80 million Americans are under heat warnings or advisories.

Temperatures across the middle parts of the United States were “dangerously high” on Wednesday, the National Weather Service warned, as another heat wave took hold. And it won’t end soon.

About 80 million people were sweating through excessive heat warnings or advisories that stretched across Kansas, Oklahoma, Missouri, eastern Tennessee and Arkansas, according to Zack Taylor, a Weather Service meteorologist.

Thursday could be even hotter, with highs in the upper 90s and low 100s, the Weather Service said. Nightfall will bring little relief, with temperatures remaining in the upper 70s and low 80s. And high humidity levels will make parts of the Central Plains feel about 10 degrees warmer than it actually is.

“These multiday heat waves can become more dangerous because the body is unable to cool off at night,” Mr. Taylor said. “And it’s just repeating, day after day after day of high heat and humidity.”

The Central Plains and the South could break daily temperature records this week. On Thursday, Memphis is expected to break a record high of 102 degrees, which was set in 1875.

This heat wave follows one that settled over roughly the same region for three weeks in June, but it will very likely move more slowly across the country, Mr. Taylor said. Much of the Central Plains may not get any relief until next week. And although hot stretches during the summer are “not terribly unusual,” he said, the back-to-back heat waves are becoming a concern.

The heat dome that is trapping the hot air over the country was centered on Wednesday in western Kentucky, Illinois and Indiana. Some scattered thunderstorms could fall there, said Michael York, a meteorologist with the Weather Service in Kentucky. “But it doesn’t look like they will really cool down anything,” he added.