Call it momentum. Or the call of country music. Or: Who could turn down Carrie Underwood?

Mystic Lake Casino is so carried away with its new temporary amphitheater that on Thursday it announced two extra shows, both surefire sellouts -- Underwood Sept. 3 and Alan Jackson Sept. 17. Another concert is already sold out: the Beach Boys on July 31.

After staging a free outdoor concert last July 4 with the Oak Ridge Boys in a parking lot, Mystic Lake decided to try a temporary amphitheater this summer.

"With the huge feedback we got, that put the idea in our heads: What else could we do?" said Mystic Lake's vice president of marketing, Bryan Prettyman. "Our goal is to do a little something for everyone. The entertainment [at Mystic's four-year-old, 2,100-seat indoor showroom] has been well received by our casino guests and a lot of first-timers."

The nine-concert series opens this weekend with Jason Mraz on Sunday and another free July 4 show with country vets Diamond Rio. "This is our learning phase," he said.

The 8,357-seat amphitheater has a temporary stage, rented seats, portable toilets, two concession tents and two big HD video screens. The stage has a roof; the seats don't (although they do have backs). Rain or shine, the shows go on. All seats, which are anchored on a gently sloped lawn, are reserved. There are box offices in the casino and outside the amphitheater (where the will-call windows are).

While this is the largest amphitheater in the Twin Cities, the Mystic venue is much smaller than the 15,000-plus facilities proposed over the past two decades for various metro suburbs. Another thing that distinguishes the Mystic Amphitheater: no alcohol, a casino policy set by the Shakopee Mdewakanton Sioux Community.

After the series, Mystic Lake will evaluate whether to build a permanent amphitheater of similar size. "Ultimately, it's the tribe's decision," Prettyman said.