ActionScript 2.0 :: Top Down Falling Particles Reveal Dynamic Mask/logo?

I have plenty of ways of creating falling snow like effects, but a client has just comitted me to an effect as a test of my ability without talking to me first, that I am a little stumped by. use AS2 or AS3 for this project because of the filters and tween effects available to me.I need to have a top down falling snow effect (its actually supposed to be flour falling from a sieve but hey...). All the snow effects I know, fall down sideways on, from top to bottom, and then they disappear off the page (the flakes).I need the particles (movieclips) to fall downwards from a birds eye view, and 90% of them to disappear, but the remaining 10% to gradually (over about 5 seconds) build up a white logo (shape) in the middle of the screen.So I am guessing that I am going to have to be clever with dynamic masks to get the shape appearing a little bit at a time, and then I also need some random motion, and scaling as the particles fall downwards (and away) from the eye.