CNN  — 

A strong storm system drenched some areas in California with snow and heavy rains as forecasters warned of mudslides and debris flows in areas ravaged by wildfires.

More than 30 million people are under flood or flash flood watches from California to Arizona, including in San Francisco, Los Angeles and San Diego.

The heavy rain poses a risk of mudslides and flooding in areas already devastated by last year’s wildfires – which included the most destructive in state history. Burned areas lack vegetation to hold the water in the ground, Ventura County Fire Capt. Stan Ziegler said.

Mandatory evacuation orders affecting more than 1,000 homes have been issued for the Holy Fire burn area in Southern California, according to Cal Fire Riverside.

A pineapple express, also known as an atmospheric river, is a narrow corridor of concentrated moisture in the upper atmosphere. The series of storms originates in the Hawaiian Islands, pulls in a significant amount of moisture from the Pacific Ocean and takes aim at the West Coast.

Central areas of California have already been hit hard, receiving 3 to 8 inches of rain in a day in many places, according to CNN meteorologist Haley Brink.

The San Francisco area broke a daily record for rainfall Wednesday with 2.13 inches, Brink said.

Flooded streets

A school bus got stuck in floodwaters Wednesday in Petaluma, about 40 miles north of San Francisco, according to CNN affiliate KGO. The station reported that two high schoolers driving in their pickups stopped to help get a student with special needs off the bus.

In Northern California, a woman has been hospitalized with minor injuries after the Sausalito house she was in slid off its foundation early Thursday, down a hill and then crashed into another home, according to Capt. Chris Tubbs of the Southern Marin Fire District. Officials received a call at about 3 a.m. and arrived to find the house had moved about a block’s distance. The woman was on the second floor of the first home.

About 50 homes have been evacuated as heavy rains pound the area, Tubbs said. He added they plan to assess the area to determine the extent of damages, secure gas and power lines, and hope to have a better idea of what happened once the sun comes up.

In the south, the rainfall created flooded streets in Fontana in San Bernardino County.

Snow and wind, too

In Northern California’s Shasta County, where both a flood watch and winter storm warning are in effect, residents are being urged not to travel because of falling trees and downed power lines, the sheriff’s office said.

Of the state’s 102,000 power outages as of Thursday afternoon, about 23,000 were in Shasta County, according to the website

The sheriff’s office warned residents there to “prepare for several days of power outages.” The state’s largest energy supplier, Pacific Gas and Electric, said weather conditions will lead to powerlines and other equipment being damaged. As of Thursday, PG&E had 84,495 customers in the state without power.

The Shasta County undersheriff declared a local state of emergency Wednesday due to the “significant damage” caused by a recent snowstorm.

An additional 3 to 6 feet of snow could fall on parts of the county as a winter storm warning remains in effect through Saturday, according to the National Weather Service.

The strongest wind gusts have been recorded at 75 mph in the Mount St. Helena area, according to the National Weather Service. In wind-prone areas, gusts as high as 90 mph are possible.

CNN’s Pierre Meilhan and Stella Chan contributed to this report.