Frontier Airlines is adding daily flights from Atlanta to Los Angeles, Las Vegas, New Orleans, New York LaGuardia, Cincinnati and Minneapolis, as it more than doubles its presence here this spring.

That will make Atlanta the third-largest city of operation for Frontier, behind its hub in Denver and its base in Chicago.

The ultra low-cost carrier last month said it will add flights from Atlanta to Miami, Austin and Indianapolis in March, and the six additional routes, which start April 30, will give 16 destinations served from Hartsfield-Jackson.

The announcement follows an expansion announced last week by ultra low-cost carrier Spirit Airlines, which is launching flights to nine more cities from Atlanta.

Ultra-low cost carriers are expanding in Atlanta following Southwest Airlines’ acquisition of AirTran and shrinking of its operation here, leaving less competition against dominant Delta Air Lines. The shift is prompting airlines to jockey for travelers in Atlanta.

Southest’s pullback left gates available on Concourse D that other carriers can use to expand. Frontier’s president Barry Biffle said Frontier now has two gates on Concourse D and access to a third. He said further expansion at Hartsfield-Jackson will depend on gate availability.

Biffle called Southwest a “mid-cost carrier,” and said that in Atlanta, “fares are relatively high compared to the average,” creating opportunity for a carrier like Frontier.