It’s unusual to see “critical” fire weather this time of year, but a combination of offshore winds gusting over 60 mph and relative humidities as low as 12 percent acted as a breeding ground for flames, especially in Southern California.
The greater Los Angeles metro area was under a wind advisory on Wednesday, with areas including Santa Barbara and San Luis Obispo under a red flag warning. That’s where “rapid fire spread, long range spotting, and extreme fire behavior which would threaten life and property” were possible if a fire were to ignite Wednesday.
Already, new fires have broken out in the Eldorado National Forest, including the Union Fire, which was contained after charring 22.5 acres. Half a dozen fires are ongoing in national forests in the Santa Cruz Mountains well southwest of San Jose. They’re burning in the same area torched by the CZU Lightning Complex fire in September, one of the state’s largest on record. At least 120 families were evacuated.
Other fires dotted the map surrounding Los Angeles. Several additional new fires had erupted in the Sierra Nevada. The Towsley Fire in Los Angeles County was 17 percent contained and had burned 167 acres as of Wednesday morning.
Instigating the strong winds and wildfires was a wave of low pressure scooting down the Golden State coastline and intensifying offshore. Counterclockwise winds around the area of low pressure channeled cool air southwestward over the crest of the Sierra Nevada, accelerating it downhill with gusts in spots topping hurricane force, or 74 mph.
Reinforcing it was high pressure over the Great Basin of Nevada, which provided a pressure gradient — or change of air pressure with distance — that boosted winds blowing from land to sea, or offshore.
A few gusts even topped 90 mph, including a gust to 97 mph on a PG&E electric transmission tower in the mountains of Sonoma County. Other nearby towers gusted to 95 mph in Forest Lake, and 82 mph in Middletown. East of the San Francisco Bay area, Mount Diablo gusted to 84 mph early Tuesday morning, while mountains bordering Los Angeles, including near Warm Springs and Magic Mountain, gusted to 88 mph and 86 mph, respectively.
Alpine Meadows, located near Lake Tahoe, clocked a gust to 137 mph early Tuesday morning.
“We look at pressure gradients, which is the difference in pressure [with distance] between a few different locations, to determine how strong the wind will be,” Lisa Phillips, a meteorologist at the National Weather Service forecast office in Los Angeles, said in an interview.
“We looked at a compilation of all of our past gradients, and [Monday] afternoon was the second-highest we’ve recorded between LAX and [Daggett, a town of 200 in eastern San Bernardino County].”
Gusts between 35 and 50 mph buffeted Los Angeles County, with the strongest winds clocked in the higher elevations. The winds brought a surge of cool, dry air that sapped vegetation of moisture. At LAX, relative humidity fell from 74 percent to 13 percent in seven hours as the dry winds poured in.
Even stronger winds blasted the higher elevations, with a gust to 69 mph at Eastmont Road in the Sierra Nevada foothills. Winds hit 83 in Fort Tejon, along Interstate 5 between Los Angeles and Bakersfield. Pelato Peak gusted to 74 mph, while Miami Mountain, in the central Sierra Foothills, saw a gust to 77 mph.
Farther south, a gust of 85 mph was recorded atop Big Black Mountain in San Diego County, while some places even in the neighboring lowlands saw powerful gusts. Hellhole Canyon measured winds to 80 mph.
“We knew it was going to be a really strong event, but it was one of the worst winds we’ve had in a long time because of the duration,” said Curt Kaplan, also a meteorologist at the Weather Service in Los Angeles, who lives in the foothills in Ventura County near the Reagan Library. “Wind gusts are usually three to six hours, max, but this lasted 12 hours at least.”
The 20-year Weather Service veteran said he’s seen plenty of wintertime Santa Ana wind events, but couldn’t recall one of this magnitude.
“Boy, this one had everything going for us,” he said. “Some of the stronger events have been associated with these upper lows coming around, [which is] not your typical setup for Santa Ana winds.”
The errant upper-level disturbance even brought about some showers on the back side of the low, with a couple reports of light rain in Los Angeles County, but not enough to benefit firefighters or make a dent in the deepening drought across the state.
“Yesterday was one of the more unusual weather days in memory,” wrote the Weather Service in Los Angeles early Wednesday in an online forecast discussion. “A day that started off with a deep marine layer cloud deck … transitioned to one of the strongest Santa Ana in the past 20 years and finished off with clouds and showers.”
David Gomberg, a fire weather forecaster with the Weather Service in Los Angeles, said this event has been unique. “By this time of year, we’ve normally had some rain on the field, but this year it’s a different story because we’ve been so dry,” he said. “I think what was impressive too was just how widespread it was.”
This wind event comes just after California endured the worst wildfire season on record, with five of the top six largest wildfires in state history occurring in 2020. Climate change is making large fires, and days with high fire danger, much more likely by increasing temperatures and drying out vegetation more rapidly. This trend was driven home by the explosive fires that engulfed the entire West Coast last fall, fouling the air across the West.
As the atmosphere continues to warm, longer fire seasons are becoming routine, with conditions favoring more extreme fire behavior when fires do occur.
Considering the strength of the winds and dry conditions, Californians may have been lucky that fires were not even worse on Tuesday.
Andrew Freedman contributed to this report.