Northern California is experiencing the first days of what weather forecasters are warning will be a long series of torrential rainstorms that could cause serious flooding across the northern one-third of the state. The relentless storms are being driven by a feature in the atmosphere you have probably never heard of: an atmospheric river.

Oh, and another atmospheric river created the worst flooding since the 1960s in western England and Wales this past week, where more than 1,000 homes had to be evacuated.

An atmospheric river is a narrow conveyor belt of vapor about a mile high that extends thousands of miles from out at sea and can carry 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. For more details, see this feature story that Scientific American has just published, written by two experts on these storms.

Scientists discovered atmospheric rivers in 1998 and have only recently characterized them fully enough to allow forecasters to warn of their arrival. They can strike the west coasts of most continents, but California seems to be a prime target. As many as nine small atmospheric rivers reach the state each year, each lasting two to three days, including the famous “pineapple express” storms that come straight from the Hawaii region of the Pacific Ocean. Ironically, although the storms are dangerous, they are also vital; they supply 30 to 50 percent of California’s rain and snow—in the span of about 10 days a year.

Megaflood Overdue?

The real scare, however, is that truly massive atmospheric rivers that cause catastrophic flooding seem to hit the state about once every 200 years, according to evidence recently pieced together (and described in the article noted above). The last megaflood was in 1861; rains arrived for 43 days, obliterating Sacramento and bankrupting the state. The disaster is largely forgotten, but the same region is now home to more than six million people. Simulations of a 23-day storm there indicate that more than $400 billion of damage and losses would occur, far surpassing the $60 billion estimates for Hurricane Sandy’s effects. New research also shows that climate change may make these storms more likely to occur.

You may begin to hear the term “atmospheric river” more often. The Weather Channel is using it, in quotation marks, in warnings for northern California, as well as the coasts of Oregon and Washington. And some popular media are beginning to adopt the verbiage as well.