i dont know enough code yet to do this myself, but im guessing it would be like..
Snow()
//Probably some overlay thing here that would affect the whole map, So it would make a snowing icon with a layer = 5. no idea of how to do that.
Sleep(3000)
//Something to make a random choice
if random choice is bla bla blah
//another overlay for the whole map, Snow on the ground this time.
else
ClearWeather()
I know its hard for someone as new to coding as me, but i really want to learn..