I'd be thinking this through science. As we all know, snow is
rain and vice versa, except they have different solid states due
to temperature changes. Temperature changes can be associated to
winds. A quick Google for "what causes wind" gave me an answer
from Answers.com: "The pressure gradient force, which arises
from the uneven heating of earth's surface. Once it is in
motion, the Coriolis force arises from Earth's rotation to
deflect it (right in the northern hemisphere, left in the
southern hemisphere)." Aye, not the best source for scientific
questions, but it gives the idea. So, theoretically, you only
need wind now. It could be random, or, might as well be somehow
scripted to you environment which heats due to some events. Take
wind now, apply to your current position, and randomly or
through a scripted way, make clouds out from it. The denser the
cloud, the more the probability and density of rain. If
temperature is low enough, rain becomes snow. Move the cloud
around and make it "not so dense" with t