That was actually super quick :D I already had a tornado and tornado shader from KriptoFX's Realistic Effects Pack 4, and I love whipping up particles and quick prototype 3d bits.
The fun part was programming the tornado to regenerate its energy based on the terrain splat maps containing sand, and take damage while over water or climbing too high, allowing me to paint actual gameplay with the terrain. It's something I've used for dynamic footstep sounds, but I think was usage is much cooler.