Southern California is about to get flooded with relentless rain, and fears of mudslides are becoming serious. Major news stations, weather channels, Web outlets and social media are all suddenly talking about the “atmospheric river” that will bring deluge after deluge to California. What is this thing? How rare is it? And how big of a threat could it be? Here are some answers. And see our graphics, below, taken from a brilliant and prescient feature article written by Michael Dettinger and Lynn Ingram in Scientific American in January 2013.

Not interested? In 1861 an atmospheric river brought storms for 43 days and turned California’s Central Valley into an inland sea 300 miles long and 20 miles wide. Thousands of people died, 800,000 cattle drowned and the state went bankrupt. A similar disaster today would be much more devastating, because the region is much more populated and it is the single-largest food producer in the U.S.

So maybe 1861 was an oddity. Not really. Geologic core samples show extreme floods like the one in 1861 have happened in California about every 200 years, since A.D. 200. The next disaster could be coming around the bend. The west coast has actually been slowly constructing large, specialized, meteorologic observatories that can sense atmospheric rivers as they develop, so forecasters can give early warnings.

An atmospheric river is a conveyor belt of vapor that extends thousands of miles from out at sea, carrying as much water as 15 Mississippi rivers. It strikes as a series of storms that arrive for days or weeks on end. Each storm can dump inches of rain or feet of snow. Meteorologists sometimes call small occurrences “pineapple expresses,” because they tend to flow in a straight line from around Hawaii toward the U.S. west coast. The graphic below explains the details.

Credit: Don Foley

Several regions of central California have been frequent targets in the past two millennia. Here’s the record from core samples showing that every 200 years or so a catastrophic atmospheric river many times greater than any pineapple express occurs.

Credit: Jen Christiansen (graphic); XNR Productions (map)

The flow pattern of the atmospheric river now battering the west coast is classic. The University of Wisconsin–Madison maintains a terrific Web site that shows the flows in real time, updated every five minutes. If you want to know more about these monster storms, check out the feature article by Dettinger and Ingram.