A significant and unusually early heatwave is building across the south-western US this week, bringing temperatures more typical of early summer than mid-March.
In Phoenix, Arizona, the National Weather Service (NWS) forecasts temperatures could exceed 100F (38C) as soon as Wednesday, the earliest arrival of triple-digit heat to the city in nearly four decades.
A strong, slow-moving high-pressure system - often called a heat dome - is trapping hot air over the region, pushing temperatures 20–30F above normal in parts of California, Nevada, and Arizona.
The heat is expected to persist for several days, with limited overnight cooling adding to the strain. Millions are likely to be affected.
Highs are expected to reach well above 90F in many areas, including typically cooler coastal regions, while inland and desert locations could exceed 100F, with some spots approaching or surpassing 110F, challenging long-standing March records.
