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

Some of the highest temperatures on Friday are expected in towns close to the Nebraska-Kansas border, with those in Kansas's St. Francis and Oberlin, and McCook, Nebraska, likely to rise to 105 degrees and over before the end of the day, according to the National Weather Service.

Central Pennsylvania is bracing for a significant and dangerous heat wave set to begin Sunday, with temperatures climbing into the 90s and heat index values potentially reaching dangerous levels, prompting an Extreme Heat Watch for multiple counties.