The weather is also expected to hinder search teams sifting through ash and rubble for the remains of victims of the deadliest wildfire in the state's history.

As much as nearly 20cm of rain is forecast to fall by Friday in areas around the town of Paradise, a community of nearly 27,000 people 280km northeast of San Francisco that was largely incinerated by the Camp Fire. The blaze killed at least 81 people and left hundreds missing.

The storm will help firefighters still battling the fire but create more suffering for many residents left homeless by the disaster.

Some of the homeless, whose numbers have not been determined, are camping out in parking lots rather than staying in emergency shelters.

Forecasters said the rains, which in some areas are likely to be accompanied by winds of up to 72km/h, might also cause rivers of mud and debris to slide down flame-scorched slopes stripped of vegetation. The fire has burned across 62,000 hectares of the Sierra foothills.

Mass evacuations since the fire erupted on November 8 have removed most people from harm's way of any debris flow, according to National Weather Service hydrologist Cindy Matthews.

She also said the volcanic soil and relatively shallow slopes in the fire zone mean the ground is unlikely to become saturated enough for hillsides to give way to landslides.

Authorities in Southern California, however, warned residents in areas burned by wildfires in the foothills and mountains northwest of Los Angeles of mud-flow hazards from rain this week. One of those blazes, the Woolsey Fire, killed three people.

Evacuees also face increasingly chilly weather.

Authorities in Butte County have been gradually allowing residents evacuated during the fire back to see what is left of their homes.

The remains of two more victims were found in Paradise on Tuesday, raising the death toll to 81. The Butte County Sheriff's Office has tentatively identified 56 of the victims.

A missing-persons list compiled by the sheriff's office grew again to 870 names on Tuesday night after detectives got through a backlog of voicemail messages, the office said.

Buffer lines have been carved around 80 per cent of the fire's perimeter and full containment is expected by the end of the month.

Smoke from the fires has drifted across the country to the East Coast, where it left a brownish-orange haze that was credited for unusually vibrant sunsets on Monday.

Australian Associated Press