A dangerous heat wave threatened to bring record-breaking temperatures to parts of the U.S. South on Friday, with the National Weather Service warning Americans from Texas to Florida to limit time outdoors over the long Juneteenth weekend.

The agency issued excessive heat warnings affecting millions of people in parts of southern Louisiana and Texas through the holiday weekend, saying the heat index could reach 110 degrees. South Florida and Mississippi are under heat advisories for Friday.

People working or partaking in outdoor activities face a heightened risk of heat-related illnesses due to the humidity, except during early mornings and evenings, according to the advisory. Children and pets should not be left unattended in vehicles.

Cooling centers opened in New Orleans to help locals contend with the conditions, WWL-TV reported.

"Outside of any rain and storms, no question about it - it will be HOT!" the weather agency's New Orleans outpost said in a tweet.

While Southern states are accustomed to high heat, extreme weather conditions have become more frequent across the country and with more intensity, driven by climate change, scientists say.

Officials advised that people drink fluids to keep hydrated, stay in air-conditioned rooms and limit time in the sun.

Over the Juneteenth weekend, thermometers could push well into the triple digits in southern Texas, where the holiday originated with African Americans celebrating the end of slavery in 1865.

"High temperatures in the 100s to 110s will represent a 15-25 degree anomaly for this time of year over that area," the weather service said.

On Thursday, the mercury nearly reached 100 degrees Fahrenheit (32 Celsius) in Houston and the heat index peaked at 110 degrees, according to local media.

(Reporting by Tyler Clifford in New York; editing by Jonathan Oatis)