A historic heat wave is currently impacting the western United States, with cities in California, Arizona, and other states experiencing record-breaking high temperatures for March. Phoenix, Arizona, is forecast to have multiple days with temperatures exceeding 100°F, which is highly unusual for this time of year. The heat wave extends beyond the Southwest, affecting the Plains with temperatures in the 90s as far north as Nebraska and Colorado. This extreme weather event is caused by a strong high-pressure system, known as a 'heat dome,' that is unseasonably potent.