By Brendan O’Brien, Rich McKay

(Reuters) -A massive high-pressure system known as a heat dome that has stalled over the U.S. Southwest was pushing temperatures in the region well above 100 degrees Fahrenheit (38 degrees Celsius) on Thursday, leaving millions of Americans to swelter in the coming days. 

Some 31 million people from Northern California, south through Arizona and east into Texas, were under excessive heat warnings and heat advisories issued by the National Weather Service through Saturday. The same region suffered under weeks of extremely hot weather last summer. 

One of the hottest spots is likely to be the Las Vegas strip, where the high temperature is expected to reach 112 F (44 C), which would mark a record for the Nevada city on June 6.

“It’s going to be a hot one out there today!” the NWS in Las Vegas said on X, urging people to drink fluids, wear loose fitting clothing and stay indoors if possible. 

In Death Valley, California – which features the lowest point in the country and is believed to be one of the hottest places on Earth – the temperature could reach 121 F by the afternoon, an ominous sign before the official start of the summer. 

Matthew Lamar, a park ranger at Death Valley National Park, keeps cool by pouring cold bottles of water over his head. 

“It feels good but doesn’t last long. It evaporates quickly,” he said, noting that tourist traffic has slowed considerably as the temperature rises. 

In Phoenix, the high temperature was to reach 114 F, forcing officials to open cooling centers at libraries and to close some popular hiking trails during the day.   

“The hot temperatures continue and a few records may even be broken over the next couple of days,” the NWS in Phoenix said in a post on social media platform X. 

A heat dome, the cause of this week’s conditions, is a ridge of high-pressure air in the upper atmosphere that stalls and traps hot air while keeping cooler air away even at night.

Phoenix was one of several cities in the region that experienced their hottest summers on record in 2023. Arizona’s capital city endured the high temperatures exceeding 110 F for 55 straight days, a record. Last summer, 645 people died in the Phoenix area due to heat-related illnesses.

© Reuters. A homeless person refresh himself from water from a pipe line as temperatures are expected to soar above 110 degrees Fahrenheit (43.3 degrees Celsius) during the summer's first heat wave, in Phoenix, Arizona, U.S., June 6, 2024. REUTERS/Carlos Barria

In response, city firefighters this summer will begin using ice immersion to care for heat stroke victims. The technique calls for rescue personnel to pack patients in ice on their way to quickly lower their body temperature, fire officials said during a demonstration for local media. 

Forecasters say it was difficult to link the record-breaking heat experienced by the U.S. Southwest in recent years to human-induced climate change, but such extremes are becoming more frequent because of global warming.