Extremely powerful winds and parched vegetation will create a “particularly dangerous” fire weather situation for more than 11 million people in Southern California starting late Wednesday, the National Weather Service is warning.

Santa Ana winds, blowing from land to sea, will push air with relative humidity values in the single digits into Los Angeles and Ventura counties, and as far south as San Diego County. The issuance of this “particularly dangerous situation” red flag warning is rare and reserved for the most severe fire weather conditions. This warning applies specifically to the Los Angeles and Ventura counties’ mountains and Santa Clarita hills, while other areas are under more conventional red flag warnings.

According to the Weather Service, any new fire that occurs during this period “will likely have very dangerous fire spread that could potentially threaten life and property.” That could include fires that spread embers ahead of the main blaze by one to three miles, enabling them to leap across highways and other barriers.

With winds forecast to be between 25 to 40 mph, with gusts to 70 mph possible for Los Angeles and Ventura counties’ mountains as well as valleys and coastal areas, any new blazes would probably cause extreme fire behavior. Southern California Edison is planning to preemptively cut power to about 270,000 customers, particularly in Ventura County, to prevent its electrical lines from starting a blaze. Several of California’s most destructive and deadliest fires in recent years have been set off by power infrastructure buffeted by strong winds.

San Diego Gas and Electric warned that 91,953 customers could experience “prolonged” Public Safety Power Shutoffs to prevent electrical equipment from sparking a fire during high winds. Even though some rural mountain areas subject to the shutoffs had received 2 to 3 inches of rain in early November, they have quickly dried and are now vulnerable to fire spread.

“The rain in early November was significant in San Diego County,” Alex Tardy, a meteorologist with the National Weather Service in San Diego said in an email. “However, this [has been] basically erased since then with no rain and now the third Santa Ana wind.”

Vegetation is now back to near-record dry levels, as it was in October, he said, and there is not much hope for relief in the near future.

The fire danger means that millions who are under stay-at-home orders because of surging coronavirus cases have to be prepared to leave their homes on a moment’s notice if a blaze threatens, a contradictory public safety message that could complicate warning and response efforts.

As Santa Ana winds blow from east to west over the high terrain, air forced down mountain slopes dries out. The air squeezes through canyons and mountain passes and accelerates, creating damaging wind gusts. As the air moves from high to low elevations, relative humidity levels can plunge to extreme lows, in the single to lower-double digits.

Fire danger is predicted to be unusually high for this late in the fall, because the region has not yet had a widespread, heavy rain event that would effectively end the 2020 wildfire season. The Storm Prediction Center has placed much of Southern California under a “critical” fire danger, the second-most severe risk category, with the San Diego mountains upgraded to the highest threat level of “extremely critical” for Thursday.

San Diego County will see widespread windy conditions, and gusts over 75 mph could be recorded in the windiest locations.

The multi-day offshore event is expected to unfold in three bursts, with very strong winds Wednesday night through Thursday, followed by weaker offshore flow but extremely dry air into the weekend. Another round of intense winds is possible early next week.

California normally would be moving into the heart of its rainy season, but a vast area of high pressure over the West has been shunting storms well to the north of the state, causing it to fall deeper into a drought.

The 2020 wildfire season in California has been unrelenting, due to record warmth, a deepening drought, and an abundance of lightning strikes and strong offshore wind events. Santa Ana winds triggered major fires in Orange County in late October, forcing tens of thousands of people to evacuate on short notice. Now, vegetation is even drier, which could make this week’s event more dangerous.

August, September and October ranked as the state’s hottest months since records began in 1895.

The lack of rain means that the driest period of the year is 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 Oct. 1, which marks the start of the water year in the state.

While autumn is typically considered the most intense part of the fire season, Southern California’s Santa Ana winds tend to be stronger and more frequent during the cool season of November through February.

Continued dry conditions can lead to disaster. For example, in December 2017, the Thomas Fire ignited in Ventura and Santa Barbara counties from power lines during high winds. It burned for more than a month, scorching 281,893 acres and destroying 1,063 structures.

The extension of the dry season deeper into this fall is consistent with a delayed arrival of autumn rains over the past several years, an effect predicted to emerge in California because of human-caused climate changes.

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. Again, 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. The state is now 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 contributed to this report.