The weather also may be behind the death of a Pacific Gas & Electric worker in West Sacramento who was killed after his truck crashed into a traffic signal pole during the stormy weather.
Flights were delayed at San Francisco's airport, and in the city's affluent Pacific Heights neighborhood, traffic was blocked for hours after a large tree crashed down, smashing a car and obstructing a busy street.
A flash flood watch will remain in effect for most of the San Francisco Bay Area extending to the Santa Cruz Mountains throughout the weekend. A constant barrage of downpours could lead to standing water and overflowing drains, said Diana Henderson, a forecaster with the National Weather Service in Monterey.
The North Bay was seemingly hit the hardest, as parts of Sonoma County received more than 7 inches of rain and areas in Napa County received nearly 6 inches, Henderson said.
"It's not a super storm by any measure, but this is pretty significant," Henderson said. "We should see periods of moderate to heavy rains."
With rain expected all weekend long, Tony Negro, a contractor from Penngrove, Calif., in Sonoma County, said he is worried about water flooding his workshop.
"I'm on my way to get some sand bags," he said.
Thousands of people were without power in that area after an outage that also affected the San Francisco-Oakland Bay Bridge. The suspension span of the bridge was briefly in the dark as traffic was backed up longer than usual because of rain and strong wind gusts.
Also, a mudslide shut down a stretch of Highway 84 east of Fremont, the California Highway Patrol reported. There was no estimate on when it would reopen.
In Sacramento, an empty big-rig jackknifed in the southbound lanes and struck the median divider on Interstate 5 south of downtown Friday morning, the CHP said.
"I would definitely say it's weather-related. The reports came in that he hit a water puddle and hydroplaned and couldn't correct," CHP Officer Mike Bradley said. "A lot of high-profile vehicles, especially the lighter ones, are getting windblown and having some problems maintaining their lane."
No one was injured in the crash on I-5, California's main north-south highway. But a second vehicle also was damaged and had to be towed, while workers cleaned up diesel fuel spilled from the tractor-trailer.
In West Sacramento, police say wet conditions may have been a factor when a PG&E worker died after he lost control of his vehicle and slammed into a traffic pole. PG&E workers at the scene told KCRA-TV that the driver had been working overtime and was returning from Clarksburg in Yolo County.
Henderson said rain in the region is expected to taper Saturday, but return later that night into Sunday. The storms could create the possibility of rock and mud slides in areas already saturated and affected by wildfires this summer.
In Los Angeles, conditions were wet and gloomy as downtown skyscrapers disappeared in low-hanging clouds.
Elsewhere in the West, a state of emergency was declared in Reno, Sparks and Washoe County in Nevada due to expected flooding as a storm packing heavy rain and strong winds swept through the area. Reno city spokeswoman Michele Anderson said public servants would be working overtime through the weekend to control what's expected to be the worst flooding there since 2005. The National Weather Service issued a flood warning along the Truckee River.
The weather also prompted cancellations of Christmas parades and tree lightings in Sparks and Truckee, just across the border from California.
Also, a storm rushed through southern Oregon this week, lingering inland over the Rogue Valley and dropping record rainfall. It largely spared coastal Curry County and its southernmost city, Brookings, which were still recovering from a storm this month.
"We are still vigilant for landslides and road closures and trees down, but so far - knock on wood - we are still good to go," Curry County Sheriff John Bishop said.
Forecasters said the region should expect more storms over the next few days.