More than 57 million people in the American South and Southwest were under an excessive heat warning on Saturday afternoon — the most severe category for heat conditions — as temperatures across the Gulf Coast and parts of the Southwest soared to record-breaking levels and were expected to remain high through early next week.

The warnings reached as far north as Southern Illinois and the region surrounding St. Louis, which the National Weather Service said was expected to have its seventh day of heat indexes over 100 degrees.

A heat index factors in humidity — which can make the air feel swampier and more suffocating — to determine how hot it really feels even at a deceptively lower air temperature.

“Extreme heat and abnormally high overnight temperatures will persist in the South” over the weekend, forecasters with the Weather Prediction Center said early on Saturday morning, adding that “widespread record-high and -low temperatures are likely to be tied or broken across the Gulf Coast.”

Another 54 million people were under a heat advisory as of Saturday afternoon including in the Southeast and Pacific Northwest.

Forecasters warned residents that they should “not underestimate” the health risks of extreme heat, which can result in serious illness or death.

Dallas, Fort Worth and Waco, Texas, “set new daily record highs (again)” on Friday, reaching 110 degrees, according to the Weather Service office in the region.

A high of 108 was possible for Saturday in Dallas and Fort Worth, which would break a record for the day of 106, reached in 1999 and 2011. The city of Austin is expected to reach a high of 106 on Saturday.

The heat index in the New Orleans region is also expected to reach “oppressive” levels on Saturday, forecasters said, hitting 118 degrees in Covington and 115 in Baton Rouge and New Orleans.

A high of 102 in New Orleans and 105 in Baton Rouge were forecast for Saturday, which would break daily temperature records in both cities.

Forecasters in Phoenix said residents could expect a high of 113 on Saturday, adding that “a stretch of record-hot temperatures” will begin on Sunday and continue through early next week, reaching 115 degrees on Monday and Tuesday.

Memphis recorded a high of 102 on Friday, breaking the daily record of 101 set in 1943. It then recorded a minimum of 80 on Saturday, a degree above the record set in 2014.

Heat indexes in excess of 110 are also expected on Saturday in Little Rock, Ark.; Macon and Columbus, Ga.; Lafayette, La.; Tulsa, Okla.; Tallahassee, Fla.; and Gulfport, Biloxi and Jackson, Miss.

Punishing heat conditions in the South have been relentless this summer, compounded by suffocating humidity and a scarcity of rainfall.

While any single weather event can be hard to tie directly to climate change, scientists have no doubt that heat waves around the world are becoming hotter, more frequent and longer lasting.

The 2018 National Climate Assessment, a major scientific report by 13 federal agencies, noted that the number of hot days was increasing, and that the frequency of heat waves in the United States had jumped to six per year by the 2010s from an average of two per year in the 1960s.

The season for heat waves was also now 45 days longer than it was in the 1960s, according to the report.