https://www.scientificamerican.com/article/warning-scale-unveiled-for-dangerous-rivers-in-the-sky/

Several times a year an atmospheric river—a long, narrow conveyor belt of storms that stream in relentlessly from the Pacific Ocean—drops inches of rain or feet of snow on the U.S. west coast. Such a system triggered floods and mudslides in central and southern California this past weekend. Yet only in the past few years have weather forecasters been able to predict roughly how strong the imminent storms may be.

Now they have a new rating system to help them—and the public—understand what an incoming atmospheric river might unleash; on Tuesday researchers unveiled the “atmospheric river scale.” It ranks severity and impacts, from category 1 (weak) to category 5 (exceptional). “Without a scale, we really had no way to objectively communicate what would be a strong storm or a weak one,” says Martin Ralph, director of the Center for Western Weather and Water Extremes at Scripps Institution of Oceanography, who led the work.