Forecasters now say 8 to 15 inches of rain could fall in the mountains and foothills of Southern California.

The National Weather Service warns the storm could drop unprecedented rain over a wide area from Saturday night through early next week, with the period of most concern from Sunday into early Monday.

Another atmospheric river set to arrive Monday could worsen already severe flooding, as the extra rain and snowmelt threaten to overflow rivers and streams at lower elevations.

Forecasters say the result could be massive flooding and a potential for avalanches across stretches of the state.

An atmospheric river is a relatively narrow channel of wind, hence the "river," that transports water vapor from the tropics to the West Coast, where it's producing rain and heavy winds.

Flooding was reported across the San Francisco Bay Area while the National Weather Service's Sacramento office warned of "potentially historic rain."