Major cities across the United States are preparing for extreme temperatures caused by a heat dome that hit the U.S. Plains on Friday and is expected to expand to much of the rest of the country over the coming days.

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

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.