Pacific Gas & Electric Corp. has begun to cut power to about 179,000 customers in 17 counties in Northern California.
The utility announced what it calls "Public Safety Power Shutoff (PSPS) events" Wednesday morning after warning customers earlier of the likelihood of its actions to reduce the potential for wildfires.
"Based on the latest weather readings, PG&E will be turning off power in portions of our service area as outlined below," read the announcement.
The Wednesday afternoon blackouts involved parts of Alpine, Amador, Butte, Calaveras, El Dorado, Nevada, Placer, Sierra, Tehama, Yuba, Lake, Mendocino, Napa and Sonoma counties.
Portions of San Mateo county, just south of San Francisco, will have power outages early Thursday morning, as will Kern county in the Central Valley.
The power outages could last longer than 48 hours.
"Once the weather subsides and it is safe to do so, PG&E crews will begin patrolling power lines, repairing damaged equipment and restoring customers," the utility's announcement said.
Some customers will experience power outages even though their locality is not experiencing "critical fire weather," in other words, high winds that can damage power lines.
"This is because the energy system relies on power lines working together to provide electricity across cities, counties and regions," PG&E explained.