Every now and then folks like to subject Dallas to an F5 tornado for the sake of a terrifying what-if short film. The first in line remains the best: the Discovery Channel's Perfect Disaster "Supertornado" episode, which is only missing Helen Hunt and Bill Paxton; if that's what'll happen to the Ballpark in Arlington, imagine what fate awaits Jerry Jones's EnormDome. The Weather Channel also did its part a while back -- turns out, those downtown tunnels may be good for something after all.As men