The Weather Service’s forecast office in Los Angeles stated in a discussion early Wednesday morning that it expects to issue advisories and warnings for strong winds that will go into effect on Thanksgiving Day through Friday, covering both inland areas of Southern California as well as portions of the coastal Los Angeles metro area.
Santa Ana winds blow from east to west, or from the land to the sea, which allows air to descend from higher elevations and dry out as it does so. As the air squeezes through canyons and mountain passes, it speeds up, resulting in powerful wind gusts.
As a result of the air compression as it moves from high to low elevations, relative humidity levels can plunge to dangerous territory, in the single to lower double digits. Fire danger is predicted to be unusually high for this late in the fall, since the region has not yet had a widespread, heavy rain event that would effectively end the 2020 wildfire season.
The trigger for this Santa Ana wind event will be a potent dip or trough in the jet stream that is diving southeast toward the Great Basin. This feature, along with an accompanying area of low pressure at the surface, will initiate howling north to northeasterly winds first in the Interstate 5 corridor on Wednesday night, then expanding southward down into Ventura County valleys and Santa Monica Mountains by Thanksgiving morning.
The strongest winds are expected in inland areas of Los Angeles and surrounding counties on Thursday afternoon, where winds may gust to 60 mph at times Thursday and Friday, with coastal areas predicted to see wind gusts potentially approach 50 mph as well.
In preparation for the strong winds, Southern California Edison is warning about 76,400 customers that they could be affected by prevented power outages Thursday and Friday. The utility aims to de-electrify certain power equipment to avoid having its equipment spark any blazes during the period of strongest winds.
Southern California’s biggest wildfire events tend to be associated with Santa Ana winds, since they bring dry air that can help enable the start and spread of blazes, while the wind can make fighting fires difficult to impossible.
The 2020 wildfire season in California has been unrelenting, due to record warmth, a deepening drought, an abundance of lightning strikes and extreme offshore wind events.
Santa Ana winds resulted in major fires in Orange County in late October, forcing tens of thousands to evacuate on short notice. Now, though, vegetation is even drier, due to the lack of significant rainfall, which could make the Thanksgiving fire weather even more concerning.
The months of August, September and October ranked as the state’s hottest since records began in 1895.
Without enough rain, high fire danger is continuing into November and December in Southern California, and is now overlapping with peak Santa Ana wind season. Many lower elevation locations, along with the mountains of Los Angeles and Ventura Counties, have received less than a quarter inch of rain since October 1, which marks the start of the water year in the state.
For example, only 0.08 inches of rain has fallen in Camarillo in Ventura County; that’s just 5 percent of the normal 1.77 inches that the area should have received since October 1.
While autumn is typically considered the most intense part of the fire season, Southern California’s Santa Ana winds are actually stronger and more frequent during the cool season — November through February.
Matt Shameson, a fire meteorologist with the U.S. Forest Service in Riverside, said Santa Anas typically strengthen after mid-November.
“Usually after November 15 we have enough rain that it doesn’t matter, but that’s not happening this year,” he said. “I don’t see any rain on the horizon for the next two weeks.”
The northern part of the state could see rain in early December, but Southern California is likely to see more wind than rain from that system, Shameson said.
In fact, the month of December looks dry, though he is hoping that a wetter pattern develops in January.
The cool season fire danger is reminiscent of December 2017 in Southern California, and is consistent with a delayed arrival of autumn rains over the last several years, an effect that has been predicted to emerge in California due to human-caused climate change.
The Thomas Fire in Ventura and Santa Barbara counties was ignited by power lines during high winds on December 4, 2017. It burned for more than a month, scorching 281,893 acres and destroying 1,063 structures.
Recent studies have shown that warming and drying fall seasons are amplifying the fire threat, as the number of extreme fire weather days increases and very dry conditions extend later into the year. This trend is the result in part of human-caused climate change and has also been seen in other parts of the world.
One study, for example, found that climate change has doubled the days during the fall with extreme wildfire conditions in parts of California since the 1980s.
California is in the midst of its worst wildfire season on record, with about 4.2 million acres burned, more than double the acreage in the previous record-breaking year. At least 10,488 structures have been destroyed and 31 people killed. Five of the top six largest fires on record in the state have occurred this season.
Diana Leonard is a science writer covering natural hazards, especially wildfires. She holds master’s and doctorate degrees in geography from the University of California at Berkeley, where she studied physical geography and climate change.