What Does Atmospheric River Mean? How the ‘Pineapple Express’ Impacts the West Coast

Throughout the year, a weather system known as an “atmospheric river” can impact the West Coast of the U.S., causing flooding, heavy snow and possible mudslides.

The National Oceanic and Atmospheric Administration defines an atmospheric river as a huge plume of subtropical moisture that moves with the weather, carrying water roughly equivalent to the average flow at the mouth of the Mississippi River.

“Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics,” the NOAA states.