An unusual heatwave struck the western United States on Friday, alarming climate scientists who described it as a once-in-500-years event. According to reports from Los Angeles, temperatures broke previous records throughout the week and were expected to remain high through the weekend. The heatwave, initially centered in the western cities, was moving eastward across the country.

In desert areas near the California-Arizona border, four locations recorded 44.4 degrees Celsius on Friday, setting a new national record for March. Weather.com reported that 65 cities, from Arizona and California to Idaho, experienced new March temperature highs. Death Valley reached 40 degrees Celsius, while typically cool San Francisco hit 29 degrees. The U.S. National Weather Service issued an extreme heat warning for large parts of the Southwest, including Los Angeles, southern California’s coastal areas, and Las Vegas.

The World Weather Attribution group stated that such an event would have been nearly impossible without human-induced climate change, emphasizing its extreme rarity even in a warming climate.