Billings Gazette: Tornadoes in Montana this year have been more frequent and more powerful than at any point over the past decade, as evidenced by twisters that destroyed an arena in Billings and killed two people in the state’s northeastern corner. While an unusually wet spring and early summer helped extend this year’s storm season, meteorologists can’t pinpoint just what’s caused the spike. Montana averages seven tornadoes a year, which ties it with New York for the nation’s 19th-lowest …