The largest utility in California began turning off the lights for millions of customers in the northern part of the Golden State early Wednesday, a shutoff that could last for days due to what officials are calling an unprecedented wildfire danger that could lead to "explosive" blazes.

Pacific Gas & Electric said early Wednesday it implemented the first phase of a Public Safety Power Shutoff ahead of the "widespread, severe wind event," impacting 513,000 customers in 22 counties at midnight. A second phase will occur at noon, impacting 234,000 customers in seven additional counties.

“The safety of our customers and the communities we serve is our most important responsibility, which is why PG&E has decided to turn the power off to customers during this widespread, severe wind event," Michael Lewis, PG&E’s senior vice president of Electric Operations, said in a statement. "We understand the effects this event will have on our customers and appreciate the public’s patience as we do what is necessary to keep our communities safe and reduce the risk of wildfire."

CALIFORNIA WILDFIRE RISK MAY CAUSE POWER OUTAGES IN 30 COUNTIES FOR MORE THAN 600,000 CUSTOMERS

Officials said the third phase of shutoffs is being considered for the utility's southernmost service area that would impact up to 42,000 customers. The power outages were expected to affect about 2 million people overall in parts of 34 northern, central and coastal California counties.

"The decision to turn off power was based on forecasts of dry, hot and windy weather including potential fire risk," the utility said. "Based on the latest weather forecasts and models, PG&E anticipates that this weather event will last through midday Thursday, with peak winds forecasted from Wednesday morning through Thursday morning and reaching 60 to 70 mph at higher elevations."

The National Weather Service San Francisco Bay Area said red flag warnings have been posted throughout the region, and the combination of strong winds, dry vegetation, and low humidity should leave area residents ready to evacuate, if needed.

"This is a recipe for explosive fire growth, if a fire starts," the agency said. "Have your bag ready."

Gusts of 35 to 45 mph were forecast to sweep a vast swath of the state, from the San Francisco Bay area to the agricultural Central Valley and especially in the Sierra Nevada foothills, where a November wildfire blamed on PG&E transmission lines killed 85 people and nearly destroyed the town of Paradise.

"Safety is our top priority, and we don't want to jeopardize anybody with an outage if it wasn't our last resort," Deanna Contreras, North Bay spokesperson for PG&E, told KTVU.

It could take as many as five days to restore power after the danger has passed because every inch of power line must be checked to make sure it isn't damaged or in danger of sparking a blaze, PG&E said.

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The outages aren't limited to just fire-prone areas because the utilities must turn off entire distribution and transmission lines to much wider areas to minimize the risk of wildfires. The Marin County Sheriff's Office warned motorists on Wednesday morning that traffic lights were out at certain intersections.

"Please use caution and treat outages at traffic lights as 4-way stops," police said.

The California Transportation Department said Wednesday morning that two tunnels in the San Francisco Bay Area that depend on electricity to function will remain open during the outages. Caltrans crews worked through the night to install generators at the Caldecott Tunnel linking the East Bay to San Francisco and the Tom Lantos Tunnel on State Route 1 in Pacifica.

The agency said that generators should be working by midday Wednesday when Pacific Gas and Electric is expected to shut off power in the Bay Area.

Area residents spent Tuesday at places like gas stations and going to ATM's, which will be inoperable when power gets cut in addition to hardware stores to get items such as generators.

“You don't really think it's going to happen but the winds are heading this way so it's time to get prepared,” Noel Bilodeau of San Jose told KTVU.

Bilodeau was making a trip to an Ace Hardware store in San Jose, stocking up on flashlights, lanterns and water in the hours before the planned outages.

“It’s the duration of the shutdown that's concerning to me," Bilodeau told KTVU. "If it takes five to seven days to get the infrastructure back up and running, that's a big concern on how people are going to function.”

Residents of the Sonoma County city of Cloverdale, population 9,300, were preparing for the possibility of zero power and downed internet and cellphone lines that could last for days. Mayor Melanie Bagby blasted PG&E, accusing the utility of failing to upgrade its equipment.

"It's inexcusable that we're in the situation that we're in," she told The Associated Press. "We pay our bills, and we gave PG&E a monopoly to guarantee we would have" reliable power.

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But others like Linda Gaskin, who lost a home in a previous wildfire, understood the need for the lights to go dark.

“I mean it is an inconvenience. I understand because of the Camp Fire and we did lose a home there. So, I understand the precautions,” she told FOX40.

Several counties have activated their emergency centers and authorities urged people to have supplies of water for several days, to keep sensitive medicines such as insulin in cool places, to have a full gas tank for emergencies and to check the food in freezers and refrigerators for spoilage after power is restored.

PG&E set up about 30 community centers offering air conditioning, restrooms, bottled water, and electronic charging stations during daylight hours.

In the southern part of the state, Southern California Edison is considering power shutoffs to nearly 174,000 customers in nine counties as early as Thursday as Santa Ana winds loomed, while San Diego Gas & Electric said it has notified about 30,000 customers in backcountry areas.

Jessica Tunis, who lost her 69-year-old mother Linda Tunis in the 2017 North Bay Firestorm, said the utility's strategy needed to be weighed with the potential consequences if power was left on.

"If they are not sure they can't prevent a massacre overnight, then please shut the power off," she told KTVU. "I'd rather be hot and my food go bad, than dozens more die."

The Associated Press contributed to this report.