Texas Prepares for a Punishing Heat Wave

Millions of people across Texas were bracing on Thursday for a spell of punishing heat that officials said would likely last for days, break records and raise the risk of wildfires and heat-related illnesses.

More than 33 million people in Texas, as well as Florida and Louisiana, were under heat advisories early Thursday, according to the National Weather Service. Many of those affected were in southern and eastern Texas, and some of the advisories were scheduled to remain in effect through Friday night.

Parts of Texas, including the Dallas-Fort Worth area, were expected to see heat index readings of 105 to 112 degrees Fahrenheit on Thursday or Friday, the Weather Service said in one advisory. An index of up to 119 degrees was possible in some southern countries, the agency said early Thursday. The heat index measures how hot it feels outside, accounting for temperature and humidity.

Later this week or early next week, the heat index in South Texas and the western Gulf Coast could rise as high as 120 degrees, the agency said in a forecast. Some daily high temperature records may fall, it added, and the weather could pose health risks for people who spend extended time outdoors or without air conditioning.

The Weather Service said that parts of Arizona, New Mexico and Texas were also expected to experience critical fire weather on Thursday and Friday, indicating a higher-than-average risk of wildfires.

There was heavy rain in the forecast, too, for parts of Texas and the Southeastern United States. More than seven million people were under flood watches across the region early Thursday, mostly in Alabama, and there was a risk of tornadoes.

It’s not unusual for officials in Texas to issue heat advisories around this time of year, said Monte Oaks, a meteorologist at the Weather Service’s San Antonio office. They typically do so when high temperatures combine with other factors, including high humidity and westerly winds that blow hot air from high-altitude deserts, he added.

In this case, Mr. Oaks said, the humidity is high because Texas had a wetter and stormier spring than usual. That has left parts of the state looking lusher than they normally do in June, he said. But it also means that the hot ground is “cooking a lot of the moisture” and releasing it into the air.

“We’re not off the charts hot,” he added, speaking by phone late on Wednesday night. “We’re kind of normally hot after a long period of fairly mild weather in the months of April and May.”

Electricity demand is expected to rise in the state later this week because of the hot weather, the Electric Reliability Council of Texas, which manages about 90 percent of the state’s electricity load, said in a statement on Wednesday night. But there is enough supply to meet the demand, the company added, and it does not expect an “energy emergency.”

Global warming is making dangerously hot weather more common, and more extreme, on every continent.

In Texas and neighboring Mexico, it is making the levels of heat forecast over the next few days at least five times more likely, according to an analysis on Wednesday by Climate Central, a nonprofit research collaboration of scientists and journalists.