The heat wave has been deadly, with officials blaming it for dozens of weather deaths, including those of two athletes and a football coach who died during football practices this week in Florida, Georgia and Texas.
If forecasts hold, it will be the 15th straight day of 100-plus temperatures in Dallas, CNN reported.
Texas also set another record for electricity demand on Wednesday, the third consecutive record-setting day.
"We are expecting another record-high electricity demand [on Thursday], so we are continuing our call for conservation," said Kent Saathoff with the Electric Reliability Council of Texas, which manages the state's power grid.
Temperatures soared to 114 degrees in Little Rock, Ark., making Wednesday the hottest day on record for the city, KLRT-TV, Little rock, reported.
In the St. Louis suburb of Pine Lawn, Mo., a 19-year-old woman was charged with parental neglect as she, her two children and her boyfriend either fell asleep or passed out on a basketball court, KTVI-TV, St. Louis, reported. A passerby called emergency services after spotting the four.
"The caller apparently told our officers there were four dead people in the heat on the basketball court," Pine Lawn Police Chief Rickey Collins said. "This is definitely a careless situation from a parent. We're not going to go lightly on it."
In Oklahoma City, a quick, robust summer storm kicked up debris that injured three women, the city's emergency services department said. The injuries were minor but did require a trip to the hospital, officials said.
The extreme heat was forecast across the central and southern Plains through the weekend and into next week, AccuWeather.com said.