The northern half of the state was being hit at dawn with strong rain, 85-mph wind and heavy snow in the Sierra Nevada, National Weather Service forecaster Andrew Rorke said.
In Southern California, the storm was gathering strength off the coast and was expected to strike the region by mid-afternoon, Rorke said.
"We're watching it really blossom on satellite," he said.
Homeowners rushed to stack sandbags around houses lying below fire-ravaged hillsides in Southern California, while Northern California residents -- like those along the Gulf Coast before a hurricane -- scurried to stock up on last-minute provisions. Forecasters warned the high wind and other extreme weather would last through the weekend.
In the eastern Sierra ski town of Mammoth Lakes, resident Barbara Sholle went to the supermarket after receiving a call from the town's reverse-911 system. She waited an hour to pay for her groceries amid a crush of residents.
"People were waiting in line for shopping carts," she said.
The storm system began dumping rain and snow Thursday in parts of Northern California. Power outages, damaged electrical lines and downed trees were reported in the Sacramento area by nightfall.
The U.S. Forest Service issued an avalanche warning for Mount Shasta, in the Cascade Range in far Northern California, while the National Weather Service issued a rare blizzard advisory for the Sierra Nevada.
The storm system brought high wind warnings along the coast. Ocean tides were expected to swell to 30 feet, leading the Coast Guard to caution boaters to remain in port.
"If you don't have to go out this weekend, it might be a nice weekend to stay at home after the holidays," said Frank McCarton, chief deputy director of the California Office of Emergency Services.
As the storms barreled into California, forecasters were expecting a freeze in the East to subside. After a freezing day virtually everywhere east of the Mississippi River, temperatures in the East were to climb Friday.
Florida's citrus growers might have been spared major damage from the cold snap, which produced flurries in the Daytona Beach area, but it will be Saturday or later before strawberry farmers know the extent of their losses.
Plant City farmer Carl Grooms surveyed his fields Thursday afternoon and spotted numerous plants with berry damage. It was a sight he suspected other farmers east of Tampa witnessed in their fields hours after temperatures dropped below freezing.
"If I've got damage, I'm sure they do too," Grooms said. But his plants seemed intact, preserving hopes that his fields would bounce back.
A serious freeze would have been devastating to the Florida's citrus trees, already struggling from years of diseases and hurricanes. But most groves are in central and South Florida, where temperatures hovered in high 20s and low 30s. Trees can be ruined when temperatures fall to 28 degrees for four hours.
"It could have been far, far worse," said Terry McElroy, a spokesman for the state Department of Agriculture and Consumer Services.
Associated Press writers Gillian Flaccus in Orange County, Calif., Anthony McCartney in Tampa, Fla., and Tony Winton and Jennifer Kay in Miami contributed to this report.