Although firefighters have seen a letup in the high winds and heat that plagued Central and Northern California for much of the week, another round of thunderstorms that will deliver little rain but lightning discharges that could start even more blazes is anticipated as early as Sunday and lasting through Tuesday.

In seven days, the California blazes have charred nearly a million acres, according to Cal Fire, more than tripling the area burned during a typical fire season (a little over 300,000 acres). The area of land burned is larger than Rhode Island.

The largest blaze in the state, known as the LNU Lightning Complex, had spread to a staggering 314,207 acres across Napa, Lake, Solano and Sonoma counties by Saturday morning. It was only 15 percent contained, and firefighters report that “extreme fire behavior” is making battling the blaze difficult.

The size of the blaze puts it behind only the Mendocino Complex Fire of 2018, which burned about 459,000 acres, on the state’s list of largest fires on record since 1932. The fire complex, composed of several blazes burning in proximity, has destroyed 480 structures and threatens 30,500 more, according to Cal Fire, the state firefighting agency.

Blazes burned through Big Basin Redwoods State Park, the state’s oldest and home to treasured redwood trees between 800 and 1,500 years old. California State Parks wrote that the park, which officials closed, suffered “extensive damage.”

A second large fire, known as the SCU Lightning Complex, now ranks as the third-largest blaze in state history, at 291,968 acres. This beats the Rush Fire of 2012, which burned about 272,000 acres.

More than 500,000 acres have been charred within a 100-mile radius of San Francisco, which is twice the land burned during the entire 2019 California fire season, according to UCLA climate scientist Daniel Swain.

Here are key figures on the latest blazes:

• The LNU Lightning Complex in Sonoma, Lake, Napa and Solano counties has burned 314,207 acres, up from 46,000 acres Wednesday, and is 15 percent contained. This fire is now the second-largest on record in California. This complex includes the Hennessey Fire, which has charred 261,793 acres in Napa County. The complex has destroyed 480 structures and threatens 30,500 more.
• The CZU August Lightning Complex in Santa Cruz and San Mateo counties has burned 63,000 acres, up from 10,000 acres Wednesday, and is 5 percent contained. It has burned 97 structures and threatens nearly 24,000 more. About 77,000 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 291,968 acres, up from 85,000 acres Wednesday, and is 10 percent contained. This makes the fire complex the third largest in California history.
• The River Fire in Monterey County has consumed 42,583 acres, up from 10,000 acres Wednesday, and is 12 percent contained.

California has seen a significant uptick in large wildfire activity due to a combination of climate change, land-use practices and other factors. Large fires have also increased across other parts of the West, which climate studies tie to human-caused climate change that alters the timing of precipitation, makes summers hotter and vegetation drier and leads to more days with extreme fire weather conditions that enable new fires to spread rapidly.

Many of the fires began a little more than a week ago, as a heat wave and rare outbreak of thunderstorms 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.

More than 100,000 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.

The blazes have spread smoke all the way to the Great Plains and have made air quality in parts of the Golden State deteriorate to the point where it ranks as some of the dirtiest air worldwide. Small particles in smoke can enter people’s lungs, harming those with asthma and other preexisting conditions.

The 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. Cal Fire has said it has 13,500 personnel deployed to fight these blazes, but they are not enough to bring them under control, leading Gov. Gavin Newsom (D) to issue an urgent plea for help from the entire country.

The governor also said he was reaching out to Canada and Australia for help. “We have more people, but it’s not enough,” Newsom said.

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 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.

Cal Fire 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.

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.

Conditions for fighting these blazes are less than ideal and are predicted to deteriorate. On Saturday, though humidity has increased along the California coast, “for inland areas and elevations above 1000 feet, where much of the fire fight is, receiving much humidity recovery is a struggle,” wrote the National Weather Service forecast office in San Francisco.

Late in the weekend, the Weather Service 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.” A fire weather watch is in effect for Sunday through Tuesday due to the potential for more thunderstorms that could ignite still more blazes and generate erratic winds.

Confidence “is growing in the chance for thunderstorms in and around the Bay Area and Central Coast late this weekend,” the Weather Service wrote, expressing the most concern for the period between Sunday night and Monday morning.

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.

Studies show that climate change is lengthening the fire season in the West and leading to larger blazes than would otherwise occur. The 2018 National Climate Assessment, published by the Trump administration, projected those trends are likely to continue in the next several decades.

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 in 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.

UCLA’s Swain wrote online that the abundant lightning strikes that were seen in Northern California were able to spark more than 350 fires. ″Unusually dry vegetation following an extremely dry winter and in the midst of a record heat wave acted as tinder — and many of these lightning strikes ignited wildfires,” he wrote in an analysis of the event. “But the number of fires is actually not the most problematic aspect of this event — it’s the astonishing speed with which these fires grew and their relative proximity to many heavily populated areas,” Swain wrote.

Newsom attributed the barrage of blazes to climate change during a video appearance at the Democratic National Convention this week.

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

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.

Other fire weather indicators also spiked to unusually high levels for this time of year at the time these blazes ignited.