Atmospheric rivers play a critical role in transporting water throughout the atmosphere by advecting (moving) moisture from the tropics to higher latitudes. When fully developed, they can deliver widespread rain and snow to areas all along the west coast. If an atmospheric river originates in Hawaii, like shown below, it is called a 'Pineapple Express'.
Atmospheric rivers form over the Pacific Ocean. As a low-pressure system develops and matures, it can form bands of extreme moisture. The speed, intensity, and strength of a given atmospheric river is dependent on the strength of the system itself.
As the system impacts land, large amounts of water in the forms of rain and snow are intensified. All this moisture is dropped onto land. When mountains are present, the moisture is forced upward. This lift forces even more moisture out of the system.
A typical atmospheric river transports an amount of water equivalent to that discharged at the mouth of the Mississippi River. However, the strongest atmospheric rivers can carry up to 15x that amount!