CreditJim Wilson/The New York Times

SAN FRANCISCO — Large swaths of central and Northern California were without electricity on Wednesday as the state’s largest utility began cutting power as a precaution against sparking wildfires.

The utility, Pacific Gas & Electric, said that as of early Wednesday morning power had been cut to around 500,000 of the 800,000 customers who will be affected. Once fully implemented on Wednesday the outages will span from the doorstep of Silicon Valley to the foothills of the Sierra.

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Extreme winds are expected on Wednesday and Thursday. Peak wildfire season has arrived in California, with a combination of high winds and low humidity creating conditions for potentially catastrophic fires.

Live power lines owned by PG&E were blamed for several large wildfires in recent years, with strong winds and untrimmed vegetation hastening the damage.

Karleisa Rogacheski, a meteorologist in the National Weather Service’s Sacramento office, said that Wednesday’s winds were expected to be the strongest since the North Bay fires two years ago. “The weather pattern and fuel conditions are pretty darn close,” she said.

In San Jose, the country’s 10th largest city, officials warned Wednesday morning that many traffic signals would go dark at noon, when the second wave of power cuts was due to take effect.

Motorists will be left to navigate intersections on their own in most affected areas, officials said, a daunting prospect in a city that has gnarled traffic on the best of days.

“Avoid driving if at all possible,” Charles Jones, the vice mayor of San Jose, said in a news briefing.

Kip Harkness, the city’s deputy city manager, said he was most concerned about elderly residents and those who relied on medical equipment. He urged residents to check on their neighbors.

“This event can be very scary and it can affect a lot of people,” Mr. Harkness said.

PG&E estimates that 38,000 customers will be affected by the power outage in Santa Clara County where San Jose is located. But an entire apartment building can be considered a single customer. Mr. Harkness estimated that as many as 200,000 people could be without power in San Jose alone.

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.

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

But re-energizing 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.

On Wednesday morning, Pacific Gas & Electric 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 Ms. Bennyi mentioned too.

On Wednesday morning, the main PG&E website pointed customers to a specific page for the outages, but the page appeared to be broken.

In Santa Rosa, a city of 175,000 in Sonoma County, Daisy 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 Ms. 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 on 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.”

Shoppers emptied supermarket shelves of batteries, water and other essentials, with many hitting the stores on 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, had charged their phones, laptops and external batteries on Wednesday morning. They took an inventory of their few supplies, which amounted to little more than granola bars, oatmeal and fruit, said Mr. Christie, 49, who works for a winter sports equipment company. “We haven’t really stocked up on anything,” he said.

Ms. Kay, 39, who works for, was checking social media for the latest updates on the imminent power outage, which was expected in the afternoon. She 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.”

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

But Mr. Christie and Ms. Kay, who live close to local farms, said they were not too worried.

“We can walk a mile down the road and have fresh peas and tomatoes,” said Mr. Christie, an avid outdoorsman. If anything, he was rather excited for an excuse to relax midweek. “I’m just stoked to go surfing,” he said. “Once the northeast winds start blowing, that’s really good conditions for waves.”

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 on 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.

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.

Jill Cowan contributed reporting from Los Angeles, and Dan Levin and Adeel Hassan from New York.

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