I added the snow using the brush tool and after aplying it I used motion blur filter to make more realistic
step 3 of 4
In this step I added the snow inside the building using the same brush tool this timw with different opacity.
step 4 of 4
After creating a new layer with a specific gradient map, i created the snowflakes and again I used the motion blur.
Special thanks to Suresh for "Apply Awesome Realistic Snow Effects to Your Photos tutorial".