Firefighters working in searing weather struggled to contain a Northern California wildfire that continued to grow Sunday and forced the closure of a major highway, one of several large blazes burning across the U.S. West amid another heat wave that shattered records and strained power grids.

Two firefighters died Saturday in Arizona after a plane they were in crashed during a survey of a small wildfire in rural Mohave County. The aircraft was helping perform reconnaissance over the Cedar Basin Fire near Wikieup, a tiny community of about 100, when it went down around noon.

The two firefighters were the only people on board, officials said. The National Transportation Safety Board is investigating the crash.

In California, officials asked all residents to reduce power consumption quickly after a major wildfire in southern Oregon knocked out interstate power lines, preventing up to 4,000 megawatts of electricity from flowing into the state.

The California Independent System Operator, which runs the state’s power grid, said Saturday the Bootleg Fire took three transmission lines off-line, straining electricity supplies as temperatures in the area soared.

“The Bootleg Fire will see the potential for extreme growth today,” the National Weather Service in Medford, Oregon, tweeted Sunday.

Pushed by strong winds, the Bootleg Fire exploded in size to 224 square miles (580 square kilometers) as it raced through heavy timber in Oregon's Fremont-Winema National Forest near the Klamath County town of Sprague River.

To the southeast, the largest wildfire of the year in California was raging along the border with Nevada. The Beckwourth Complex Fire — a combination of two lightning-caused blazes burning 45 miles (72 kilometers) north of Lake Tahoe — showed no sign of slowing its rush northeast from the Sierra Nevada forest region after doubling in size between Friday and Saturday.

Late Saturday, flames jumped a major highway, U.S. 395, which was closed near the small town of Doyle in California's Lassen County. Flames threatened rural properties and forced evacuations in Nevada’s Washoe County. “Take immediate steps to protect large animals and livestock,” the Truckee Meadows Fire Protection District tweeted.

The blaze, which was only 9% contained, increased to 131 square miles (339 square kilometers). Temperatures in the area could top 100 degrees (37 Celsius) again Sunday.

It was one of several fires threatening homes across Western states that were expected to see triple-digit heat through the weekend as a high-pressure zone blankets the region.

Death Valley in southeastern California's Mojave Desert reached 128 degrees Fahrenheit (53 Celsius) on Saturday, according to the National Weather Service's reading at Furnace Creek. The shockingly high temperature was actually lower than the previous day, when the location reached 130 F (54 C).

If confirmed as accurate, the 130-degree reading would be the hottest high recorded there since July 1913, when Furnace Creek desert hit 134 F (57 C), considered the highest measured temperature on Earth.

The National Weather Service warned the dangerous conditions could cause heat-related illnesses.

Palm Springs in Southern California hit a record high temperature of 120 F (49 C) Saturday. It was the fourth time temperatures have reached 120 degrees so far this year, the Desert Sun reported.

In California's agricultural Central Valley, 100-degree temperatures blanketed the region, with Fresno reaching 111 degrees F (44 C), just one degree short of the all-time high for the date.

Las Vegas late Saturday afternoon tied the all-time record high of 117 F (47 C), the National Weather Service said. The city has recorded that record-high temperature four other times, most recently in June 2017.

NV Energy, Nevada’s largest power provider, also urged customers to conserve electricity Saturday and Sunday evenings because of the heat wave and wildfires affecting transmission lines throughout the region.

In Southern California, a brush fire sparked by a burning big rig in eastern San Diego County forced evacuations of two Native American reservations Saturday.

In north-central Arizona, Yavapai County on Saturday lifted an evacuation warning for Black Canyon City, an unincorporated town 43 miles (66 kilometers) north of Phoenix, after a fire in nearby mountains no longer posed a threat.

A wildfire in southeast Washington grew to almost 60 square miles (155 square kilometers) as it blackened grass and timber while it moved into the Umatilla National Forest.

In Idaho, Gov. Brad Little declared a wildfire emergency Friday and mobilized the state’s National Guard to help fight fires sparked after lightning storms swept across the drought-stricken region.

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Associated Press writers Daisy Nguyen in San Francisco, Martha Bellisle in Seattle and Paul Davenport in Phoenix contributed to this report.