you could do it in after effects without expressions. (i hate scripting for some reason, so i;m always looking for ways around it!)
put something in the background of your scene, like a little sphere or boxes or anything simple. make the bg color black and the boxes and spheres white. hide everything else and render it. your new render will just have little white shapes bouncing around in a black void. in aftereffects, track them and apply the tracked data to the BG layer. (for an even simpler, faster way, just render the wireframe and track some gridpoints as far away from the camera as you can!)
don't forget to render your tornado with an alpha so you can put it over easily!
