An atmospheric river, unheard of in the meteorological world before 1998, is carrying water in a narrow conveyor belt of vapor from the equator, up to 8,000 feet above Earth, and dumping it on Northern California in a series of storms.

“It is basically as the term suggests, a river of water in the atmosphere,” Sonoma County Water Agency chief engineer Jay Jasperse told the Santa Rosa Press Democrat. “We are in one of the prime areas to receive it.”

Between 15 and 20 inches of rain had fallen as of Monday and at least another 5 inches was expected within days.

As many as nine atmospheric rivers hit the state each year, sometimes lasting just a few days. The ones best known to the public are called the “Pineapple Express,” because they form near Hawaii and slam through the state like a freight train. They have also been called the “Hawaiian Firehose.”

Forty-two atmospheric rivers have been identified in California since studies of the phenomenon began 14 years ago. They were blamed for seven floods on the Russian River between 2000 and 2006, one of which saw 10 inches of rain fall in two days. They also supply between 30% and 50% of California’s rain and snow, and ultimately the state’s fresh water supply. And they do it in about 10 days.

Although the regular downpours can be destructive, they are puny compared to the megacalamity that seems to accompany them in the state every 200 years. In 1861, it rained for 43 straight days, wiped out Sacramento and bankrupted the state.

The U.S. Geological Survey (USGS) designed a scenario to predict what a three-week atmospheric river would do to the state now and its report was not optimistic. Property damage would cost $400 billion, six times estimates for Hurricane Sandy, and business interruption would cost the state another $325 billion. All together, that’s three times the USGS estimate for damage from a severe Southern California earthquake.      

–Ken Broder

To Learn More:

Mysterious Atmospheric River Soaks California, Where Megaflood May Be Overdue (by Mark Fischetti, Scientific American)

Atmospheric “Rivers” Drench North Coast with Rain (by Bob Norberg, Santa Rosa Press Democrat)

Overview of the ARkStorm Scenario (U.S. Geological Survey)

More Rain-Drenched Days Ahead in North California (by Terry Collins, Associated Press)