Los Angeles Times

Rapid spread for blaze in Mariposa

Oak fire, at 1,600 acres and uncontaine­d, exhibits extreme behavior

- BY GREGORY YEE AND CHRISTIAN MARTINEZ

A brush fire in Mariposa County showed extreme behavior as it exploded in size to 1,600 acres in less than five hours Friday, prompting authoritie­s to expand evacuation zones.

Around 3:30 p.m., about an hour and a half after it started, the Oak fire was 60 acres and 0% contained, according to the California Department of Forestry and Fire Protection. An hour later, it had grown to 611 acres. By 5:38 p.m., the blaze was 1,300 acres and remained uncontaine­d, Cal Fire said.

Shortly before 7 p.m., the blaze had grown to about 1,600 acres, according to a Mariposa County fire map.

Some residents evacuating the Oak fire posted photos to Twitter of a pyrocu

mulus cloud ballooning into the atmosphere. The cloud top reached altitudes of 25,000 to 30,000 feet early Friday night, according to Andy Bollenbach­er, a meteorolog­ist with the National Weather Service in Hanford.

Pyrocumulu­s clouds are formed by plumes of smoke rising vertically over winddriven fires. Heated air from the fire rises rapidly, creating even gustier conditions and making suppressio­n more difficult. Under certain conditions, that fast-rising air can create a fire tornado. The black pyrocumulu­s clouds carry soot, ash and other pollutants as high as 10 miles into the atmosphere.

In the most extreme cases, a wildfire can create its own weather when smoke forms what is known as a pyrocumulo­nimbus cloud. Like a thundersto­rm, such clouds produce lightning and potentiall­y stronger winds, which can start and spread more fires in a chain reaction.

Though conditions in the Oak fire area were not yet that severe, they helped drive the fire’s rapid growth, Bollenbach­er said. Relative humidity was very low — about 7% to 8% — and temperatur­es hovered around 95 degrees. Winds came out of the northwest at 5 to 10 mph and gusted to about 20 mph, he said.

Lighter winds Saturday could bring limited relief and tamp down the extreme spotting behavior observed Friday, but temperatur­es and humidity will stay about the same, Bollenbach­er said.

The fire tore through an area with extremely dry fuels near “subdivisio­ns nestled in the foothills amid dense vegetation,” Daniel Swain, a climate scientist at UCLA and a California climate fellow at the Nature Conservanc­y, said on Twitter.

Less than 10 miles east of the blaze, the Washburn fire was still burning in Yosemite National Park. That fire has scorched nearly 5,000 acres since igniting July 7 and was 79% contained as of Friday.

The Oak fire started around 2 p.m. in the area of Highway 140 and Carstens Road, near Midpines, Cal Fire said.

Authoritie­s issued an evacuation order and closures for Carstens, Buckingham Mountain and Plumbar Creek roads and for Triangle Road to Highway 140, according to the Mariposa County Sheriff ’s Office. Jerseydale and all side roads were later added to the evacuation and closure list.

Deputies received reports of people trapped in the evacuation zone either without vehicles or otherwise not able to leave, said Kristie Mitchell, spokespers­on for the Sheriff’s Office. Units were helping them evacuate, but it was not clear how many people in the path of the fire needed assistance.

An evacuation center has been set up at Mariposa Elementary School at 5044 Jones St., the Sheriff ’s Office said. Those with animals have been told to go to the Mariposa Fairground­s and Exposition Center at 5007 Fairground­s Road.

 ?? Pacific Gas & Electric ?? A PYROCUMULU­S CLOUD, formed by smoke rising vertically over wind-driven fire, balloons Friday in Mariposa County southwest of Yosemite National Park.
Pacific Gas & Electric A PYROCUMULU­S CLOUD, formed by smoke rising vertically over wind-driven fire, balloons Friday in Mariposa County southwest of Yosemite National Park.
 ?? Pacific Gas & Electric ?? THE OAK FIRE tore through dry fuel, aided by low humidity and temperatur­es hovering around 95 degrees. Conditions are expected to change little on Saturday.
Pacific Gas & Electric THE OAK FIRE tore through dry fuel, aided by low humidity and temperatur­es hovering around 95 degrees. Conditions are expected to change little on Saturday.

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