How do atmospheric rivers deliver so much rain to the coast?

Atmospheric rivers are not rivers in the ocean, but in the sky. They can transport enormous quantities of water vapor from tropical or subtropical regions toward higher latitudes. When this moist air is forced upward by mountains or meets cooler air, the vapor condenses and falls as intense precipitation. A single strong atmospheric river can produce flooding, landslides, or major snowpack in winter. On the U.S. West Coast and in many other regions, they are a major source of annual rainfall and snowfall.