Carrie Underwood is the latest artist to join Toby Keith's tornado relief concert on July 6 in Norman, Okla. Her performance will be aired via satellite from the Grand Ole Opry House in Nashville to the Gaylord Family-Oklahoma Memorial Stadium in Norman. The event is already sold out. In addition to Keith, previously announced performers include Garth Brooks, Trisha Yearwood, Willie Nelson, Sammy Hagar, Ronnie Dunn, Mel Tillis and John Anderson. Proceeds will benefit the United Way of Central Oklahoma's tornado relief fund.