The storm, also known as a "Pineapple Express" because it develops from a ribbon of moist air moving across the Pacific Ocean, was forecast to dump as much as 10 inches (25 cm) of rain in coastal mountains.
National Weather Service meteorologist Austin Cross said more than three inches (7.5 cm) of rain had been already recorded in the hills of western Sonoma County by early Friday afternoon.
Fire crews responded to flooding in Siskiyou County, near the Oregon border, placing sandbags to protect homes and minor mudslides were reported in Washington state. Flash flood advisories were also issued for Sonoma, Napa and Marin counties in Northern California.
High winds caused more than 80 flights be canceled and hundreds more delayed at San Francisco International Airport, knocked down trees and caused scattered power outages.
According to Pacific Gas and Electric, more than 114,000 homes and businesses lost power, although the majority of them had been restored by late-afternoon.
Cross said heavy rains were expected to fall into Friday night and again on Sunday after a brief lull on Saturday.
Forecasters say that while the soaking would provide some relief to the drought-stricken state, it was not expected to make a significant impact, in part because the warm weather system would not add to mountain snowpacks.
The multi-year, record drought has prompted California officials to sharply reduce water supplies to farmers and impose conservation measures statewide.
Brian Fuchs, a climatologist with the U.S. Drought Monitor, said that experts were optimistic when California was hit with several strong storm in December but those hopes largely evaporated when January saw very little precipitation.
January is typically the state's wettest month, he said, with little rain falling between mid-April and December.
Fuchs said the drought's intensity was lessened in some areas, including Marin County, after December's rains and a small section of the state, on the eastern edge of San Bernardino County, was no longer considered to be in drought.
"It's one little corner of the state, a desert region that had more precipitation than they usually do and that was enough," he said, adding that it would not make a major impact on the larger crisis.
The skies were clear blue for days before this one arrived. Not a jet trail of any type to be seen.
Perhaps they got tired of messing with the weather, or ....??