Atmospheric rivers are powerful columns of moisture that act like a fire hose, tapping into the warm, humid air of the tropics which storms can then dump in the form of torrential rain and heavy snowfall on the land.
These types of storms are characteristic of winter along the entire West Coast. Not all of them are harmful. Weaker atmospheric rivers bring much-needed rain and snow to the region during the wettest months of the year.
This rain and snow is vital to the western water supply. Fills water reservoirs that are critical resources during hot, dry summers.
But atmospheric rivers can quickly become more dangerous than useful. The excessive rainfall unleashed by these powerful storms increases the chances of flooding, especially when a series of storms occur in sequence with little time to dry in between.
Last week's atmospheric river broke multiple rainfall records in California and prompted road closures and water rescues.
Atmospheric rivers are not unique to the West Coast. They can also affect the eastern US and occur frequently in parts of Europe, Africa, Australia and New Zealand.
