Temperatures across the Southern Plains were expected to be in triple digits on Monday, continuing a long stretch over recent weeks that have led to several heat-related deaths.
On Sunday, the nation's high was 109 degrees at Ardmore, Okla.
The Dallas area saw 107 F on Sunday, the hottest day of the year. A high of 105 F was expected for Monday, which would mark the 11th straight day of triple-digit temperatures there.
Highs around Dallas reached 104 degrees Saturday. The National Weather Service said July ended as the fourth-hottest in history for the Dallas area.
Authorities in Dallas County have confirmed three heat-related deaths in what has become one of the most sweltering Texas summers on record. The Dallas County Medical Examiner's Office says the deaths began in June, and forecasters say the worst may still be ahead.
Dallas County authorities say the heat-related deaths were two men, ages 49 and 84, and an 82-year-old woman.
Forecasters expect the high around Dallas on Monday to reach 107 degrees, which would be a record for this year.
NBC affiliate KXAS reported that the cooling stations designated by Fort Worth were closed on Sunday, as were the Salvation Army's community centers and the stations Dallas set up in recreation centers.
Anita Foster of the Salvation Army said people seeking relief could try spending time at the mall or at movie theaters.
While the Salvation Army's community centers were closed, their shelters were doing double duty on Sunday as cooling centers. On Sunday, it had room for 200 additional people in both its Dallas and Fort Worth locations.
The heat also took its toll on six Fort Worth police officers, who were treated for heat-related conditions after an extremely hot Friday in which temperatures exceeded 103 degrees, KXAS reported.
The officers were treated after trying to chase down two burglary suspects.