LOS ANGELES (Reuters) - The heart of Yosemite National Park was closed on Wednesday as a fierce wildfire burning just to the west jumped fire lines overnight, pouring thick smoke into the valley and forcing visitors to pack up camp and flee.

The Sierra Hotshots, from the Sierra National Forest, are responding on the front lines of the Ferguson Fire in Yosemite in this US Forest Service photo from California, U.S. released on social media on July 22, 2018. Courtesy USDA/US Forest Service, Sierrra Hotshots/Handout via REUTERS

Extreme temperatures, erratic winds and low humidity are supercharging the blaze and dozens of other fires were also burning in the U.S. West, with temperatures set to stay above 100 degrees Fahrenheit (37 C) into Thursday in many parts of California, Oregon, Arizona and Nevada, the National Weather Service said.

Heavy black smoke from the Ferguson Fire, which erupted on July 13 in the Sierra Nevada Mountains some 170 miles (275 km) east of San Francisco, prompted Yosemite park officials to shut down the main visitor hub of Yosemite Valley as well as Wawona and Mariposa Grove.

“Basically all the smoke is funneling into that valley and being held down by the inversion layer,” U.S. Forest Service spokesman Jim Mackensen said, referring to the atmospheric condition that prevents warmer air and smoke from rising.

The smoke reduced visibility and posed health risks to visitors in the popular tourist destination as well as park employees, Mackensen said.

The Ferguson fire, burning in steep, rugged terrain at the western edge of the park that has proven difficult for crews to access, killed firefighter Braden Varney on July 14 when the bulldozer he was using to cut containment lines overturned in a ravine.

Seven other firefighters have been hurt battling the flames, which were 25 percent contained as of Wednesday afternoon. More than 3,400 personnel using 16 helicopters and 59 bulldozers were fighting the conflagration.

The blaze is one of some 60 major wildfires burning in the United States this week that have so far scorched an area of about 1.2 million acres (485,620 hectares). Most are in western states, with blazes also in central Texas and Wisconsin, according to the National Interagency Fire Center.

As of July 25, wildfires had burned through 3.94 million acres (1.59 million hectares) this year, above the 10-year average for the same calendar period of 3.54 million acres (1.43 million hectares), it said.

Record temperatures in Southern California helped drive a fast-moving blaze toward mountain resort communities around 90 miles (145 kms) east of Los Angeles in the San Jacinto Mountains.

The Cranston fire was believed to have been started by arson around noon and had blackened 3,000 acres by 7 p.m. (10 p.m. ET), according to the Inciweb website. The blaze had burned several homes and threatened hundreds more, forcing the evacuations of Idyllwild, Pine Cove and Fern Valley. A man was arrested after witnesses said he started the fire, local media reported.