Dangerous Heatwave Hits the US West Coast This Spring

A significant and unusually early heatwave is building across the southwestern 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 100°F (38°C) as soon as Wednesday, marking the earliest arrival of triple-digit heat to the city in nearly four decades.

A strong, slow-moving high-pressure system, often termed a heat dome, is trapping hot air over the region, pushing temperatures 20–30°F above normal in parts of California, Nevada, and Arizona.

The heat is expected to linger for several days, with limited overnight cooling compounding the strain. Millions across the region are likely to be affected.

Highs are anticipated to reach well above 90°F in many areas, including typically cooler coastal regions. Inland and desert locations could see temperatures exceeding 100°F, with some areas nearing or surpassing 110°F, posing challenges to long-standing March records.

NWS has issued heat advisories for parts of California, including the Bay Area and central coast, where temperatures may hit around 90°F. Residents are being urged to check on the elderly and other at-risk populations while rescheduling outdoor events to avoid peak heat.

Forecasters highlight that this extreme early-season heat, coupled with high tourism rates, presents substantial dangers, particularly for those unaccustomed to such heat and visitors from cooler climates.

The prolonged heatwave also threatens California's already below-average snowpack, accelerating melt during a period when it typically builds or declines slowly. Rapid runoff could heighten the risk of summer water shortages as snowpack contributes roughly a third of the state's water supply and could impair landscapes susceptible to wildfires.

Despite the heat, wildfire risk remains relatively low in many parts of the Southwest due to lighter winds, which could provide a buffer for several communities. In contrast, areas towards the east, near the Rockies and High Plains, may experience favorable conditions for wildfires due to dry air, parched vegetation, and persistent breezes.

This heatwave comes on the heels of recent blizzards and frigid conditions in the Midwest and Great Lakes and severe thunderstorms and tornado threats in the East Coast.

As the week progresses, above-average temperatures are expected to spread across much of the United States.