September is keeping summer going strong with heat warnings expected to cover 26 million people, mostly in the Southwest, by Wednesday.

Temperatures in cities such as Los Angeles, Las Vegas and Phoenix could be 10 to 20 degrees above normal starting Tuesday and peaking Thursday, according to federal and NBC News forecasters.

A mass of warm air pushing down on the Earth’s surface, known as an upper-level ridge, is strengthening over the Southwest and blocking cool air from the Pacific Ocean,…