SAN FRANCISCO — Nearly 771,000 acres of largely unpopulated land have burned across California during the past week, according to Cal Fire, as dozens of lightning-sparked wildfires moved quickly through dry vegetation and threatened the edges of cities and towns.

The fires have been most severe in the state’s northern and central regions, where about 600,000 acres have burned in the past week. Some of the blazes have grown so large that they now rank among the biggest 10 wildfires on record in the state, and they’re still expanding.

Evacuations surged during the day and night Thursday as authorities worried that high heat and gusty winds could cause the fires to spread rapidly. By midday, several of the major fires had more than doubled in size, in some cases jumping across major highways, as crews struggled to contain the blazes. By Friday morning, the largest blaze, known as the LNU Lightning Complex, had charred 219,000 acres, having expanded by 4,000 acres overnight amid cooler conditions.

Another blaze, known as the CZU Lightning Fire, has forced the evacuation of more than 64,000 people, according to Cal Fire, the state’s firefighting agency. Some may not be able to return to their homes for weeks, officials said.

The fires have been blamed for at least five fatalities.

Many of the fires began days ago, as a heat wave and unusual storms produced more than 20,000 lightning strikes. The resulting fires — and “complexes” of many small fires — have merged into major conflagrations in many parts of the state.

Officials have urged everyone in California to prepare a bag with a change of clothes and necessities, and be ready to evacuate their homes at a moment’s notice.

This is just the beginning of the state’s wildfire season, something that has been a constant threat during the past four years of record-setting blazes for their breadth and lethality. Despite the familiarity, the current fires and their speed and thick smoke have presented a new terror amid a global pandemic — poor air quality and concerns about evacuating masses of people to crowded shelters, and that some might not heed the warnings.

The fires, spread across hundreds of miles, have presented an overwhelming challenge to the crews trying to battle them as California has issued a nationwide call for help.

Dozens of major wildfires continued to blaze across the state on Aug. 19 and 20. (The Washington Post)

Arjun Mendiratta has lived in La Honda, Calif., for two years, in a small community surrounded by tall trees south of San Francisco in the Santa Cruz mountains. He and his wife packed their 1½-year-old son in the car Wednesday night and drove to Half Moon Bay, a town on the coast, where they stayed in a hotel for the night. Already packed, they decided to leave their home after receiving an evacuation warning message as fires approached; they did not want to wait for a full evacuation order and risk getting caught in traffic.

“We thought, ‘Let’s just go while we can,’ ” Mendiratta said, noting that not all of his neighbors plan on leaving, hoping better weather will come before the flames do.

In Santa Cruz County, and in San Mateo County, where La Honda is located, about 48,000 people were ordered to evacuate because of a fire threatening communities there, part of the CZU Lightning Complex. The blaze had already burned 50 structures, fire officials reported. On Thursday evening, the University of California at Santa Cruz was under mandatory evacuation and had declared a state of emergency.

Tens of thousands of people have been asked to evacuate and make difficult decisions about where to go. In the past, they might have stayed with friends or family, but now they need to calculate the risk of exposure to the novel coronavirus.

Wherever people go, they are likely to face other hardships. California has been enduring a record-breaking heat wave that has prompted rolling blackouts because of high electricity demands for air conditioning and other uses. Most of the area is also experiencing severe or moderate drought.

The largest of the lightning-related fires was north of San Francisco, covering Napa and Sonoma counties. On Thursday, that mass of fires, the LNU Lightning Complex, had grown to 219,000 acres and was 0 percent contained, according to CalFire. Approximately 30,000 structures were at risk of burning and 480 had been destroyed.

The blaze near Vacaville, known as the Hennessey Fire and part of the LNU Lightning Complex, has been one of the most destructive, burning down homes and claiming the life of a PG&E worker who was assisting first responders. This same blaze burned down the La Borgata Winery and Distillery in Vacaville. Mandatory evacuations remained in effect for the north part of the city Thursday.

Cal Fire reported three additional civilian fatalities associated with the LNU Lightning Complex on Thursday evening.

“The public needs to be prepared and have a plan and have a go bag,” said Brice Bennett, public information officer for CalFire. “This is showing us we can have wildfires anywhere.”

CalFire is at normal staffing levels, with approximately 12,000 firefighters working Friday, Cal Fire reported said. According to Bennett, state has asked for 375 fire engines from out of state for help, in part to give current units a chance to rest.

In Central California, a pilot on a firefighting flight near Fresno died when his helicopter crashed. The National Transportation Safety Board is investigating.

California has trained volunteer inmates to fight wildfires as part of its Conservation Camp Program, which was started during World War II. The inmates train and live in camps across the state, but because of the coronavirus, the number of available volunteers is down. There are usually 2,200 inmates qualified to fight fires on the front lines in the camps, but now the California Department of Corrections and Rehabilitation says there are 1,659.

“The decrease in conservation camp population is attributed to a combination of expedited and standard releases,” said Aaron Francis, a CDCR spokesman.

CalFire says it anticipated the impacts of the coronavirus and hired additional firefighters ahead of the wildfire season.

