(Reuters) – Brutally hot temperatures, fierce winds and arid conditions will sweep across the U.S. West on Wednesday, and the weather may contribute to an already deadly wildfire season.

Temperatures above 100 degrees Fahrenheit (37 C), winds gusting up to 50 miles (80 km) per hour and humidity levels in the teens are in the forecast for many parts of Oregon, California, Arizona and Nevada on Wednesday and into Thursday, the National Weather Service said in a series of advisories.

The service warned that the weather could lead to more of the fires in the region, which have killed nine firefighters and destroyed more than 2,500 homes.

One of the largest, the Ferguson Fire, forced the Yosemite Valley and other parts of Yosemite National Park to close on Wednesday as smoke filled the air in the popular tourist destination.

The Ferguson Fire, which has been burning since July 13 and has claimed the life of one firefighter, had charred about 37,795 acres (15,295 hectares) to the south and west of the park. It was 26 percent contained as of Tuesday night, according to the California Department of Forestry and Fire Protection.

The park’s Yosemite Valley, Wawona and Mariposa Grove are to be closed at least through Sunday by the fire operations, the National Park Service said.

More than 3,400 personnel using 16 helicopters and 59 bulldozers have been battling the blaze, which has caused six injuries and led to evacuations in parts of the region.

In all, 73 major wildfires are burning in the United States in an area of about 700,000 acres. Most are in western states, with blazes also in central Texas and Wisconsin, according to the National Interagency Coordination Center.

As of July 24, wildfires had burned through 3.94 million acres this year, above the 10-year average for the same calendar period of 3.54 million acres, it said.

(Reporting by Brendan O’Brien in Milwaukee, editing by Larry King)