A firefighter extinguishes portions of a controlled burn during the Glass Fire near Calistoga, Calif., on Oct. 2. (Philip Pacheco/Bloomberg News)

Northern California faces what may be its most perilous fire weather event of the devastating 2020 wildfire season, beginning Sunday and lasting into Tuesday. The weather pattern will feature fierce dry winds blowing from higher elevations down to valley floors, including the San Francisco Bay area, at a time when vegetation is at record dry levels.

In preparation for the dangerous fire weather conditions, Pacific Gas & Electric Co. plans to preemptively cut power to at least 1 million residents to avoid having its power lines spark yet another deadly blaze. This would be one of the largest deliberate power cuts taken since the shut-offs began as a fire deterrence measure, rivaling an event in October 2019.

The upcoming extreme weather event is the result of an unusually early season Arctic outbreak across the West. A deep dip or trough in the jet stream will be carved out over the Great Basin as cold Arctic air pours southward, which is a setup well-known to California forecasters for leading to firestorms.

The steep air pressure gradient that is forecast to develop between coastal parts of Northern California and northern Nevada will set air into motion beginning Sunday afternoon.

The National Weather Service forecast office in San Francisco is name-checking past fire events to describe the risk level of the coming event, including the deadly 2017 wine country fires and the 2019 Kincade Fire, the latter of which burned nearly 80,000 acres in Sonoma County.

But it may be even more extreme than those events, because winds are expected to reach the surface more easily and also to spread all the way to the coast, even impacting the Highway 1 corridor on the immediate coastline.

“We are expecting the wind to get all the way down to the valley floors and even to the coastal areas,” said Roger Gass, a meteorologist with the Bay Area’s National Weather Service office. “If a fire were to start anywhere in the Bay Area, it would likely rapidly spread in these types of conditions.”

The Weather Service in Sacramento is also sounding the alarm, with red flag warnings posted across virtually all of Northern California for Sunday through Tuesday for what the office is calling the “strongest event of the year so far.”

“It’s going to be a tinder box out there, and any new ignitions are going to spread rapidly,” said Chris Hintz, a meteorologist with the Weather Service in Sacramento.

Brian Garcia, warning coordination meteorologist at the Bay Area’s Weather Service office said that people should be on alert and ready to leave at a moment’s notice, with go-bags packed, and noted that a fast-moving fire might not come with an official warning.

Potential for a worst-case scenario

The high wind event is poised to strike Northern California at its most vulnerable point in the fire season: late fall, when the state has gone the longest without rain. Heat waves in August and September accelerated the drying process, and now trees, shrubs and other vegetation are record dry for this time of year.

One fire weather index used to gauge the risk level is called the energy release component (ERC), which is a measure of how hot a fire will burn once the moisture content of both living and dead vegetation is taken into account.

The ERC increases as fuels dry out through the season. Much of Northern California now shows near- or record high ERC levels, indicating the potential for fires to spread quickly and burn ferociously, especially given strong and gusty winds.

“It’s very clear that it is going to be critically dry across the entire Bay Area starting Sunday evening,” Gass said. “We’re expecting relative humidity into the teens to reach all the way to the coast, which is pretty uncommon.”

With the offshore flow, any moisture from coastal air will be blown out to the Pacific Ocean.

The PG&E shut-offs will affect much of the state. The utility company’s equipment has been blamed for several destructive fires in California in recent years, including the deadliest blaze on record in the state, which was the Camp Fire in 2018.

The Weather Service has hoisted a wind advisory for the Bay Area for Sunday through Monday for winds up to 50 mph in valleys. Winds could gust as high as 70 mph in hilly terrain, with downed trees and power lines expected even in urban areas around San Francisco Bay, the Weather Service said.

Other parts of the state are under high wind watches for even stronger winds, including parts of the Los Angeles metro area, where land-to-sea winds, known there as Santa Ana winds, are expected to be fierce from Sunday into Tuesday.

In its comparison with previous destructive fires, the Weather Service San Francisco office brought up the Camp Fire, which destroyed much of the town of Paradise, Calif., killing more than 80 people.

“The air mass appears to be much drier with humidity values forecast to plummet into the single digits and teens. There will be no marine layer, so even the valleys will be bone dry,” forecasters wrote. “And as has been noted throughout the week, this will all occur on top of record dry fuels. So, yes, it has similarities to the 2018 Camp Fire as well.”

In a news update, Cal Fire, the state firefighting agency, called the forecast winds during the upcoming weather pattern “the strongest event this year.”

With power outages an issue, Weather Service to use NOAA Weather Radio

The Bay Area’s Weather Service office plans to send fire-related alerts to NOAA weather radios, if specific counties within their forecast area request such a move. It will also post “Civil Emergency” messages on the agency’s Bay Area website that contain evacuation orders and other county wildfire messages.

NOAA weather radios, which run by battery, can be an important source of emergency information when cellular service and electricity are disrupted by fires, wind damage or preemptive power shut-offs.

Although they are not yet in wide use in Western states, these radios may prove invaluable in an age of worsening wildfires and hasty evacuations and power outages.

“It’s an old-school technology that is essentially having a rebirth at this point,” Garcia said. “To me, this is excessively important in the West.”

A home burns as the Camp Fire rages through Paradise, Calif., on Nov. 8, 2018. (Noah Berger/AP)

Climate change and fire timing

Typically California’s most destructive blazes occur in the fall, when bone dry conditions overlap with weather patterns that produce strong land-to-sea winds. This year, officials may be battling an element of fire-warning fatigue, given that the state has had its worst fire season on record, with 4.1 million acres burned, 31 people killed and at least 9,200 structures lost.

Scientific studies also show that by increasing air temperatures and drying out soils and vegetation, climate change increases the frequency and severity of days with extreme fire risk. This is true in the West, but also in other parts of the world, according to a recent review of the scientific literature on this topic.

Land management practices, along with the building of homes closer to forested areas that are susceptible to fires, are other significant factors driving wildfire trends in the West, but they don’t explain the major uptick in large fires in recent years.

One recent study found that climate change has doubled the days during the fall with extreme wildfire conditions in parts of California since the 1980s.

So far, five of the state’s top 20 largest wildfires have burned this year, including the largest, known as the August Complex, which is the state’s first “gigafire” on record — measuring more than 1 million acres.

It is still burning, and there is no rain in sight for California through at least the end of the month.