The effects of the coronavirus and the wildfires are tangled in other ways. Smoke from the fires is a complicating factor, as it and the virus can attack the lungs and could have a greater impact on people with certain preexisting health conditions. The massive blazes are sending plumes of smoke and ash into the skies surrounding populated areas, including San Francisco, fouling air quality for hundreds of miles.

“We’re very worried about that combination this fire season, about wildfire smoke exposures and a raging pandemic,” said John R. Balmes, a medical professor at the University of California at San Francisco and a member of the California Air Resources Board.

Bay Area residents for months have been told that outdoor spaces are safer than being indoors during the coronavirus outbreak. Now, they’re being asked to stay indoors if possible.

The cloth masks that have now become habit for many Californians when they venture outside are largely ineffective against the tiny smoke particles filling the air, Balmes said. Balmes and other UCSF doctors recommend finding N95 masks with vents, since they’re not in demand by health-care workers, or even trying used N95 masks.

He recommends that people shelter in place, staying at home with their windows closed and ventilation systems set to recirculate air. Staying indoors with windows closed is a big ask during a heat wave, especially in areas such as San Francisco, where many people do not have air conditioning.

Here are key figures on the latest blazes:

  • The LNU Lightning Complex in Sonoma, Lake, Napa and Solano counties has burned 219,067 acres, up from 46,000 acres Wednesday, and is 7 percent contained. This fire is now the 9th-largest on record in California. This complex includes the Hennessey Fire, which has charred 192,000 acres in Napa County. It has destroyed 480 structures and threatens 30,000 more.
  • The CZU August Lightning Complex in Santa Cruz and San Mateo counties has burned 50,000 acres, up from 10,000 acres Wednesday, and is 0 percent contained. It has burned 50 structures and threatens nearly 21,000 more. About 48,000 people have been evacuated.
  • The SCU Lightning Complex of about 20 fires, affecting locations in Santa Clara, Alameda, Contra Costa, San Joaquin and Stanislaus counties, has consumed 229,968 acres, up from 85,000 acres Wednesday, and is 10 percent contained. This makes the fire complex the 7th largest in California history.
  • The River Fire in Monterey County has consumed 39, 464 acres, up from 10,000 acres Wednesday, and is 9 percent contained.

Elevated fire risk lingering

Because of increasing humidity and generally light winds, favorable weather for fire spread are not expected Friday into the weekend. However, the National Weather Service office in San Francisco cautioned when sea breezes move inland, the “sudden increase in winds may push the fires during the late afternoon to early evening, bringing brief but potentially erratic and dangerous growth.”

Late in the weekend, the forecast office in San Francisco wrote, Tropical Storm Genevieve, near the Baja Peninsula, could release a pulse of moisture and instability to increase the potential for more lightning “that may lead to further ignitions.” Temperatures are also expected to increase during the weekend as well. The National Weather Service forecast office in San Francisco said Friday that it plans to issue a fire weather watch for late this weekend into early next week due to the potential for more thunderstorms that could ignite still more blazes.

Smoke will remain a persistent issue because of the ongoing fires, “so air quality will be a real problem through at least the end of the week,” the Sacramento office wrote.

Lightning, plus an intense, late-season heat wave, served as triggers

The California wildfires, along with other blazes in the West, have sent a blanket of smoke across at least 10 states and southwestern Canada, with tendrils of smoke extending over the Pacific Ocean as well. Air quality alerts are in effect for parts of California, where the tiny particles in the dense smoke will be capable of aggravating respiratory conditions and worsening preexisting health conditions.

The fires stem from an unusual confluence of extreme weather events, set against the backdrop of human-caused global climate change, which is causing more frequent and severe heat waves in the region as well as larger wildfires across the West.

Appearing via video from a forest near Watsonville, Calif. Gov. Gavin Newsom (D) attributed the barrage of blazes to climate change.

“The hots are getting hotter, the drys are getting drier,” Newsom said. “Climate change is real. If you are in denial about climate change, come to California.”

The immediate trigger of most of the more than two dozen large fires burning in the Bay Area was an unusual August thunderstorm outbreak, which lit up the night skies above San Francisco on Sunday and Monday and moved inland, where lightning discharges struck trees and grasses at a time of year when vegetation is at its driest.

Between midnight Saturday and midnight Wednesday, there were 20,203 cloud-to-ground strikes in California, according to Chris Vagasky of the company Vaisala, which operates the National Lightning Detection Network. The total number of lightning discharges, which includes lightning that jumped from cloud to cloud without hitting the ground, was equivalent to 11 percent of California’s average annual lightning activity, he said via a message on Twitter.

The storms were the result of moisture moving north from former Tropical Storm Fausto near the Baja Peninsula and the sizzling heat across the state.

The long-lasting and intense heat wave has played a key role in these blazes. Multiple monthly heat records have been set in the past 10 days, including in Death Valley, Calif., where one of the hottest temperatures on Earth, a high of 130 degrees Sunday, was recorded.

One measure of fire risk is known as the evaporative demand drought index, or EDDI. It measures the “thirst” of the atmosphere and can help predict fire risk. In part because of the heat’s ability to speed up evaporation, the EDDI in Central and Northern California preceding these fires soared to record levels, indicating a high fire risk.

Freedman and Samenow reported from Washington.

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