"This is a big storm, and slow moving," Eric Lamoureux of the state Office of Emergency Services told AFP. "We need to monitor it and be ready to respond."

Rivers along California's north coast were under flood watch and thousands of sandbags had been handed out to shopkeepers and homeowners to erect protective barriers around buildings.

Four homes were destroyed in landslides during rains earlier in the week and cities have been vexed by flooded streets during downpours.

A boulder tumbled from famed Telegraph Hill in San Francisco, causing temporary evacuation of some homes there. Mobile homes in some low-lying areas have been moved to safer ground.

"That is about the extent of the damage so far," Lamoureux said. "Everybody is getting wet."

High winds that arrived with the rain caused average delays of more than two hours for flights arriving at San Francisco International Airport, according to officials.

A combination of wind, high tide and heavy rain had officials worried about erosion to levees in the California Delta, a vast network of waterways near the city of Sacramento.

The levees were being constantly patrolled and water releases from reservoirs were increased to keep up with the rainfall, said Michael Miller of the state Department of Water Resources.

"We are looking at a major weather event," Miller told AFP at mid-day Friday. "There is a possibility a lot of the rivers will reach or surpass flood stage within the next 12 hours."

State National Guard, Conservation Corps, and Department of Forestry units were on alert, Lamoureux said.

The storm was expected to stretch inland to the Sierra mountains and separate storms are forecast to douse Southern California, Miller said.

Mountain areas in Los Angeles County recently hit by brush fires were particularly vulnerable.

"Flash flooding and mud and debris flows will be a very real threat for the burn areas," the National Weather Service said.

A light storm was forecast for Friday and Saturday and a more powerful storm was expected to hit Southern California on Sunday, likely dumping two to four inches of rain in coastal and valley areas and four to eight inches in the foothills and mountains.

The forecast could mean the annual New Year's Day Rose parade in Pasadena will be "a little soggy" for the first time in five decades, Miller said.

In Northern California, revised predictions by meteorologists said rainfall would total from three to 10 inches (7.5 to 25 centimeters) along the coast from San Francisco to the border of the state of Oregon by the end of the New Year's weekend.

The Russian River that flows through California's chic wine country is expected to breach its banks. The river is the most flood prone in the United States, according to water officials.

The ground has been saturated by rains in recent weeks, meaning soggy hillsides could give way and water will run off instead of being absorbed into soil.

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