LAS VEGAS >> 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 (47.2 degrees Celsius) 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 (48.8 degrees Celsius) during the day, and remain in the 90s F (above 32.2 C) 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 (37.8 C).

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

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 (43.3 degrees Celsius) or higher temperatures on Friday, hitting 116 degrees Fahrenheit (46.6 degrees Celsius) 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.

“We’re getting a lot of heat-related illness now, a lot of dehydration, heat exhaustion,” said Dr. Ashkan Morim, who works in the ER at Dignity Health Siena Hospital in suburban Henderson.

Morim said he has treated tourists this week who spent too long drinking by pools and became severely dehydrated; a stranded hiker who needed liters of fluids to regain his strength; and a man in his 70s who fell and was stuck for seven hours in his home until help arrived. The man kept his home thermostat at 80 F (26.7 C), concerned about his electric bill with air conditioning operating constantly to combat high nighttime temperatures.

Regional health officials in Las Vegas launched a new database Thursday to report “heat-caused” and “heat-related” deaths in the city and surrounding Clark County from April to October.

The Southern Nevada Health District said seven people have died since April 11, and a total of 152 deaths last year were determined to be heat-related.

Besides casinos, air-conditioned public libraries, police station lobbies and other places from Texas to California planned to be open to the public to offer relief at least for part of the day. In New Mexico’s largest city of Albuquerque, splash pads will be open for extended hours and many public pools were offering free admission. In Boise, Idaho, churches and other nonprofit groups were offering water, sunscreen and shelter.

Temperatures closer to the Pacific coast were less severe, but still made for a sweaty day on picket lines in the Los Angeles area where actors joined screenwriters in strikes against producers.

In Sacramento, the California State Fair kicked off with organizers canceling planned horseracing events due to concerns for animal safety.

Employers were reminded that outdoor workers must receive water, shade and regular breaks to cool off.

Pet owners were urged to keep their animals mostly inside. “Dogs are more susceptible to heat stroke and can literally die within minutes. Please leave them at home in the air conditioning,” David Szymanski, park superintendent for Santa Monica Mountains National Recreation Area, said in a statement.

Meanwhile, the wildfire season was ramping up amid the hot, dry conditions with a series of blazes erupting across California this week, Wade Crowfoot, secretary of the Natural Resources Agency, said at a media briefing.

Global climate change is “supercharging” heat waves, Crowfoot added.

Stefan Gligorevic, a software engineer from Lancaster, Pennsylvania visiting Las Vegas for the first time said he planned to stay hydrated and not let it ruin his vacation.

“Cold beer and probably a walk through the resorts. You take advantage of the shade when you can,” Gligorevic said. “Yeah, definitely.”

Watson reported from San Diego. AP reporters John Antczak in Los Angeles, Anita Snow in Phoenix and Susan Montoya in Albuquerque, New Mexico contributed to this report.