Heat Wave Spreads From Texas to Bake the Southeast U.S.

A scorching early-summer heat wave that has baked much of Texas and Oklahoma for the past week spread across the Gulf Coast on Tuesday, with dangerous heat forecasts reaching all the way to the Florida Keys.

“In the South, we’re resilient, but it still hurts,” said Kevin Ardoin, who grows watermelons on a family farm about 40 miles north of Lafayette, La., where the heat index — a measure of how the air feels that takes into account both temperature and humidity — had already climbed to 107 degrees Fahrenheit before noon.

Temperatures have been brutal lately, Mr. Ardoin said as he stood under a tent in a wide-brimmed straw hat. He and his brothers cope by starting their field work early and pausing during the hottest part of the day, before going back out in the afternoon. “We definitely respect the heat, because it’s dangerous,” he said.

More than 55 million people in the United States were under some form of heat advisory, watch or warning on Tuesday, according to New York Times estimates using National Weather Service advisories and LandScan population data.

In Austin, Texas, where the heat index climbed to 118 degrees last week — the highest on record in the city — officials were braced for daily high temperatures to remain above 100 “for the foreseeable future,” said Kevin Snipes, the city’s emergency management director.

Ambulance calls and emergency-room visits for heat exhaustion have risen in Austin and other cities, including Tulsa, Okla., where electricity was out for tens of thousands of people for several days last week after heavy storms that were followed by triple-digit heat.

The high temperatures have already proven fatal for some. A teenage boy from Florida and his stepfather, who were hiking in Big Bend National Park in southern Texas on Friday, died as temperatures there rose to 119 degrees Fahrenheit — the second-highest mark ever recorded in the state. And a 17-year-old boy died on Wednesday after suffering heat-related fatigue on a trail in Palo Duro Canyon State Park near Amarillo, Texas.

“We are in extreme heat right now,” said Thomas VandenBerg, a park ranger at Big Bend.

As people across Texas sought to stay cool on Tuesday, demand for power from the state’s independent electrical grid reached 80,144 megawatts, an unofficial high for June and just shy of the record of 80,148 megawatts set on July 20 of last year. The Electric Reliability Council of Texas, which manages the grid and had warned of potential problems during the heat, did not report any strain.

The unusual early-summer temperatures — daily highs in the low 90s are more typical for much of the region in late June — are the result of a stubborn “heat dome” of high pressure that has lingered over much of Oklahoma, Texas and northern Mexico for days.

Determining whether a particular heat wave is tied to climate change requires analysis. But even so, scientists have no doubt that heat waves around the world are becoming hotter, more frequent and longer lasting. The 2018 National Climate Assessment, a major scientific report by 13 federal agencies, noted that the frequency of heat waves in the United States jumped to six a year by the 2010s, from an average of two a year in the 1960s.

Forecasters expect the current heat dome to shift slowly to the north and east during the week, extending the brutally hot weather to parts of Louisiana, Arkansas, Mississippi, Alabama, Missouri, Tennessee, Georgia and Florida. The heat index will push well into the triple digits during the day in some of these locations, and temperatures won’t cool much during the evening.

The pattern could continue across much of the South through the Fourth of July holiday, with perhaps the largest number of people affected on Wednesday and Thursday.

On the oak-canopied streets of Mobile, Ala., where the city was under an excessive heat warning on Tuesday as the heat index rose above 100, Randy Price sold fruits and vegetables in his usual spot southwest of downtown. Customers gathered in sweaty, bedraggled lines, clutching $10 bills and damp handkerchiefs.

Many would be tempted by the sweet Sugar Baby watermelons, Mr. Price predicted while drinking Milo’s sweet tea from a dripping gallon jug. On a scorcher like Tuesday, he will easily sell 300 watermelons, he said. “I just drink lots of liquids, try to hide under the tent and hope I sell out before it gets too hot.”

Mary Beth Gahan contributed from Dallas. John Keefe, Ivan Penn and Jacey Fortin also contributed.