A series of storms will ride down the West Coast of the U.S. bringing lower elevation rain and heavy mountain snow to California. An atmospheric river (narrow green stripe) will set up on the West Coast of the U.S. by the middle of the week producing heavy rain and mountain snow for much of California. An atmospheric river is a narrow flowing column of moisture responsible for producing significant rain and/or snow. These are most common along the West Coast of the U.S. with developing Pacific storms. West Coast StormAn upper-level low sliding down the California coast will induce the atmospheric river allowing for continuous heavy rain and snow for days.