Atmospheric River Formation

An atmospheric river forms when a narrow, elongated corridor of the lower atmosphere, often just a few hundred kilometres wide but thousands of kilometres long, concentrates an unusually large flux of water vapor drawn from the warm, moist tropics and channels it toward the mid-latitudes ahead of a frontal system. Where such a corridor makes landfall against rising terrain, the forced ascent of the moisture-laden air can release extreme rainfall or snowfall in days, the mechanism behind both many of the U.S. West Coast worst floods and a large share of its annual water supply, depending on intensity.