Atmospheric Rivers transport a lot of water vapor/moisture into the west coast. Between 30 and 50% of the annual precipitation out west comes from Atmospheric Rivers. The amount of water vapor is at least 15 times the average flow of water at the mouth of the Mississippi River and when they make landfall, this water vapor can be released in the form of rain or snow. Additionally, atmospheric rivers can be at least 350 miles (563 km) wide and up to 1,600 miles (2575 km) long.
