The National Weather Service said the heat wave that brought temperatures of 54 Celsius to California's Death Valley on Sunday had likely peaked, with more moderate temperatures expected by the end of the week.
"However, excessive heat warnings remain in effect from California to Utah since readings will still be well above normal for Monday," the weather service said in an advisory, predicting triple-digit temperatures again in the desert southwest.
The unseasonable heat wave, triggered by a lingering high pressure system, is already the third for the region this year, an anomaly that some experts have attributed to climate change.
The high temperatures come as forests and brushwood are already bone dry across the West from years of severe drought, contributing to what authorities say could be an intense fire season.
Already in Oregon, the so-called Bootleg Fire had burned through more than 153,000 acres (nearly 240 square miles) in and around the Fremont-Winema National Forest. Crews have made little progress containing the blaze.
The flames were burning along a high voltage power corridor connecting Oregon's power grid with California's, worrying officials in both states that electricity could be knocked out to thousands of homes and businesses.
The agency that manages California's power grid, the California Independent System Operator, issued a "flex alert" urging residents to conserve power between 4 p.m. and 9 p.m. local time.
Hundreds of residents in the Klamath Falls area, in south-central Oregon, were under mandatory evacuation orders, and in an unusual step, county officials said residents who defy them could face citations or arrest.
Along California's border with Nevada, the Beckwourth Complex Fire had grown to 89,600 acres (140 square miles) as of Monday morning, with about 23% containment, according to the state's fire incident reporting system.
ARIZONA PLANE CRASH
In Arizona, federal authorities on Monday were investigating the crash of a small plane that went down while surveying a wildfire in the northwestern part of the state, killing both crew members.
Authorities said the Beechcraft King Air C-90 aircraft was part of the initial effort to contain the Cedar Basin Fire when it crashed at about noon on July 10 near Wikieup, approximately 139 miles northwest of Phoenix.
The fire has burned 734 acres and is 75 percent contained as of Monday morning, fire officials said.
The crew members were identified as Jeff Piechura, 62, a retired Tucson fire chief, and Matthew Miller, 48, a pilot with Falcon Executive Aviation Inc. The Department of Interior Office of Aviation Services, in conjunction with the National Transportation Safety Board and the Federal Aviation Administration, are investigating the accident.