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

Some 30 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.

“It’s just too hot,” said Michelle Reynolds, 52, who was with her dog, Scott, at a park in Modesto, Calif. “It’s not only hot for us, it’s also hot for the animals.”

Reynolds said the seasonal heat seems to be getting worse every year, sounding resigned to the uncomfortable conditions.

“You just never know how the climate is going to change,” she said. “We just go day by day. That’s all you can do.”

The high temperature is expected to reach 112 F (44 C) on the Las Vegas strip on Thursday, which would be a record for June 6. That is still cooler than the expected conditions in Death Valley, California, where the temperature is forecast to reach 121 F by the afternoon.

In Phoenix, Arizona’s capital city of 1.6 million people, the high temperature was to reach 114 F, forcing local 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.

Several cities in the region experienced their hottest summers on record in 2023. In Phoenix, the high temperature reached 110 F for 55 straight days, a record for the city where 645 people died in the metro area due to heat-related illnesses last summer.

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.

A heat dome is the cause of this week’s dangerously hot conditions, according to NWS forecasters. A heat dome 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.

Forecasters urged residents across the region to stay indoors, drink plenty of fluids and if able, to check on neighbors and loved ones.