have reports of broken power poles, downed lines and damaged equipment in the Appleton area,” said Brendan Conway, a company spokesman. “We have all available crews working to restore power as quickly and safely as possible as we also prepare for more storms throughout the day.”

The storms in Wisconsin were also producing large hail, with one preliminary report near Gresham noting that ice the size of a grapefruit had fallen from the sky.

By 5 p.m. local time, nearly 450,000 homes and businesses were without power in Wisconsin, Illinois, Indiana and Michigan, according to Poweroutage.com, which tracks utilities.

Gordon Hintz, the top elected official in Winnebago County, Wis., told The A.P. he was not aware of any deaths or injuries, but the authorities were watching for that possibility. “Obviously, the footage looks devastating,” Mr. Hintz said. “Clearly some significant damage done with power lines down and things like that.” He also told The A.P. he had heard reports of destroyed buildings.

Thunderstorm watches were issued for most of eastern Wisconsin, down into Illinois and Indiana, across the Great Lakes and across the western shores of Michigan. In addition to tornadoes, the Weather Service warned that the storms could bring damaging winds, sometimes gusting as strong as a hurricane’s.

By midafternoon, another intense storm was moving out of Northern Illinois into Indiana and was expected to produce strong winds, damaging hail and potentially a tornado or two. This system was likely to continue sweeping across Indiana through the afternoon and the evening.

Flooding was becoming a threat as more storms formed over the same areas, and several flash-flood warnings were in place across the region.

The worst of the storms were more likely in the Midwest on Monday afternoon, but there was a chance a few thunderstorms would reach into in the Northeast, including New York City.

How to prepare