“Atmospheric river” is a term that has been mentioned in the weather news lately. An atmospheric river is a relatively long, narrow region in the atmosphere that moves water vapor outside of the tropics. These columns of upper level moisture move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When the atmospheric river makes landfall, it often releases this water vapor in the form of rain or snow. In the U.S. a particular type of atmospheric river called the “Pineapple Express” transports water vapor from near the Hawaiian Islands to California. The big question in the Central Plains is how much of that moisture remains after the system crosses the mountains.
Mary Knapp, Weather Data Library