Maps show where extreme humidity and 90- or 100-degree temperatures will affect around 255 million people across the United States.

A few locations could approach or top 100 degrees, including Denver, which happens only twice a year on average, said Weather.com meteorologist Jonathan Erdman, who added that "New York City and Philadelphia could also flirt with the century mark during the peak of the heat wave next week.

A “significant and extremely dangerous heat wave” is expected to send temperatures soaring across much of the United States in the coming days, stretching from the Central Plains to the East Coast, according to forecasters with the Weather Prediction Center.