St. Louis, Omaha and Des Moines were among the cities that saw heat indexes rise toward triple digits, with similar temperatures expected on the East Coast by Monday.

The temperature made it to 91 on Saturday in Philly, and by the end of the workweek, that kind of heat may seem like a cool shower. By then, said Zack Taylor, chief of the analysis section at the National Oceanic and Atmospheric Administration’s Weather Prediction Center,