LOS ANGELES -- The first wave of a powerful Pacific storm spread rain and snow early Friday through much of California, where communities endangered by a wildfire just weeks ago now faced the threat of mud and debris flows.

Authorities in the foothill cities of Glendora and Azusa east of Los Angeles kept a wary eye on the barren slopes as rains moved through. Small debris flows covered one Glendora street but no property damage occurred, police said. Mandatory evacuation orders were issued for about 1,000 homes in the area on Thursday.

A new evacuation covering 200 homes was ordered Friday morning in the nearby foothill city of Monrovia, where a wildfire denuded 170 acres of slopes above the city.

Numerous traffic accidents occurred on slick or flooded roads across Southern California, and a 16-kilometre stretch of Pacific Coast Highway west of Malibu was closed as a precaution against possible rockslides from a fire-scarred section of the Santa Monica Mountains.

In Los Angeles, rising water forced police to close major roads crossing the Sepulveda Basin. The flood-control area for the Los Angeles River on the San Fernando Valley floor is maintained as a wildlife refuge and recreation centre but is otherwise kept clear of development.

A Fire Department swiftwater team rescued two men and two dogs from a perch on a tree trunk in the fast-moving Los Angeles River.

Power outages hit about 24,000 customers, the Los Angeles Department of Water and Power and Southern California Edison said.

Forecasts called for the storm to last through Saturday in California, bringing some relief amid a long-running drought, and to spread east into similarly parched neighbouring states. Phoenix, Arizona, was expecting its first noticeable precipitation in two months.

Rain was also falling in the central coast counties, in the San Francisco Bay region and in the Central Valley. Some arriving flights at San Francisco International Airport were delayed by more than four hours, according to the Federal Aviation Administration. Weather conditions at Los Angeles International Airport caused cancellations of nine arrivals and 10 departures, officials said.