A dangerous stretch of extreme heat is gripping much of the Southeastern United States this week, with the most intense combination of high temperatures and humidity expected to persist through at least Wednesday. Forecasters said relief isn’t likely to arrive until the weekend. Temperatures are expected to climb into the upper 90s and low 100s.

More than 250 million people in the U.S.—nearly three quarters of the population—are experiencing moderate, major or extreme risk of heat effects on July 28, according to the National Oceanic and Atmospheric Administration’s National Weather Service.