Start and finish
I copied the original image to a new layer above. I then convereted it to Black and White, lowering the saturation of yellow and green.
Using curves to increase contrast and lighten the image even more
You now have 4 layers, Remove visibility for the original layer and merge the visible layers.
6. Lower the opacity on the snow layer to about 77%
On the original layer, select the sky , change to the snow layer make sure you are on the Mask, and fill the selection with black.
Changes after filling the sky with black on the mask.
8. On the mask, paint with black to show the original image where you don’t’ want it covered with snow, use a lower opacity brush to reveal only some of the original layer.
9. You can see where you have painted on the mask, by holding down the ALT key and clicking on the mask.
10. Create a new layer at the top, fill with black, add some noise (mono) create a Treshold layer, I set mine to about 95. Make sure it has a clipping mask so it will only affect the layer below.
Working with the 2 bottom layers visibility off;
Go to filters and choose Pixalate and Crystaline, try a value of 10.
Add some blur, motion or gaussian.
Select the threshold layer and merge down.,
return the visability to the 2 underlaying layers, and make sure the snow falling layer is set to SCREEN.
Make a new layer from the falling snow layer, enlarge it a little a move it around, flip it or whatever you want so the snow shows in a different place.
16. Keep making more layers, All set SCREEN, until you are happy with the snow, you may want to go back to some of the earlier layers and add some more blur, Gaussian or motion.
Make the lower layers visible again, and make any add more snow if needed.
Select the sky and make a hue/saturation layer above the original layer. You may have to double click on it and rename it, as you can’t add adjustment layers to a background layer.
Lower the saturation and the light.
Add an other Hue/Saturation layer, and lower the saturation until you get the effect you are after.
If you find you want some colour back, just paint on the mask of the last layer with a soft grey brush.