This rig consists of particles goaled to a nurbs cylinder which has controls to adjust the size, shape and movement of the tornado. There are expressions driving the particles around and up the cylinder. I've used this rig for sprites, instanced debris and particle emitting fluids. All the custom attributes are located on the particles for easy editing:
~ emission rate and location.
~ goal weight and goal offset per particle.
~ up and around speed per particle.
~ release controls (loose debris) per particle.