I would like a scene for my Snowball fight mod that at the start would randomly make big bits of snow fall, then it would just make flakes fall, like in Icelands.

For the big chunks of snow, you could try copying TLB's "coming of god" mod and editing the sprites and some of the variables. Because lots of small snowflakes would make the game lag a lot, you could make a background of snow falling, though I'm not sure if you can animate backgrounds.

How come no one remembers LordTim's Lua'd weather system?

Isn't there an old map with falling snow somewhere around this forum? Probably not compatible with this build, but it should give help.