More than 40 million people in the American South were under an excessive-heat warning Saturday -- 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 this 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.
An additional 39 million people were under a heat advisory Saturday in parts of 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)" 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 Electric Reliability Council of Texas, or ERCOT, has asked the state's 30 million residents five times this summer to voluntarily reduce power usage because of the high temperatures creating high demand for electricity.
"There's going to be a front that starts making its way down here, the high is only going to be 103 degrees [today]," weather service meteorologist Ted Ryan said with a laugh. "But Monday and Tuesday highs are going to be in the mid-90s, which is right around normal ... 95 is going to feel pretty good for a lot of us."
Ryan said highs above 100 are likely not at an end with temperatures probably reaching above that level during September.
The heat index in the New Orleans region was also expected to reach "oppressive" levels 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.
"This is the hottest summer we've ever recorded," National Weather Service meteorologist Phil Grigsby said in Louisiana.
Forecasters in Phoenix said residents could expect a high of 113 on Saturday, adding that "a stretch of record-hot temperatures" will begin today and continue through early this week, reaching 115 degrees 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 were also expected Saturday in Little Rock and Tulsa.
Information for this article was contributed by Chris Cameron of The New York Times; and by Ken Miller of The Associated Press.