Visitors to Las Vegas on Friday stepped out momentarily to snap photos and were hit by blast-furnace air.

But most will spend their vacations in a vastly different climate — at casinos where the chilly air conditioning might require a light sweater.

Meanwhile, emergency room doctors were witnessing another world, as dehydrated construction workers, passed-out elderly residents and others suffered in an intense heat wave threatening to break the city’s all-time record high of 117 degrees Fahrenheit this weekend.

Few places in the scorching Southwest demonstrate the surreal contrast between indoor and outdoor life like Las Vegas, a neon-lit city rich with resorts, casinos, swimming pools, indoor nightclubs and shopping.

Tens of millions of others across California and the Southwest, were also scrambling for ways to stay cool and safe from the dangers of extreme heat.

“We’ve been talking about this building heat wave for a week now, and now the most intense period is beginning,” the National Weather Service wrote Friday.

Nearly a third of Americans were under extreme heat advisories, watches and warnings. The blistering heat wave was forecast to get worse this weekend for Nevada, Arizona and California, where desert temperatures were predicted to soar in parts past 120 degrees Fahrenheit during the day, and remain in the 90s F overnight.

Sergio Cajamarca, his family and their dog, Max, were among those who lined up to pose for photos in front of the city’s iconic “Welcome to Fabulous Las Vegas” sign. The temperature before noon already topped 100 F.

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“I like the city, especially at night. It’s just the heat,” said Cajamarca, 46, an electrician from Brooklyn Park, Minnesota.

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His daughter, Kathy Zhagui, 20, offered her recipe for relief: “Probably just water, ice cream, staying inside.”

Meteorologists in Las Vegas warned people not to underestimate the danger. “This heatwave is NOT typical desert heat due to its long duration, extreme daytime temperatures, & warm nights.

Everyone needs to take this heat seriously, including those who live in the desert,” the National Weather Service in Las Vegas said in a tweet.

Phoenix marked the city’s 15th consecutive day of 110 degrees Fahrenheit or higher temperatures on Friday, hitting 116 degrees Fahrenheit by late afternoon, and putting it on track to beat the longest measured stretch of such heat.

The record is 18 days, recorded in 1974.

“This weekend there will be some of the most serious and hot conditions we’ve ever seen,” said David Hondula the city’s chief heat officer. “I think that it’s a time for maximum community vigilance.”

The heat was expected to continue well into next week as a high-pressure dome moves west from Texas.