Heat alerts were issued for millions across parts of the western U.S. Sunday as an unusually prolonged heat dome reached its peak.

Most of the U.S. is about to swelter under an unusually large, strong and long-lasting heat dome. The heat wave is set to start this weekend and last at least a week.

The heat dome’s immense size means it will bring sweltering conditions to as much as two-thirds of the continental United States