The state’s largest power utility, Pacific Gas & Electric, said it had cut power to 500,000 customers soon after midnight. A second round of cuts affecting 250,000 more customers in the hills surrounding the San Francisco Bay Area had been scheduled for noon but was delayed.

“It’s all dependent on weather conditions,” said Jeff Smith, a spokesman for the company. Smith could not give a new time for the next round of blackouts, though a police department in the East Bay of San Francisco said the new power shut-off time was 8 p.m.

While PG&E; said that hundreds of thousands of customers would lose power, an entire apartment building can be considered a single customer. Once the two phases are complete, around 2.5 million people will be without electricity, according to one estimate.

The power company described Wednesday’s cuts as a precaution, hoping to prevent its electrical equipment and power lines from sparking blazes in dangerous conditions.

These are not your average California winds.

Meteorologists compared the winds forecast for Wednesday to those that propelled fires through wine country two years ago.

Hannah Chandler-Cooley, a meteorologist in the National Weather Service’s Sacramento office, said a strong weather system moving through the Great Basin was causing the high winds across Northern California.

“Northerly winds first and foremost really dry out the area,” she said, a risky combination with grasses and vegetation in the summer and fall, when there has not been much rain.

Chandler-Cooley said the region rarely gets north winds, which blow for only a few days just a few times a year, and can cause trees to topple or limbs to fall — often on power lines. Adding to her concern are forecasts of “pretty extreme winds,” with sustained speeds of 20-30 mph and gusts at 35-45 mph or higher in some areas, like mountain canyons.

While the power outage may help prevent sparks, she urged residents to practice fire safety, recalling that several wildfires last year were caused not by downed power lines but by human activity. The Carr fire spread after sparks from the wheel rim of a car fell on dry grass.

The utility is trying to avoid a repeat of the deadly fires that have ravaged the state.

The company has been found responsible for dozens of wildfires in recent years, including the state’s deadliest, an inferno in and around the town of Paradise last November that killed 86 people.

Over the summer the utility turned off power to less-populated areas in Northern California, but this shut-off is by far the company’s most extensive, affecting large parts of the San Francisco Bay Area.

More than half of all counties in California — 34 out of 58 — are expected to be affected by the power cut, according to PG&E;, one of the country’s largest utilities.

The outages have upended daily life for many in the Bay Area.

Officials in Santa Rosa, about 50 miles north of San Francisco, said they had responded to multiple traffic collisions, including five with injuries, at intersections without power.

“Please slow down and treat all intersections without power as a four-way stop,” city officials said on Twitter.

For residents in assisted-care homes or dependent on medical equipment in their own homes, the power cutoffs added to their daily challenges and stoked the worries of family members.

“My father is in a nursing home in Santa Rosa working on backup power,” Daisy Pistey-Lyhne of Santa Rosa said. A sister with a disability, who lives in a nearby apartment complex, still had power Wednesday, but Pistey-Lyhne was busy trying to make contingency plans.

In Oakland, Stacey Milbern, who uses a ventilator for breathing and needs other medical devices, spent more than two hours Tuesday afternoon calling PG&E; to try to find out whether her home would lose power and whether she would get additional assistance.

She learned that her home did fall in an area affected by the outage. But she still had her power Wednesday afternoon. “Honestly, I have so much privilege. I work full-time from home,” she said. “But for the everyday disabled person, it’s so scary.”

As of late Wednesday afternoon, PG&E; spokesman Jeff Smith could not confirm whether any Bay Area hospitals would lose power during the second phase of planned shut-offs, which are expected to take place Wednesday evening.

“There may be some hospitals that are in the footprint, but we won’t know for sure,” Smith said. “We’re working extensively with locations like hospitals to ensure that they have an emergency plan.”

Turning the power back on could take as long as five days.

PG&E; anticipates that it will begin turning power back on starting Thursday, when winds subside.

But reenergizing power lines is a tricky process, even after the winds subside. Sumeet Singh, a PG&E; vice president, said in a briefing Tuesday night that technicians will need to inspect “every inch” of line before restoring power. That could take as long as five days, he said.

