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

In Austin, Texas, where the heat index climbed to 118 degrees Fahrenheit last week — the highest on record in the city — officials were preparing 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.

“We are in extreme heat right now,” said Thomas VandenBerg, a park ranger at Big Bend. The growing demand for electricity to cool homes and businesses has also put a strain on the Texas’ independent power grid, though it appears to have held so far.

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 — a measure of how the air feels that takes into account both temperature and humidity — 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.

John Keefe and Jacey Fortin contributed.

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