Hi Striker02, I'll take a stab at an answer and others can correct me if I'm wrong on any points. What you see on a radar for a supercell with a possible tornado is what's called a 'hook echo' or 'bow echo'. what that shows is that the system has rotation on a big scale.
Radar does not actually 'pick up' or 'detect' debris or dust and does not 'see' rain as such (although if there is so much debris it will), but rather the amount of precipitation within and around the system. The darker the colours the heavier the precipitation - for example white to grey light, yellow medium then orange, red, black for heavy precipitation. Even outflow bands of rain can be picked up on the radar image as faint lines of half circles or full circles in front of the actual storm echo - that sometimes shows in which direction the storm is heading.
Somebody may be able to post a radar image of a hood echo but as far as i know the vacant area within the visible 'hook' is the updraft section of the storm with the tornado usually forming on the rh or lh side of that. The radar won't show a 'tornado' as such as they're things that are created by the host storm itself and not the other way round, but it can show 'where' the tornado is most likely to be formed with images bounced back to the radar from within the system.
Hope that helps. I think i'm sort of right without all the technical stuff, but you'll get more technical answers no doubt long after this!
Mike
