Dry autumn winds long connected to costly blazes
Fire rode the wings of the wind this week to bring disaster to Northern California.
It is the dread dry autumn wind with many names — Santa Ana and Diablo among them. It drives fire before it and turns even small blazes into nearly unstoppable firestorms.
In Northern California, the autumn wind is called the Diablo. It is a classic part of the fall season and is caused when cold air develops under high atmospheric pressure in inland areas in Nevada and the Great Basin and is drawn toward the coast by lower barometric pressure.
It’s called the Santa Ana in Southern California. The winds always blow from high pressure to low, like a river of air.
The cold air warms and compresses as it moves through passes and down mountain slopes at speeds that can top 70 mph. It usually comes from the north or northeast — the exact opposite of the normal summer pattern of westerly winds.
The strong, gusty winds pass over country that typically has not had rain for months. The land is dry as dust, with humidity often below 10 percent. As a result, any small fire that flares up can be driven by the gale-force winds and spread.
The wind-driven fires feed on dry fuel on the ground and in forests, getting big quickly and moving fast. It is now a wildfire and almost impossible to stop.
“You cannot get in front of it. It will run you over,” said Jan Null, a meteorologist with Golden Gate Weather Services, who has studied weather and fires for more than 30 years.
Null noted that the Tubbs Fire — one of the largest of many that broke out over the weekend — began near Calistoga on Sunday night. No one has determined a cause, but by 3:30 a.m. Monday it had roared into a Santa Rosa neighborhood 10 miles away. “Very fast for a fire,” Null said. Robert Fovell, an emeritus professor of atmospheric and ocean sciences at UCLA, said this time of year in California is optimal for such fires. “You can’t dismiss the chances,” he said.
The key is the state’s Mediterranean climate, which has wet winters and dry summers, where no rain falls for months. It creates a situation ripe for burning in the transition seasons, particularly autumn, when the land is dry. The seasonal switch in the wind patterns makes conditions worse.
The Santa Ana winds — named for the Santa Ana pass in Orange County — are blamed for hundreds of huge fires in Southern California over the years.
The Diablo winds — so named because they come from the direction of Mount Diablo in Contra Costa County — helped turn the East Bay Hills fire of 1991 into a disaster that killed 25 people and destroyed 3,000 structures. It was “a classic Diablo wind event,” according to a Federal Emergency Management Agency report.
That fire was listed as the largest fire loss in U.S. history by FEMA. “The key ingredient was the Diablo wind condition which combined with other critical fire risk factors to create an irresistible destructive force,” FEMA reported.
In 1990, a similar fire in Santa Barbara, driven by an offshore wind locals call “a sundowner,” burned 427 buildings after roaring out of control and jumping a six-lane freeway.
Santa Ana winds drove other big Southern California fires in 2003 and 2007. More than 1,000 square miles burned, 2,232 structures were lost, and 14 people were killed in the 2003 fires that dropped clouds of ash over downtown San Diego. In 2007, 25 fires raged out of control in Southern California for days.
“If ever there was a region and a climate designed to be conducive to fires, it would be Southern California,” Fovell said. “That is true in Northern California to some degree. The difference is that you have more fog up there.”
Fires go back well into the recorded history of California. The Spanish governor of Alta California warned of fires in a message in 1793, and references to Santa Ana winds are found in the earliest Los Angeles newspapers.
Fovell’s research found that strong winds and raging wildfires had been part of the Los Angeles Basin for more than 5,000 years, dating back to the original inhabitants.
The fire pattern in the Bay Area was different. Many early fires were caused by arson, and the catastrophic 1906 San Francisco fire was triggered by an earthquake.
However, a wind-whipped fire — helped by a northeast gale — caused extensive damage in fall 1923 in Berkeley, and a massive fire driven by wind from Mount Tamalpais nearly destroyed Mill Valley in 1929.
The National Weather Service is forecasting a resumption of strong northeasterly winds in the Bay Area and the North Bay through Friday evening and into Saturday.