California’s wildfire crisis will enter an unprecedented new stage Wednesday as PG&E plans to begin cutting power to about 800,000 customers, shutting down the electric lines that have sparked many of the state’s worst blazes and setting off a chaotic scramble of people preparing for an outage that could last a week in some places.

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As word spread Tuesday of the preemptive shut-off — which is set to hit the Bay Area at noon — those in the locations expected to go dark stockpiled water and canned food and emptied store shelves of batteries for flashlights and cell phones. They made a run on gas pumps, causing lines that sometimes extended for blocks. And they hurried to help loved ones whose medical needs require electricity.

Transportation officials worked, mostly successfully, to keep open roads, bridges and tunnels, including the eight-lane Caldecott Tunnel that links Oakland and Berkeley to the suburbs to the east. Meanwhile, firefighting agencies staffed up. And school leaders canceled classes Wednesday on dozens of campuses, including UC Berkeley.

Of the Bay Area’s nine counties, all but San Francisco was to be hit by the mass outage intended to stop trees from crashing into Pacific Gas and Electric Co. lines when fast, dry winds blow in after several months without sustained rainfall. Across the state, 34 counties will be affected, some starting in the early morning and others not until noon or later on Wednesday.

Major transit agencies, including BART, said they expected to run a normal schedule during the outages, though BART said some station escalators may not work.

PG&E’s plan spurred anger across the state, which flared when the company’s website — showing details on the outages — crashed repeatedly amid heavy traffic.

Dana Dickey, a Shasta County resident, had her power shut off for three days recently after a wildfire. She was bracing for another shut-off on Wednesday, but wasn’t convinced it was necessary. Dickey also said that PG&E’s clearing of vegetation near power lines had been inadequate, and that she had trouble getting information from the company’s website during the previous shut-off.

“I’m not happy with PG&E for their high costs, lack of transparency and seemingly putting their shareholders over their customers,” she said. “I don’t think they are handling things well and need to have a better public plan for the future.”

Some of the Bay Area’s largest cities will see neighborhoods blacked out, among them San Jose, Oakland, Fremont, Santa Rosa, Hayward, Berkeley and San Mateo.

The utility’s estimates for outages included 32,680 customers in Alameda; 51,310 in Contra Costa; 9,855 in Marin; 32,124 in Napa; 14,766 in San Mateo; 38,250 in Santa Clara; 32,863 in Solano; and 66,289 in Sonoma. A single customer account might be a residence or business with multiple people, so the true number of people who lose power is likely far more than the 800,000-customer figure PG&E shared — possibly into the millions.

The outages could last until next Tuesday, and perhaps even longer, for some customers. The company will have to inspect all of the power lines it turns off, while fixing any damage it finds, and that can’t happen until the weather calms in most places on Thursday, according to PG&E. Officials said the job would require an “army” of workers in trucks, in helicopters and on foot.

For PG&E, the shut-offs will mark a high-stakes test of a program the now-bankrupt company developed after being implicated in two years of catastrophic infernos that killed dozens of people and burned thousands of homes to the ground. The crisis has raised fundamental questions about whether PG&E can deliver power safely to its customers amid a warming climate.

The outages will have will have far-reaching consequences in the Bay Area. Caltrans said at one point that the shutoffs could snarl parts of some major transportation routes, forcing the closure of the Caldecott Tunnel on Highway 24 for as long as five days and the Devil’s Slide tunnel on Highway 1 in San Mateo County.

But Sumeet Singh, the vice president in charge of PG&E’s wildfire safety program, told reporters Tuesday night that the company was working to prevent the tunnel closures by installing generators. Caldecott Tunnel and Devil's Slide were expected to be open for morning commute — though disruptions were possible.

Utility officials said the dramatic outages were fueled by a weather forecast that was shaping up to deliver Northern California’s most dangerous windstorm since the October 2017 wildfires that ravaged the North Bay’s Wine Country. Many of those fires were started by PG&E power lines, according to state investigators. The two-year anniversary of the disaster was Tuesday. Last year, PG&E equipment ignited the Camp Fire in Butte County, the worst fire in state history.

Where to get help If you need a place to go while the power is down, PG&E is opening Community Resource Centers starting Wednesday at 8 a.m. Each center will have restrooms, bottled water, air conditioning, electrical charging and seating for 100 people. Centers will be open only during daylight hours. Check our Bay Area Outage Mapon sfchronicle.com to see if your community is affected. Here’s where the centers are, by county. Alameda County Merritt College, Lot B, Leona Street, Oakland Contra Costa County Bishop Ranch Parking Lot, 2600 Camino Ramon, San Ramon Napa County Calistoga Fairgrounds, 1601 N. Oak, Calistoga San Mateo County Pasta Moon Restaurant, 845 Main St., Half Moon Bay Santa Clara County Avaya Stadium, 1123 Coleman Ave., San Jose Solano County Mission Church, 6391 Leisure Town Road, Vacaville 1001 Fairgrounds Drive, Vallejo Sonoma County Santa Rosa Veterans Memorial Building, 1351 Maple Ave., Santa Rosa Source: PG&E

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“We do not take or make this decision lightly,” Singh said.

PG&E said it would start turning off power in phases in the early morning, depending on the severity of winds, beginning in the northern part of the state. One round of shutoffs will occur from midnight until 4 a.m. and another — which includes the Bay Area — was to start at noon. Outages will occur as far south as Kern and Santa Barbara counties and as far north as Humboldt and Trinity counties, according to PG&E.

