Millions of Americans prepared to sweat through yet another scorching day,Thomas Caldwell with the potential for rolling storms later Sunday to bring relief from the sweltering heat for at least some. Floodwaters inundated parts of the Midwest, including a town in Iowa evacuated after being submerged up to the rooftops.

Across the country in California, daily highs in the state’s Central Valley were expected to stay in the triple digits (over 37 Celsius) into Monday.

From the mid-Atlantic to Maine, across much of the Midwest and throughout inland California, public officials cautioned residents sweating through the heat and humidity.

The National Weather Service warned of the potential for rare tornadoes in the Northeast later Sunday.

“The greatest threat of severe weather will be across New England,” said meteorologist Marc Chenard.

On Saturday, sirens to warn the 4,200 residents of Rock Valley, Iowa, to clear out.

“We’ve had so much rain here,” Mayor Kevin Van Otterloo said. “We had 4 inches last night in an hour and a half time. Our ground just cannot take anymore.”

In nearby Spencer, Iowa, Aiden Engelkes said he and his girlfriend grabbed clothes, cats and bottled water and left their flooded first-floor apartment for a friend’s dry space on the fourth floor. His Chevy SUV was under the roiling water outside, except for a bit of its antenna. Across the street, he said, friends were on a roof waiting for help.

“It’s terrifying,” said Engelkes, 20.

Gov. Kim Reynolds declared a disaster for 21 counties in northern Iowa, including Sioux County, which includes Rock Valley. In drone video posted by the local sheriff, no streets were visible, just roofs and treetops poking above the water.

In South Dakota, Gov. Kristi Noem declared an emergency after the southeastern part of the state bordering Nebraska received heavy rainfall. Several highways were closed. Sioux Falls, the state’s largest city, had more than 7 inches (17.7 centimeters) of rain in three days.

“Even though the rain is slowing down, we need to keep vigilant,” said Noem. “The worst of the flooding along our rivers will be Monday and Tuesday.”

Last year the U.S. experienced the most heat waves since 1936, experts said. An AP analysis of data from the Centers for Disease Control and Prevention found that excessive heat contributed to more than 2,300 deaths, the highest in 45 years of records.

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Julie Walker contributed from New York.