Flash flood warnings and evacuation advisories are in place for much of the Bay Area of California, triggered by an unusual weather phenomenon called atmospheric rivers.

So what is this strange aerial stream wreaking havoc across the West Coast? Several areas ravaged by record wildfires last year now face mounting risks from mud slides as a result of heavy rains and snow in and around San Francisco and the Sierra Nevada Mountains.

The National Weather Service in Sacramento this week warned that some inland areas in the mountain range could expect as much as 8 feet of snow. The heavy moisture comes from a system called an atmospheric river.

“Atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that transport most of the water vapor outside of the tropics,” the National Oceanic and Atmospheric Administration explained. “These columns of vapor move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River.”

When these rivers move inland and run against the mountain range, the water vapor rises with it, cools and leads to significant amounts of precipitation. The extreme amount of precipitation has triggered mudslides across the area, with growing fears that more could be on the way.

Dorothy Priolo, an official with the Monterey County Regional Fire Protection District, said several homes suffered damage because of mudslides. One person was treated for injuries after mud swept through their home.

“We are expecting there could be more earth movement here in the next couple of days,” Priolo was quoted by the Bay Area’s NBC affiliate as saying.

As much as a foot of rain could fall on some areas in the coming days. With power outages and widespread evacuation advisories, Boulder Creek Fire Chief Mark Bingham told residents via the CBS affiliate in San Francisco to take the weather seriously and get out of harm’s way.

“I would ask you - urge you - to please reconsider the decision to leave,” he said to those who may be reluctant to evacuate. “The weather is unpredictable.”

Farther east, winter weather advisories are in place for parts of Montana. The National Weather Service there said strong winds and as much as five inches of snow are possible through Thursday afternoon.

This graphic explains an atmospheric river.
This graphic explains an atmospheric river. NOAA