A teenage boy and his stepfather hiking in Big Bend National Park in Texas died as temperatures there rose to 119 degrees Fahrenheit on Friday — the second-highest mark ever recorded in the state — during a triple-digit heat wave that was forecast to spread to the Southeast this week.

“We are in extreme heat right now,” said Thomas VandenBerg, a park ranger at Big Bend, near the U.S. border with Mexico, where another hiker recently died of heat-related causes.

The dangerous early-summer heat wave has broken daily temperature records across Texas and strained the state’s independent power grid. In Oklahoma, the heat scorched a state battered by storms that left tens of thousands, mostly in the Tulsa area, without electricity for much of last week. By Monday, most power had been restored and temperatures had dropped into the 90s, but they were expected to climb over 100 again later this week.

A stubborn “heat dome” of high pressure has kept the heat index — an indication of how the air actually feels, taking into account humidity — high across much of Oklahoma and Texas for days. The system is forecast to shift slowly to the east during the week, extending the brutally hot weather to Louisiana, Mississippi and Alabama. Forecasters say the pattern could continue through the Fourth of July holiday.

The afternoon high of 119 degrees in the Big Bend area on Friday came within one degree of tying the state’s high for any date, of 120 degrees Fahrenheit, first recorded in 1936 and equaled in 1994, according to the National Oceanic and Atmospheric Administration.

A man was hiking in Big Bend with his two stepsons in the late afternoon on Friday, park officials said in a statement, when the younger stepson, who was 14, lost consciousness. His brother, 21, tried to carry the boy back to the trailhead while the stepfather rushed to his vehicle to seek help.

By the time park rangers and Border Patrol agents reached the boy, he had died, officials said. The stepfather, 31, was later found dead in his vehicle, which had crashed over an embankment.

The three had been hiking on the Marufo Vega Trail, which “winds through extremely rugged desert and rocky cliffs within the hottest part of Big Bend National Park,” the officials’ statement said. It added that hikers on the trail did not have access to shade or water, making it “dangerous to attempt in the heat of summer.”

The authorities said the three hikers were from Florida, but did not give their names. The 21-year-old brother who survived has returned home, officials said on Monday.

Several other hikers have succumbed to extreme heat in the United States in recent years. The death of two experienced hikers and their 1-year-old daughter in the Sierra National Forest of California in 2021 confounded investigators for two months, until officials determined that they had died from the effects of heat stroke and possible dehydration in 110-degree weather.

Last July, a 22-year-old man died, reportedly of heat exhaustion, after he ran out of water while hiking in the Badlands of South Dakota. In September, another hiker died, and five more were rescued after suffering from extreme heat in Arizona. In April, a man died on a hiking trail in Lakeside, Calif., after suffering symptoms of heat exhaustion.

The recent heat has been fatal in cities, too. Last week, a postal worker collapsed and died while on his route in the Lakewood neighborhood of Dallas.

The early-summer heat has been brutal even in places where residents are used to hot summers. At Main Street Mowing in the northern suburbs of Dallas, business always picks up when temperatures hit the triple digits, said Tanner Maxson, who owns the business. This year, though, the calls are coming in late June, not July or August.

“People are throwing in the towel,” Mr. Maxson said. “The phone has been ringing off the hook.”

Temperatures in the Dallas area were expected to reach 103 on Monday, with a heat index of about 110. By Wednesday, the National Weather Service expects temperatures to reach about 107. Highs in late June are typically in the low 90s.

In Austin, temperatures were also expected to reach about 103 on Monday. “No one can survive this,” said Paula Knight, 34, who runs a small business advisory group and tried — only briefly — to get some work done at an outdoor table at a coffee shop on Monday afternoon.

Still, some Texas residents said they were accustomed to the scorching heat. “This is summer in Texas,” said Petr Obrda, 79, while walking on Monday morning in north Austin.

David Montgomery contributed reporting from Austin. John Keefe also contributed.