Snow began dusting the ski slopes and peaks of the Sierra Nevada and rain fell on the far northern reaches of California.
Both were expected to go from drizzling to pounding as the weekend continues.
Snow plows and other snow-removal equipment were parked on roads in the mountains Friday ahead of the storms expected to dump up to 2 feet of snow in some spots.
The National Weather Service issued a winter storm advisory for the Lake Tahoe area from Friday night through Saturday night.
The storm moving in from the Gulf of Alaska soaked the state's Redwood Coast for several hours.
In fire-scarred regions of wine country, meanwhile, crews were working to stem the flow of ash, soil or hazardous substances into waterways in case the storms bring heavy rains there.
A mountain pass across the Sierra Nevada that runs through Yosemite National Park remained closed to traffic Friday ahead of the snow storms forecast for the weekend at upper elevations, park officials said.
Tioga Road, the soaring eastern entry to Yosemite, was closed to traffic shortly before nightfall Thursday. Officials were also closing Glacier Point Road, which offers sweeping views of Yosemite Valley.
Both roads are to be shut through Monday, when the storms are expected to dissipate and road conditions can be assessed.
Two storm systems will be moving through Northern California over the weekend, bringing rain and snow to the region.
Most of the rain was expected overnight Friday into Saturday morning.
A light snow fell overnight Thursday in the Donner Summit area and other upper elevations of the northern Sierra Nevada. By Friday afternoon, multiple Cal Trans snow plows and other equipment were parked along the side roads of Interstate 80, ready to be deployed.
In Sonoma County north of San Francisco, crews have been working long days to prevent feared mudslides and water pollution in areas hit by last month's devastating wildfires, which killed at least 43 people and destroyed more than 8,900 homes and other structures.
The work includes placing sandbags and bundles of straw to block runoff into streams, and moving to capture some of the runoff in urban areas that burned. California has declared a public-health emergency in fire areas in part out of concern that household chemicals from burned areas could contaminate soil and water.
Up to 2 feet of snow are forecast to fall in elevations above 8,000 feet, and at least a foot of snow is expected in Donner and Tioga passes and other areas above 6,000 feet, forecasters said. The storms coming in from the Gulf of Alaska will also bring rain to the San Francisco Bay Area, including North Bay counties still recovering from last month's fires.
The first storm will clear by Sunday afternoon. But a second storm system is expected to hit the area Sunday night.