Nearly half of the US population will see temperatures of at least 95 degrees over the next seven days, according to meteorologists.
More than 70 million people are under heat watches, warnings or advisories in different parts of the country, including the Midwest, the National Weather Service said Tuesday.
"Temperatures in the mid-90s to 100 degrees sounds bad, but the most significant aspect of the heat wave will be the overnight temperatures," CNN meteorologist Taylor Ward said. "Low temperatures from the Midwest to the Northeast will be in mid- to upper 70s. In some locations, the overnight temperatures may not drop below 80 degrees."
Ward said dozens of locations will set records for warm overnight low temperatures.
"Heat waves tend to bring the most severe impacts when the overnight lows are well above normal because there is no relief for people's bodies or homes to cool down," he said.
In the Southwest, the temperatures reached 115 degrees in Phoenix on Tuesday, three degrees shy of a record set in 1925. The normal temperature for this time of year is 106 degrees.
The night time low was expected to be around 90 degrees, the NWS said.
The heat advisories also cover parts of the Carolinas and extend into the Southern Plains, from Oklahoma City to Dallas, where heat indices will approach 110 degrees, the weather service said.
Much of the Midwest, including St Louis, Wichita, Omaha and Kansas City, are under heat watches, with the mercury expected to top 100 degrees over the coming days, the NWS said.