The rainy season in the west is in full swing. With multiple storms expected to bring heavy rain and snow into next week, a specific meteorological phenomenon keeps popping up. Atmospheric rivers are long, narrow regions in the atmosphere that transport water vapor from the tropics. These columns of moisture move with the weather. They can carry the same amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River. When atmospheric rivers fuel a storm system, they can cause heavy rain and snow. Stronger atmospheric rivers create very powerful winds and the threat of flooding. If they stall over vulnerable watersheds, extreme rainfall events can disrupt travel, induce mudslides, and threaten life and property. However, some are much weaker. These systems help provide beneficial rain and snow crucial to the water supply. You may have heard of the "Pineapple Express." It's a common, strong type of atmospheric river the channels water vapor to the U.S. West Coast from the tropics near Hawaii. Credit: NOAA http://www.esrl.noaa.gov/psd/atmrivers/.