I am not actually having time to make a code. But I want to share the knowledge.
There are multiple ways you can do this.
The way I think is cool:
make a html canvas
In each frame draw a circle onto the canvas and add a bit if movement to the snow. (I HOPE YOU KNOW THIS). Then make a function that changes the display property of canvas to none. So that canvas disappears.
Call the function after 30 seconds using setTimeout()
Hope it helps..