A potentially record-setting heat wave is expected to drive temperatures across the western U.S. into the triple digits as it builds this week, triggering warnings as authorities set up cooling shelters throughout the region.

Millions of people from southwestern Oregon through California, parts of Arizona and western Nevada will be under excessive heat warnings Tuesday through Friday.

The highest temperatures are expected in Furnace Creek in Death Valley National Park, where temperatures could reach 120 degrees this week. Meanwhile, in southern California's Palm Springs area and the Coachella Valley, afternoon highs could hit 118 degrees.

Federal forecasters said major cities including Los Angeles, Phoenix, Las Vegas and Portland, Oregon, were expected to surpass 100 degrees, according to the weather service. Los Angeles Mayor Karen Bass said that emergency personnel have begun preparing for the heat wave and that authorities are setting up cooling centers around the city.

"The homeless, elderly, children, and those with health issues are at the highest risk for heat illness," the weather service office in Los Angeles said on X. "Daytime highs between 95 to 105 will become common away from the coast."

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Heat wave in the Southwest isn't unusual this time of year

The looming stretch of searing heat is not uncommon for Southern California, western Nevada and southern Arizona this time of year, said Heather Zehr, a senior meteorologist at AccuWeather.

"September is actually a fairly hot month overall, being as far southwest as they are, and as dry as the climate is. ... They tend to reach their peak normal averages at the end of August and beginning of September," she said.

Zehr noted that the heat wave will be far less severe than an earlier heat wave that broke records in July across the Southwest. That blast of hot weather brought temperatures in Phoenix to 118, afternoon highs in Las Vegas to 120 and peak temperatures in Palm Springs desert area to 124 degrees. This week, each of those areas is forecast to remain several degrees below its afternoon highs in July.

The same cannot be said for the Northwest.

Zehr said southwestern Oregon typically begins to cool near the start of fall as intrusions of Arctic air push across the region. But because of the heat wave, daily record highs across the Willamette Valley, including in cities such as Portland and Medford, may be challenged as forecaster project temperatures to reach as high as 105 degrees.

"This is a little more unusual for them," Zehr said, speaking of the heat wave settling over southern Oregon.

How to stay safe during a heat wave

To stay safe during persistent heat waves, experts advise that people avoid direct sunlight, drink plenty of water and, if they must go outside, wear sunscreen or sun-protective clothing.

Seemal Desai, president of the American Academy of Dermatology, told USA TODAY that people should especially stay inside between 10 a.m. and 2 p.m., peak sunlight hours. People should drink water, not just caffeinated beverages or sports drinks that are hydrating but have other chemicals and additives, Desai said.

Kelly Olino, assistant professor of surgical oncology at the Yale School of Medicine, urged people to keep watch for the serious warning signs of heatstroke or heat-related illness: nausea, vomiting and lightheadedness.

The National Weather Service issued advisories on Tuesday reminding people to check their vehicles for children and pets before leaving them unattended because car interiors can reach lethal temperatures in a matter of minutes, the weather service said.

How extreme heat can affect your health

Experts say that while warm weather improves mental health – people get outside and soak up more vitamin D when it's nice outside – extreme heat can have the opposite effect.

"Heat has been shown to cause irritability, agitation, anxiety, cognitive impairment," Susan Albers-Bowling, a clinical psychologist at the Cleveland Clinic, told USA TODAY in June. "Research also shows that, during heat waves, there's a noted increase in suicide rate, domestic violence, substance use and crime."

Albers-Bowling said the high temperatures could disrupt sleep and render certain medication less effective. Extreme heat also can cause hormone changes, including an increase in cortisol, the stress hormone, and can reduce alertness and cognition, she said.

Contributing: Doyle Rice