PG&E;’s website was down for many people, right when they needed it.

On Wednesday morning, PG&E; customers across Northern California said they were frustrated by difficulties getting information about blackouts and when power might be restored. Many blamed the utility for cutting power before they believed it was really necessary.

“There hasn’t been even the slightest bit of wind in the entire county,” Candace Bennyi, whose power was cut in Sonoma County, wrote in an email.

“One would have expected PG&E; to at least wait to see if there was actually going to be an event that warranted such a move.”

The utility’s website was working only intermittently — something Bennyi mentioned too.

In Santa Rosa, Pistey-Lyhne woke up without power in her home, which she said was at the edge of where the power cutoff had been planned in her neighborhood.

Though Pistey-Lyhne made preparations for the blackout, she expressed concern that many others in the region were left unaware. She said that she had found out only Tuesday afternoon that the power would be shut off overnight.

“I don’t think PG&E; did a great job,” she said, despite the local government’s work on improving emergency preparedness and communications. “It was less than 12 hours’ notice.”

Residents have been stocking up on generators and water.

Shoppers emptied supermarket shelves of batteries, water and other essentials, with many hitting the stores Tuesday night and early Wednesday while power was still on.

In the small beach town of Montara, just down Route 1 from San Francisco, Heidi Kay and her partner, Steve Christie, took an inventory Wednesday of their few supplies, which amounted to little more than granola bars, oatmeal and fruit, said Christie, 49. “We haven’t really stocked up on anything,” he said.

Kay, 39, had driven to the nearby town of San Bruno to buy a few groceries after finding the Target near her office had been practically stripped bare.

The hardware store was also “out of everything,” she said, so Montara residents were taking matters into their own hands on the neighborhood social app Nextdoor.

“Everyone on there is in search of a generator,” she said. “It’s mad chaos.”

Over the summer, stores in Northern California reported higher-than-usual sales of gasoline generators. For those who missed out, PG&E; established around 30 facilities stocked with bottled water and outlets to charge electronic devices.

Lines at gas stations were 20 cars deep Tuesday night, reported Gary Bowman of Grass Valley in the Sierra Foothills area midway between Sacramento and Reno, Nevada. But Wednesday morning, as he and his wife searched for a restaurant to have breakfast in, he found the area “deader as a doornail.”

Bowman, a recently retired schools superintendent who also spent 10 years as a wildland firefighter, said he would spend the day working on his 2-acre property, where he tames the undergrowth to help keep fires at bay.

“We’ve had power outages in the deep winter when the cold and darkness are issues,” he said, “so to get something like this in October, when it’s 70s and sunny, is not so bad.”

Mass transit is running, but many schools are canceling classes.

The main mass transit systems serving the San Francisco Bay Area — BART and Caltrain — said they would maintain service.

A number of schools in San Jose and Oakland said they would close for as long as there was no power. The University of California, Berkeley, canceled classes Wednesday.

The East Bay Municipal Utility District, a water utility, said its pumping capacity would be affected by the shut-off and urged its customers to minimize water use and turn off their irrigation systems.

How much does a power cut reduce the risk of wildfire?

PG&E;, which filed for bankruptcy in January in the face of tens of billions of dollars in wildfire liabilities, has been repeatedly castigated and admonished by a judge overseeing an effort to improve the company’s safety culture and remove vegetation near its electrical lines.

The deliberate power cuts have been described by PG&E; as a way to lower the risk of fire while the company proceeds with its vegetation-trimming program. But by no means does it remove the risk of fires entirely.

Climate change, years of drought and the construction of houses and communities in wild land areas have all contributed to the spate of intense and deadly fires in California in recent years. In addition to electrical equipment, the direct causes of the fires have included lawn mowers, campfires, arson and, in one case, a man trying to plug a wasp’s nest with a metal spike.

Wildfires that ignite in extreme wind conditions can be very difficult to bring under control, firefighters say. The deadliest fires of the past two years — the one that razed Paradise last year and the wine country fires of 2017 — both occurred in similar conditions to the ones that meteorologists are forecasting this week.

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