What is an atmospheric waterway?

.FOR CALIFORNIANS it felt like familiarity. In 2014 a procession of storms, fed through atmospheric rivers, killed a minimum of 21 people and triggered almost $5bn-worth of damages across the state. On February fourth this year the very same sort of hurricane once again created chaos.

Hurricane-strength winds buffeted the central coast near Major Sur. In Los Angeles stars were soaked as they got to the Grammy awards’ red carpet. Manors in the Hollywood Hills were actually covered in mud.

A minimum of 3 people have died. What are actually atmospheric rivers, and are they a hazard beyond The golden state?