Atmospheric rivers are vital to western water supplies, yet until very recently they were poorly understood: difficult to predict and measure, and very hard for scientists to estimate where they would make landfall. These are often erroneously called “pineapple express” storms, a term that applies to only a subset of atmospheric river events that originate near Hawaii. Most atmospheric river storms begin in the more distant tropical ocean and develop into a narrow band of strong winds that funnel huge quantities of moisture toward the West Coast of the United States.