At least 40 million people were under extreme heat warnings in the central and southern United States on Sunday, as high temperatures and extreme humidity gripped large swaths of the country for the second day in a row.

Heat indices well above 100 degrees Fahrenheit (38 Celsius) extended from Minnesota to Mississippi, extending westward to southern California and Nevada. Parts of the Southwest were expected to see highs of 120 degrees Fahrenheit (49 C), according to the National Weather Service.

Forecasters said the sweltering weather was the result of another heat dome, a phenomenon in which atmospheric conditions trap heat, accentuating and prolonging already high temperatures.

“It’s unusually hot,” said Pete Miller, outreach manager for homeless services network Threshold CoC, who was handing out water bottles to residents of Omaha, Nebraska, on Sunday.

“We’re seeing a lot of people exhausted, exhausted by the heat in a way that’s really dangerous,” Miller added. “The humidity is incredible. You stand outside for 30 seconds and you get soaked. And it’s going to be like that all night.”

On Saturday, Rapid City, South Dakota, reported a record high of 112 degrees Fahrenheit (44 C). A horse race scheduled at Canterbury Park in Shakopee, Minnesota, was canceled Sunday because of the heat.

“Right now, it’s affecting a lot of people in the middle of the country,” said NWS meteorologist Bob Oravec. “Core of heat sinks southward early next week.”

Moist air was also producing other volatility around the top of the heat dome, including the risk of severe thunderstorms, flash flooding and hail around the Great Lakes on Sunday. The chance for strong storms was expected to move east in the coming days.

“If you have a heat dome in one part of the country, you will have a different corresponding climate in another part,” Oravec said. “Everything is related.”

Temporary relief will reach the central United States by midweek, but high temperatures could return by the weekend, he said.

Officials warned that children and older adults, along with people with certain medical conditions, are especially at risk of serious conditions, including heat stroke or even death. Experts recommend finding air conditioning or a cooling center, making sure you drink enough water, and limiting time in direct sun.

Extreme heat events are increased and exacerbated by climate change, which comes from the burning of fuels such as gasoline, oil and coal.

The continental U.S. endured the hottest second first half of the year on record in 2026, according to the National Centers for Environmental Information’s tracking of average temperatures.