Kern County, where more than 42,000 customers are set to lose power, won’t be affected until Thursday morning because it will be hit by a different weather event than the one that will prompt shutoffs in other parts of the company’s service area, PG&E said.

Speaking to reporters after signing a series of bills in Oakland on Tuesday, Gov. Gavin Newsom called PG&E’s planned blackouts a “massive inconvenience,” but one that was “all about reducing fire danger.”

“People should be outraged, as we are,” Newsom said. “No one is satisfied with this. No one is happy with this. But we have to get through this fire season.”

The utility has had the shut-off program in place for more than a year, but has never used it on as wide a scale as it intends to do Wednesday. PG&E expanded the program since it was first launched — now, any line can be turned off, even high-voltage ones that serve many people far away.

PG&E is not alone in looking to cut electricity because of wildfire danger this week. Southern California Edison, which powers much of the southern part of the state, was considering turning off power to more than 106,000 customers in eight counties: Kern, Inyo, Los Angeles, Mono, Riverside, San Bernardino, Tulare and Ventura.

PG&E warned customers by email, text messages and automated phone calls. Customers were urged to update their contact information online if they hadn’t done so already. PG&E has also made it possible for people who aren’t direct utility customers to get alerts about a particular location — for example, an office or a school, or an apartment building where the landlord pays the utilities.

Around the Bay Area, emergency services offices and PG&E blasted out email and text alerts, automated phone calls and news releases urging Bay Area residents to take the potential power shutdowns seriously and make preparations.

“We encourage you to find alternative energy sources for light, charging devices and other necessities,” Alameda County officials said. “Plan ahead with food and water, and make sure your grab-and-go emergency kits are ready.”

Bay Area hospitals did not expect to go dark during the shut-offs, but health systems that also run urgent care clinics, outpatient facilities and smaller doctors’ offices — which may not have the large backup generators that hospitals typically have — were preparing for the possibility of losing power.

John Muir Health, which runs hospitals and medical centers across the East Bay, was preparing to move medications that need refrigeration from clinics in Orinda, Lafayette and San Ramon to locations where power will be maintained, said spokesman Ben Drew.

All Kaiser hospitals have emergency generators that can support critical hospital systems during a power outage, said Tom Hanenburg, Kaiser Permanente’s chief operating officer for Northern California. Power outages may affect outpatient offices and require rescheduling some services, he said, and Kaiser will communicate with affected patients by phone and text.

Kaiser was also reaching out to patients who use electricity-powered medical devices at home to advise them on how to protect their health during an outage.

Some water districts asked customers to store supplies. The East Bay Municipal Utilities District, which delivers water to 1.4 million customers in Alameda and Contra Costa counties, and other water providers, including the Marin Municipal Water District, were asking customers in affected areas to store a minimum of two gallons of water per person per day — enough to last three to seven days — and to store extra for pets. Although the districts have back-up generators for water pumping and distribution, services could be affected.

At the Oakland Zoo, the staff was busy Tuesday setting up setting up generators and heading out to buy more in preparation for an outage. The zoo will close to the public if it loses power, but concern about revenue loss takes a backseat to the zoo administrator’s main priority — animal safety.

“We have some generators at the zoo, but we certainly need more,” said Nik Dehejia, the zoo’s chief financial officer. “If it’s a day or two, we’re OK. If it goes well beyond that, we’re going to be looking for help and see what we need to do.”

The zoo has three endangered species of frogs and toads, which require temperature-controlled habitats, that zoo-keepers are working to rehabilitate in the wild. “It would be devastating if those frog and toad species are lost,” Dehejia said. “We’re going to do everything we can to make sure that they receive the temperature regulation that they need.”

While people hit stores to stock up on batteries, coolers, ice and canned foods, Oakland Mayor Libby Schaaf joined the call to be prepared but criticized PG&E for potentially cutting power for as long as five days.

“This is the type of interruption to our lives that should not happen,” she said. “This type of interruption is not acceptable. We are going to do it because we believe it is in the interest of the safety of the people, but we have got to do better.”

National Weather Service forecasters issued red-flag warnings for the East Bay and North Bay hills, as well as the Santa Cruz Mountains. Cool, dry winds are expected to increase to 20 to 30 mph with gusts between 45 mph and 55 mph in the North Bay and East Bay hills — and even higher on North Bay peaks like Mount St. Helena.

The northeast winds, known as Diablo winds, will be dry but temperatures are not expected to be hot Wednesday. Temperatures will range from the 60s near the coast to the 70s inland, with a few locations hitting 80 degrees.

While some businesses in the shut-off zone had generators, many were worried about sustaining big losses. At the Casa Orinda restaurant on Tuesday, the owner had bought industrial fishing coolers capable of keeping things frozen for four days, said Claudia Tata, the general manager.

“We are in panic meat mode,” she said.

Meanwhile, dairy products would go in a makeshift cooler the restaurant’s maintenance worker built Tuesday, but they would have to cross their fingers for the sauces — those were going in the conventional freezer, Tata said. The restaurant was closing early to get all the food stashed away and then they would cross their fingers.

“If the outage is still going Saturday, c’mon over,” Tata said. “We’re going to hold a great barbecue.”

Chronicle staff writers Anna Bauman, Alejandro Serrano, Catherine Ho, Sarah Ravani, Roland Li, Jill Tucker, John King and John Wildermuth contributed to this report.

Michael Cabanatuan and J.D. Morris are San Francisco Chronicle staff writers. Email: michael.cabanatuan@sfchronicle.com, jd.morris@sfchronicle.com Twitter: @ctuan, @thejdmorris