An oppressive heat wave settled over a wide swath of the U.S. South on Thursday, bringing record-breaking temperatures that topped 100 degrees Fahrenheit (38 Celsius) in many spots and creating dangerously hot conditions from Texas to Florida.

At midday, more than 35 million people in southern Texas, Louisiana and Florida were under excessive heat warnings, watches and advisories extending through the three-day Juneteenth weekend, the National Weather Service said.

"It wouldn't be an active summer weather pattern without oppressive heat and much of Texas and the Deep South will have plenty heading into the beginning of the holiday weekend," the service said in its forecast, which included the chance of strong thunderstorms and possible tornadoes for the region.

The growing frequency and intensity of severe weather across the U.S. is symptomatic of human-driven climate change, climate scientists say.

Record-breaking heat index temperatures could reach 115 degrees F (46 C) in cities such as Dallas, Houston and Austin, the Texas state capital, where officials have opened cooling centers in libraries and park buildings and urged people to drink plenty of water and stay out of the sun.

In Austin, Balcones Roofing & Remodeling owner Karl Larsen said he was providing plenty of cold drinks and Popsicles to his roofers. He has scheduled jobs and inspections earlier in the mornings and early afternoons to avoid the hottest part of the day.

"You feel your feet sweating and your socks getting wet and the heat from the roof through your shoes," he said. "If you are inspecting a roof, you have to do it quick and choose your battles wisely. No reason to be a hero ... and put your life on the line."

At 12 p.m. CST (1700 GMT), temperature readings across southern Texas were well above 100 degrees, including in Houston where it felt like 111 degrees F (44 C) and 108 F (42 C) in Galveston.

The Electric Reliability Council of Texas (ERCOT), which operates the grid for more than 26 million customers representing about 90% of the state's power load, has said it has enough resources to meet demand.

Across southern Louisiana, similar sky-high temperatures were in the forecast. New Orleans residents spent the morning under a 96-degree F (36 C) heat index, the weather service said in a tweet. "Dat's some bayou weather right there," the agency said in the local parlance.

Triple-digit temperatures and drenching humidity will also smother South Florida over the next several days, including in cities such as Miami and Jupiter, where officials warned residents to not leave pets and children alone in hot vehicles.

"I live in Lava Land in sunny Florida," a Twitter user who goes by the name of RyDavis wrote in a post early on Thursday morning.

(Reporting by Brendan O'Brien in Chicago; editing by Jonathan Oatis)