Dry au­tumn winds long con­nected to costly blazes

San Francisco Chronicle Late Edition - - WINE COUNTRY FIRES - By Carl Nolte Carl Nolte is a San Fran­cisco Chron­i­cle staff writer. Email: cnolte@sfchron­i­cle.com Twit­ter: @carl­noltesf

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.

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