LOS ANGELES >> A Pacific storm moved into drought-stricken California on Tuesday, bringing hopes for much-needed moisture but fears of mudflows on wildfire-scarred hillsides.

Heavy rain began falling overnight in Northern California, while showers in the southern part of the state started after dawn. The strongest downpours were expected later in the day.

Storm watches were issued for a large swath of the Sierra Nevada, where a huge amount of the state’s water supply is normally stored as snowpack. Significant accumulations were predicted but not enough to be a drought buster.

Any delight over the prospect of beneficial precipitation was tempered by concerns about the threat of debris flows from the many areas of California where wildfires have burned away vegetation that would keep soil stable.

Residents prepared by placing sandbags to protect properties in the foothill city of Glendora northeast of Los Angeles, where some neighborhoods below steep mountains have long lived with concrete barriers lining streets in hopes of keeping debris flows out of homes.

Bill Golubics said he would heed warnings and evacuate his home in Camarillo, site of a fire last year that burned nearly 40 square miles of dry brush in Ventura County.

"Looks like most of my neighbors have left already," Golubics told KABC-TV, pointing out large rocks sitting on loose soil along on a slope above his home.

Voluntary evacuations also were planned in Orange County’s rural Silverado Canyon, where officials feared mudflows following a blaze that stripped hillsides bare in September.

The weather caused flights arriving at San Francisco International Airport to be delayed nearly four hours, the Federal Aviation Administration website reported.

The rain comes on the heels of a weak storm Sunday, which was enough to block Pacific Coast Highway west of Malibu with a flow of mud from a section of the steep Santa Monica Mountains denuded by a 44-square-mile fire last year.

The muck was cleared but debris basins that overflowed were still full and will keep the highway from reopening anytime soon.

"We’re going to have to wait until the next storm passes," said Patrick Chandler of the California Department of Transportation.

The back-to-back storms are helping some cities in northwest California reach normal rainfall amounts for the year, or even better, but the reservoirs and Sierra snowpack that provide much of the state’s water remain far short of what they should be after three years of intense drought.

The state Department of Water Resources reported the Sierra snowpack, which counts most for the state’s water supply, was at 24 percent of normal for this time of year.

Rain so far, at this early point in California’s wet season, has yet to make much of an impact on the state’s main reservoirs. Lake Shasta and Lake Oroville have less than 50 percent of their usual water levels for the start of December, while Folsom Lake stands at 59 percent, National Weather Service forecaster Eric Kurth said.

"The good news is there’s more storms on the way," Kurth said.