The heat wave’s intensity and early arrival shocked many climate scientists. “It is exceptionally difficult for the Earth system to produce temperatures this warm so early in the season,” wrote Daniel Swain, a climatologist with the University of California Agriculture and Natural Resources who runs the Weather West blog.

Yet not only did Western locations set new March highs; many exceeded temperature records for May. And those high temperatures kept hanging on, said Zachary Labe, a climate scientist at the nonprofit science center Climate Central, for nearly two weeks.

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“The heat dome put an exclamation point on the worst winter in a century,” said Still. “It was the worst possible way to end the winter that was already worse than normal.”