What is actually an atmospherical waterway?

.FOR CALIFORNIANS it seemed like familiarity. In 2013 a ceremony of tornados, sustained through atmospheric rivers, gotten rid of a minimum of 21 people and triggered nearly $5bn-worth of damages throughout the state. On February 4th this year the exact same kind of tornado once again ravaged.

Hurricane-strength winds buffeted the main coast near Big Sur. In Los Angeles famous people were actually soaked as they reached the Grammy honors’ ceremony. Estates in the Hollywood Hillsides were actually dealt with in mud.

At the very least 3 people have perished. What are climatic rivers, as well as are they a danger beyond California?