By Dan Whitcomb

LOS ANGELES (Reuters) – A tractor operator was killed while trying to clear brush around a massive wildfire in central Oregon and two firefighters were injured battling a blaze burning at the edge of Yosemite in California, officials in the two states said on Wednesday.

Crews responding to a report of a charred tractor near the 36,000-acre (14,600-hectare) Substation Fire burning near The Dalles, Oregon, found the unidentified driver nearby, Wasco County Sheriff’s officials said on the department’s Facebook page.

“It appears the tractor operator died as a result of exposure to the fire,” the sheriff’s office said, asking for the public’s help in identifying the victim.

In California, one firefighter broke a leg, requiring hospitalization, and a second was treated for heat-related illness, after fighting the so-called Ferguson Fire burning on the western boundary of Yosemite, said Richard Egan of the U.S. Forest Service.

The United States is facing an unusually active wildfire year, with some 3.3 million acres (1.3 million hectares) already charred this year, more than the year-to-date average of about 3 million acres (1.2 million hectares) over the past decade.

The California injuries came as crews made a major push to cut containment lines around the conflagration before thunderstorms forecast for this week further whip up the flames.

“These next 48 hours are going to be pretty critical for us in terms of containing the fire,” Egan said, adding that lightning strikes could touch off new hot spots.

The blaze has blackened more than 17,300 acres (7,000 hectares) of forest in California’s Sierra Nevada mountains, prompting the closure of State Route 140 and a Yosemite park entrance.

Fire managers have issued evacuation orders or advisories for the mountain communities of Jerseydale, Mariposa Pines, Clearing House and Incline.

Complicating firefighting efforts was an inversion layer of thick black smoke, visible for miles, that has prevented water-dropping aircraft from flying into narrow canyons.

That inversion layer, an atmospheric condition that prevents the warmer air and smoke from rising, was expected to partly clear on Wednesday evening as the storm approached, allowing aircraft to make runs at the fire, Egan said.

Firefighter Braden Varney was killed on Saturday when a bulldozer he was using to cut a fire break overturned, according to the California Department of Forestry and Fire Protection.

Varney was the 10th U.S. wildland firefighter to die in the line of duty this year, according to National Interagency Fire Center data.

California has had its worst start to the fire season in a decade, with more than 220,421 acres (89,200 hectares) blackened and six major wildfires burning statewide as of Wednesday.

In Oregon, where the Substation Fire has burned since Tuesday, Governor Kate Brown declared an emergency, prompting authorities to issue evacuation orders for communities along the Deschutes River.

The risk of large wildfires is set to ease in much of the Southwest and Rocky Mountains because of expected summer rains, but remains high in California through October.

(Reporting by Dan Whitcomb; Editing by Scott Malone and Peter Cooney)