In New York City, a tropical storm delivered record-breaking rains this weekend. Heavy downpours caused devastating flash floods in central Tennessee, tearing apart houses and killing more than 20 people. Yet, California and much of the West remained in the deepest drought in at least two decades, the product of a long-term precipitation shortfall and temperatures that are much hotter than usual.

This divide, a wetter East and a drier West, reflects a broader pattern observed in the United States in recent decades.