The heat will extend over southeast Oregon, northern California, the Mojave Desert, eastern California, and parts of Nevada and Utah, according to CNN meteorologist Michael Guy.
The ridge of high pressure is expected to level out, bringing more typical temperatures back to the area, with alerts beginning to expire Monday and some lasting to Tuesday evening.
The cooling will be a welcome relief to an area that has been baking under dangerous temperatures.
Earlier this month, temperatures reached 15 to 30 degrees warmer than normal, Guy said. Much of the West is expected to remain 5 to 10 degrees warmer than average, he added. During this part of the long duration heat wave, at least seven locations broke or tied their all-time record high temperatures. Guy said.
Las Vegas hit a high temperature Saturday of 117 degrees Fahrenheit. This was the fifth time the city reached this all-time record temperature since July 24, 1942.
Another concern is the record warm overnight temperatures.
Overnight low temperatures in portions of the Desert Southwest have failed to fall below 90 degrees. When overnight temperatures don’t cool enough it makes the human body more susceptible to the effects of heat stress, putting people at greater risk of heat stroke and death, Guy said.
55 large fires burning
In California and Nevada, residents are being asked to conserve energy in response to heat and wildfires.
“Thank you for conserving energy today between 6 p.m. and 9 p.m. to help reduce the strain on the electric grid,” the tweet said.
Transmission lines were tripped off on Friday and Saturday, California ISO said in the release, noting that it has limited “electricity flow from the Pacific Northwest to California and other states.”
Wildfires across the western US have been growing, fueled by the hot, dry weather.
More than 11,300 wildland firefighters are battling the ongoing fires, the National Interagency Fire Center (NIFC) said.
Across 12 states, 55 large fires have burned 768,307, according to the NIFC.
In California, the largest fire currently burning is the Beckwourth Complex Fire in the area of the Plumas National Forest.
“The Beckwourth Complex (contains two fires) is California’s largest current fire,” Beckwourth Complex Fire Information spokesman Mike Ferris told CNN. “There have been 3,061 people affected by the evacuation with 1,199 residences threatened.”
And the River Fire in the state has exploded in the vicinity of a route that leads to Yosemite National Park, Cal Fire spokesperson Jamie Williams told CNN.
Fire officials have received calls from “quite a few visitors and many phone calls from residents” regarding the smoke and fire information, Williams said.
Rapid snowmelt on volcanoes
During the historic heat wave, some of Washington state volcanoes experienced “significant” snowmelt in June, according to a meteorologist at the National Weather Service (NWS) Seattle office.
Paradise which is located at 5,400 feet on Mount Rainier, experienced approximately 30% of snowmelt between June 26-30, NWS Seattle meteorologist Jacob DeFlitch told CNN.
Another volcano in the state, Mount Baker also experienced “rapid snowmelt” DeFlitch said.
In contrast, the pictures taken of both volcanoes on July 10 from the NWS Seattle office rooftop it reveals much less snowpack on the peaks.
“From a visual standpoint, it shows a very significant change in such a short time period for the mountains snow pack,” DeFlitch said.
Glacier Peak has an elevation of 10,541 feet, and is located in Snohomish County, north of Seattle.
DeFlitch said it is common to see snowmelt during the summer at the volcanoes but “this is a large amount for such a short period of time.”
“The significance of the snowmelt is the magnitude and duration of the heat from the West, particularly Pacific Northwest, has experienced from late spring into early summer,” Guy said.
“In this instance, the magnitude of the heat was so strong that it was able to warm temperatures at a height that normally never gets warm and melt the snow,” he added. “This is very significant when talking about climate change and feedback loops that only exacerbate the problem to worsen, and in my perspective very concerning.”
Correction: An earlier version of this story overstated the number of people affected by heat alerts. More than 18 million people remain under heat alerts. The story also incorrectly described the heat record date in Las Vegas. It is the fifth time the city reached this all-time record temperature since July 24, 1942.
CNN’s Hollie Silverman, Stella Chan and Kay Jones contributed to this report.