I'd have snow banks created with shoveled areas already shoveled out.  Then I'd fill the shoveled out areas with a mix of fake snow and the white bead stuff they use for packing materials.

 

Each night shovel the stuff out during the performance and shovel it back after the performance.  Have the actors practice shoveling on their marks so that they only shovel the shoveled-out-and-filled-with-packing-bead-areas.

 

Now the only other problem is the falling snow.  For that do the classic solution for snow, the snow cradles/snow bags filled with plastic flakes.

 

Interesting problem. 

 

BTW--I'd do the snow banks with chicken wire covered with muslin.  Then paint it white and sprinkle both fake snow and sparkly glitter flakes over the top--or cover it with quilt batting material and fake snow and sparkly glitter flakes.  Years ago I'd have used styrofoam and carved it,  but the stuff is so toxic if it melts, and melting it is the easiest way to carve it. Been there done that.