Northern California potentially faces 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.
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 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.
Given the dry conditions and wind threat, Pacific Gas & Electric is planning preemptive power shut-offs in the Bay Area and other parts of northern California to reduce the chances that its power lines will cause a fire.
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 high wind watch for Sunday through Monday, and a fire weather watch that extends into Tuesday. 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.
“Wind damage will be likely for any fire weakened trees,” the Weather Service stated in a forecast discussion. “Downed trees, branches and power lines will be a high possibility … Make plans now for possible power outages whether they are planned or unplanned.”
In its comparison with previous destructive fires, the Weather Service 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.”
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 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 globe, 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 is another significant factor driving wildfire trends in the West, but it doesn’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.