Having true snow flakes (small physical objects) stack is a horrible idea for a game. One, the number of objects will get really high really quickly. Two, the physics calculations will get very resource intensive (also really quickly).
What you need to do is to fake it. I would advise using a mesh to simulate the top of the snow and slowly increase the height of the mesh over time as the snow falls. It wont be perfect, but it should achieve the desired effect.
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