At least 20 people, including three children apparently buried in their sleep in a mud slide, were dead or missing today after a fierce winter storm unleashed a sea of mud and water over parts of the state.

The storm struck an already waterlogged northern California Monday, flooding highways, derailing two trains, washing expensive homes off hillsides, stranding thousands of skiers at mountain resorts and temporarily closing the Golden Gate Bridge.

The storm moved eastward today.

California Gov. Edmund G. (Jerry) Brown Jr. declared states of emergency in San Mateo, Sonoma, Marin, Santa Cruz and Contra Costa counties, where hundreds of homes were destroyed or damaged. He also declared an emergency in Humboldt County, which suffered damage from rain flooding Dec. 19.

"In 32 years of working with emergency services this is the worst that I have ever seen," said William Ward, regional director for California Emergency Services.

Hardest hit was Marin County, where 12 inches of rain fell in 24 hours and damage was estimated at $100 million. At least 10 homes, one of them worth $500,000, were destroyed by mud slides, and others were still in danger of slipping off mucky hillsides.

Rescuers today found the bodies of 11 people buried by mud or killed by falling debris, bringing the total of confirmed dead to 18. Two persons were missing and presumed dead in the San Francisco area.

Workers used shovels and power equipment, including a 30-ton crane, to rip the rubble apart and remove parts of the roof of a house in Pacifica, 10 miles south of San Francisco, in the search for the three children of Barbara and Bill Velez, who were buried in their bedrooms shortly before midnight Monday when a house above them on the hillside collapsed and sent mud and debris crashing into the Velez home.

Late tonight workers found the bodies of two of the three children, Michelle, 14, and her sister, Melissa, 2. The search continued for their brother, Billy, 7, who was presumed dead.

One man was still missing and presumed dead after mud swept his home away in Marin County.

Six cars of Amtrak's San Francisco Zephyr passenger train derailed at San Pablo Monday afternoon, shortly after leaving Oakland for Chicago. The floods apparently had washed out some of the rail bed, and the tracks buckled under the train's weight.

Thirteen passengers were slightly injured, and the train's occupants had to be rescued by boat and helicopter as water swept over the track. A seven-car Southern Pacific freight train was derailed about 200 miles to the north, at Alderpoint, but no injuries were reported.

The storm hit an area of the state already drenched from heavy rains last week. The National Weather Service in Redwood City, Calif., said the storm carried an unusual amount of moisture because it originated in the subtropics, rather than in the usual source of such rainstorms for northern California, the Gulf of Alaska.

By today the worst of the storm had passed San Francisco, with some roads and some lanes of the Golden Gate reopened.

"Right now we're trying to get out of the emergency phase and into the recovery phase," said Donna Darling, an emergency services coordinator in San Francisco. "We've still got houses sliding. The county officials are overwhelmed."

An estimated 150,000 northern California homes suffered some power failures. Small Pacific Coast towns like Bodega Bay and Point Reys Station were reportedly cut off by the flooding temporarily. Workers sandbagged the banks of the Russian River north of San Francisco as it rose toward flood level.

The northern California storm dumped two feet of snow on the Sierra Nevada mountains as it moved east, adding to snowdrifts as deep as eight to 12 feet and stranding many ski enthusiasts. All major roads across the Sierra Nevada were temporarily closed, and the U.S. Forest Service triggered more than 100 intentional snowslides to reduce the danger of unexpected avalanches.

"I couldn't even find my car," Linda Bowen of South Lake Tahoe, Nev., told the Associated Press.