An unusually early heatwave is developing across the southwestern United States, with temperatures expected to reach levels typical of early summer rather than mid-March. According to the National Weather Service (NWS), Phoenix, Arizona, may experience temperatures exceeding 100°F (38°C) by Wednesday, marking the earliest occurrence of such heat in nearly 40 years. A strong high-pressure system, often referred to as a heat dome, is causing temperatures to rise 20–30°F above normal in parts of California, Nevada, and Arizona. This heatwave is anticipated to persist for several days, with minimal overnight cooling, affecting millions of residents. Coastal regions, usually cooler, are expected to see highs above 90°F, while inland and desert areas could surpass 100°F, potentially breaking March records.