Los Angeles Times
Rapid spread for blaze in Mariposa
Oak fire, at 1,600 acres and uncontained, exhibits extreme behavior
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.