Masking the snow effect?

Hello. I am learning flash…kinda new so sorry if my question is stupid.
I am doing the flash snow tutorial (version 3 for flash 8) and I wanted to know if there is a way to mask the snow effect? I want it to only snow inside of a specific graphic that has a weird shape and I am lost on how to do it.
Thanks, and I hope this is a good question.

DH