Now, a prolonged period of near-record heat is combining with periods of strong, dry winds across much of the state, from the Oregon border south to the San Francisco Bay area and, eventually, in Southern California, as well.
The periods of “critical” wildfire risks — the highest fire-threat category, are occurring Sunday into Monday for Northern California, where wind gusts to 65 mph have already been recorded in higher elevations and temperatures are forecast to soar into the triple digits away from the Pacific coast, though even downtown San Francisco could reach 90 degrees.
A weather pattern featuring a dip in the jet stream across the Great Basin and Northern Rockies, while a large northward detour in the jet stream, or ridge, builds over the West Coast, is providing the weather setup to heighten fire weather concerns even further. The ridge is unusually strong for this time of year.
The National Weather Service forecast office in San Francisco termed the weather pattern “concerning” in an online forecast discussion on Sunday morning, since air-pressure differences are funneling strong winds from north to south in some areas, and east to west in others, both of which help to dry out the air and make it possible for existing blazes to flare up and new fires to spread quickly.
California is only now getting into its traditional land-to-sea wind season, when it typically sees some of its worst wildfires.
There have been reports of one growing wildfire, known as the Glass Fire, in Napa County that has prompted evacuations. To the north, another wildfire developed in Shasta County, with its smoke plume and satellite signature indicating increasingly intense flames as it exceeded 1,000 acres Sunday afternoon.
The wind event has trended stronger than what forecasters anticipated last week, because of a stronger pressure gradient between the Northern Rockies and the West Coast, as well as airflow higher in the atmosphere that will reinforce and strengthen winds at the surface.
Given the heightened threat, Pacific Gas & Electric plans to cut power to 89,000 customers in 16 counties to reduce the risk that its power lines could cause any fires. The company has been blamed for starting California’s deadliest blaze, 2018′s Camp Fire, which nearly wiped out the town of Paradise.
Red flag warnings for high fire danger are posted across much of Northern California through 9 p.m. Monday, with the Sacramento Valley and North Bay mountains expected to see the strongest winds. Gusts between 25 and 40 mph will be common but may exceed that in some locations.
“This wind event is solidly in the moderately strong category — a notch below the Camp Fire and Wine Country events but still significant,” said Brent Wachter, a fire meteorologist with the U.S. Forest Service in Redding, referring to devastating wind-driven Northern California fires in 2018 and 2017.
While one wind burst is expected to peak Sunday morning, an even higher risk period may be Sunday night, as very dry air pours into the region and winds become more widespread and extend to the coastline — a true land-to-sea (also known as offshore) wind event.
The winds could further spread existing fires, even though they are better contained than they were in early September, when the North Complex made a swift advance through Butte County that killed 15 people. Although the vast acreage already burned this year provides some buffer against fire spread, new ignitions are still a concern, and there are many areas that could still burn intensely.
“There’s a lot of potential in this event,” Wachter said.
On Monday, the focus shifts to Southern California, when strong offshore winds, known there as Santa Ana winds, start blowing through the region’s inland mountains. A fire weather watch is in effect for the area, including parts of Los Angeles and Ventura counties.
“We’ve seen numerous heat waves — we’re now on heat wave number 13 — and they’ve really been centered on the inland areas,” Weather Service meteorologist Alex Tardy, who works in the agency’s San Diego office, said in a video update posted to Twitter.
Heat waves are becoming more likely and intense because of human-caused global warming, and some case studies have shown certain extreme heat events could not have occurred without human-caused global warming.
Because of repeated heat waves and a lack of summer monsoon rain, Forest Service data shows Southern California mountains are at or near-record-dry levels for this time of year.
The timing of the heat and high winds is the opposite of what happened earlier in September, when some of the largest blazes advanced as many as 25 miles in one night. Then, an offshore wind event immediately followed one of the worst late-season heat waves on record in the state.
Now, the strongest winds are hitting before the heat is projected to peak. However, forecasters consider the next week to be a period of concern, as heat and very low humidity could make it difficult to contain any new fires and will make vegetation extremely dry. Temperatures 10 to 20 degrees above average will persist in many locations through Friday.
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, according to a recent review of the scientific literature on this topic.
Land management practices, including the building of homes closer to forested areas that are susceptible to fires, is another significant factor driving wildfire trends in the West.
According to Wachter, forecasters are going to be watching computer models closely for any indication that strong winds could return at the end of next week.