Dick Kraklow rolled into Minneapolis earlier this week with three generations of his family and several vintage vehicles in tow, excited for an annual gathering of the Minnesota Street Rod Association that celebrates classic cars.

But instead of setting up on Saturday morning to display their collection, Mr. Kraklow, 42, and his family were loading up to drive back to Wisconsin. Several members of the group are in their late 70s, and the heat radiating off the asphalt at the state fairgrounds in St. Paul on Friday caused the family to change plans.

“We love the show,” Mr. Kraklow, a welder from Muskego, Wis., said as his uncle angled a yellow 1957 Ford Thunderbird onto a trailer. But ultimately, he said, “It’s too hot.”

Millions of Americans on Saturday faced sweltering conditions as a dangerous heat wave brought rising temperatures to the Midwest and Central Plains. By early afternoon, the National Weather Service reported that the heat index — a measure of how hot it feels that accounts for both heat and humidity — had hit 93 degrees in Minneapolis, 98 in Des Moines, 99 in Sioux Falls, S.D., and 105 in St. Louis. In Omaha, forecasters said that it would feel like it was 100 to 110 degrees this weekend — in the shade.

The most extreme heat was expected to move east and south over the next several days. New York City, Washington and Philadelphia could all break 100 degrees on the heat index by the end of the weekend. Several cities could see heat records broken. Over the entire country, more than 64 million people were under an extreme heat warning.

Climate scientists have found that climate change has made heat waves more common, more intense and longer lasting worldwide, though attributing a specific heat wave to climate change is tricky.