The Oak Fire began Friday afternoon near the town of Midpines in rural Mariposa County — roughly 75 miles from Fresno — and by the end of that day, it covered more than 4,000 acres. By Monday morning, it had burned more than quadruple that amount — 16,791 acres — outside Yosemite, according to Cal Fire, the state’s fire agency.
Officials and experts have attributed the quick spread to hot and dry conditions, as well as vegetation that could have helped fuel the flames. Authorities say the blaze continues to be driven by dense vegetation and the area’s terrain.
Still, authorities said early Monday that the fire activity “was not as extreme” as it had been in the previous two days, which allowed firefighters to “make good headway” and contain 10 percent of the fire overnight, according to a Cal Fire incident report.
Overnight, the fire perimeter had pushed toward the community of Mariposa Pines, where strike teams were able to hold the fire line, according to the report. Crews also continued working to hold the line on the northeast and south sides of the fire.
Kelly Martin, a former chief of fire and aviation management at Yosemite National Park, said in an interview Monday that several elements aligned perfectly for the Oak Fire to burn: high temperatures, abundant vegetation and steep topography.
Martin added that the urban landscape of Mariposa County, where communities are “very spread out,” makes it difficult to do prescribed or “controlled” burning, which can reduce the amount of combustible vegetation. This material, including wood debris and branches, then becomes available for burns.
“Hotter and warmer summers and more vegetation growth on the landscape, minus 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 the high temperatures, poses serious challenges for fire crews to go in and try to contain the blaze, Martin added.
Weather had little to do with the severity of this blaze, said Jeffrey Barlow, lead forecaster at the National Weather Service forecast office in Hanford, Calif. Instead, it was years of drought that dried out vegetation and turned the forest floor into a tinderbox.
“The biggest factor has been the dead fuels,” he said. “We’ve had multiple years of drought. This has led to massive tree kills throughout the Sierra.”
The situation was made worse by strong winds in the winter that toppled trees.
“We have acre upon acre of dead trees,” he said. “These trees are stacked up like a camp fire.”
Barlow said that while it has been hot and dry, winds have generally been light.
“If we had red-flag conditions and gusty winds, considering the tremendous fuel load, it [the fire] would be devastating and significantly worse,” Barlow said.
Even without strong large-scale winds, the fire has created its own localized tastes passing over the steep, desiccated terrain packed with combustible material, “like opening up the flu in our fireplace,” Barlow said.
The forecast is for a continuation of mostly hot, dry conditions in the area, he said, although forecasters are watching for the possibility of thunderstorms in the vicinity that can create erratic winds of their own.
The blaze has been generating vast amounts of smoke, which prevailing winds have pushed northeast toward Yosemite National Park and Reno.
Barlow described “very unhealthy conditions” in these areas.
The Weather Service office in Reno issued a statement warning of “degraded air quality” each night and morning in western Nevada and the Sierra through the week.
The wildfire had destroyed 10 structures and damaged five as of 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 that about 3,000 people were under evacuation orders and that nearly 2,000 were being warned they may need to leave soon.
“This fire in particular has just had a really dangerous rate of spread,” Fouts said.
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Govt. Gavin Newsom (D) declared a state of emergency for Mariposa County on Saturday. The Federal Emergency Management Agency is also providing resources to suppress the fire, Newsom said.