More than 25 million Americans were again under an excessive heat warning Saturday, with one of the nation’s largest cities opening emergency cooling centers within libraries and other public buildings for vulnerable residents.

The National Weather Service on Saturday forecast heat indexes near Houston and San Antonio as high as 120 degrees Fahrenheit.

The extremely hot temperatures “will likely tie or break records” — and would do so ahead of the official summer solstice on Wednesday, the weather service said Saturday.

Residents in Texas and Louisiana are being told to drink plenty of fluids, stay in an air-conditioned room, stay out of the sun and check in on relatives and neighbors.

“This is dangerous heat,” said AccuWeather senior meteorologist Michelle McLeod. “Some records are going to be challenged.”

Some of the nation’s largest cities will swelter through next week, including Houston and San Antonio, forecasters say.

Austin, Texas, and Corpus Christi, Texas, along with New Orleans and all of southern Louisiana will also feel the excessive heat and there’s increased risk of wildfires being sparked in some areas, the weather service said Saturday.