Forecasters warned that “dangerous heat and humidity” will spread across the central and eastern United States this week, threatening to break records for high temperatures and ending a spell of fall-like weather.
The heat wave will bring “unseasonably hot” temperatures to the Upper Midwest and Mid-Atlantic, said David Roth, a meteorologist at the Weather Prediction Center.
The extreme heat and humidity could make it feel like 105 to 115 degrees Fahrenheit in parts of the country.
“In some places in the Midwest, it could be the hottest temperatures they’ve seen all summer,” Mr. Roth said. “Not only is it late, it’s the hottest, too. So that’s a little unusual.”
The heat is expected to build in the Midwest through Tuesday, then move to the Mid-Atlantic and Southeast by the middle of the week. Temperatures will break 90 degrees and near 100 degrees in many areas in the Midwest.
Columbus, Ohio, and Chicago are expected to reach a high of 95 on Monday. Louisville, Ky., is expected to hit a high of 99 on Wednesday and Thursday, while Washington could register 100 on Wednesday.
There will be little relief at night as temperatures are expected to drop only into the 70s in the Central and Southern states.
Excessive heat warnings were in effect on Monday for parts of eastern Iowa as the heat index, which measures how hot it really feels outside, is expected to break 100 degrees. Alexis Jiminez, a meteorologist at the National Weather Service office in Des Moines, said temperatures are usually in the 80s at this time of year.
Excessive heat warnings are issued within 12 hours of “extremely dangerous heat conditions,” which means the daytime heat index feels like 105 for at least two days and the low temperature does not drop below 75.
As of Monday morning, more than 13 million people, mostly in the Midwest, were under an excessive heat warning.
In Eau Claire, Wis., which is about 90 minutes east of Minneapolis, the average high temperature for Aug. 26 is 79 degrees but it is predicted to be in the 90s on Monday, Mr. Roth said.
Forecasters anticipate a cold front will follow the high heat, bringing relief days after the scorching start to the week.
Several heat waves have enveloped parts of the United States this summer, intensifying hot Southern climates.
Mr. Roth said temperatures on Sunday were over 100 degrees in Amarillo, Texas, for the 10th day in a row, its longest streak on record. Sunday was also a record 91st consecutive day of 100-degree temperatures or higher in Phoenix. The previous record was 76 days, set in 1993.
Thousands of Americans die from heat-related illness every year, though it’s difficult to track exactly how many. The Weather Service recommends that people stay out of the sun or heat as much as possible, and drink plenty of water to avoid heat illness.
This past June was the hottest June on record globally, and July was the second-hottest July, just behind July 2023.