The Oak Fire began Friday afternoon near the town of Midpines in rural Mariposa County — about 75 miles from Fresno — and by the end of that day covered more than 4,000 acres. As of Monday morning, it had burned more than four times that amount — 16,791 acres — outside of Yosemite, according to Cal Fire, the state fire agency.
Officials and experts attributed the rapid spread to hot, dry conditions, as well as vegetation that may have helped fuel the flames. Authorities say the fire continues to be fueled by dense vegetation and terrain in the area.
Still, authorities said Monday morning that fire activity “wasn’t as extreme” as it had been for the previous two days, giving firefighters “good progress” and to contain 10% of the fire overnight, according to a Cal Fire incident. report.
Overnight, the perimeter of the fire had pushed towards the community of Mariposa Pines, where strike teams were able to hold the line of fire, according to the report. Crews also continued to work to maintain line on the northeast and south sides of the fire.
Kelly Martin, former chief of fire management and aviation at Yosemite National Park, said in an interview Monday that several elements lined up perfectly for the Oak Fire to burn: high temperatures, abundant vegetation and steep topography.
Martin added that Mariposa County’s cityscape, where communities are “widely dispersed,” makes prescribed or “controlled” burning difficult, which can reduce the amount of combustible vegetation. This material, including wood debris and branches, then becomes available for burns.
“Warmer, hotter summers and more vegetation growth across the landscape, less any natural fire, means this fire was waiting to happen,” Martin said. “With these conditions, these fires will continue to burn and threaten communities no matter what.”
The steep topography of the Midpines area, along with high temperatures, pose serious challenges for fire crews to enter and try to contain the blaze, Martin added.
The weather had little to do with the severity of this fire, said Jeffrey Barlow, chief forecaster at the National Weather Service’s forecast office in Hanford, Calif. Instead, years of drought have parched vegetation and turned the forest floor into a powder keg.
“The biggest factor was dead fuels,” he said. “We have had several years of drought. This resulted in the massive death of trees throughout the Sierra.
The situation was aggravated by strong winter winds that toppled trees.
“We have acres and acres of dead trees,” he said. “These trees are stacked like a campfire.”
Barlow said although it has been hot and dry, winds have generally been light in recent days.
“If we had red flag conditions and gusty winds, given the huge fuel load, [the fire] would be devastating and much worse,” Barlow said, estimating the fire would be at least double if those conditions occurred.
Even without strong large-scale winds, the fire created its own localized gusts passing over the steep, parched terrain filled with combustible materials, “like opening the flu in our chimney,” Barlow said.
The forecast is for a continuation of mostly hot and dry conditions in the region, he said, although forecasters are watching for the possibility of thunderstorms in the surrounding area that can create their own erratic winds.
The fire generated copious amounts of smoke, which prevailing winds blew northeast toward Yosemite National Park and Reno, Nevada.
Barlow described “very unhealthy conditions” in those areas.
The Reno Weather Service office issued a statement warning of “deteriorating air quality” nightly and mornings in western Nevada and the Sierra throughout the week.
The wildfire had destroyed 10 structures and damaged five by Sunday evening, according to the department’s website. On Monday, that figure was adjusted to no structures damaged and seven destroyed.
Cal Fire spokeswoman Natasha Fouts said Sunday about 3,000 people were under evacuation orders and nearly 2,000 were warned they may have to leave soon.
“This particular fire just had a really dangerous spread rate,” Fouts said.
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Governor Gavin Newsom (D) declared a state of emergency for Mariposa County on Saturday. The Federal Emergency Management Agency is also providing resources to extinguish the fire, Newsom said.