So lets go from this.
Ok first open the image you want to use and create a new layer like so.
Next go to Edit-Fill once you have clicked on Fill it will open this box.
Make sure Black is selected then Ok. You should have a completely Black layer.
Next go to Filter-Noise-Add Noise (as below)
A box will openset the amount as shown and make sure Gaussian is selected the OK.
You should end up with something like this.
Next go to Filter-Blur-Blur more.
Now on the top tool bar go to Image-Adjustments-levels
When the box opens set amounts as shown then press OK
Now go to the Layers pallet bottom right and change drop down to Screen as shown below.
Now you see snow but we have not finished yet, it's not very realistic is it?
Go to Filter-Blur-Motion Blur
and when the box opens set these amounts. It gives it some movement.
Making Sure that Layer 1 is selected(showing blue) in the layers pallet go to Layer-Duplicate Layer on the top toolbar.
Now you have twice as much snow, but we don't want them all the same size cos snow isn't like that.
So go to
Edit-Transform-Rotate 180 degrees.
Next go to Filter-Pixelate- Crystallize.
Again a box will open set the cell size to 15 as shown( nice big snow flakes) press Ok
Go to Filter-Blur-Motion Blur as before only set the distance to 16 instead of 8.
Nearly there now.
Go to Layer-Merge Down this will make both snow layers into one layer.
Duplicate Layer 1 again this will brighten the snow you have created.
This may look slightly over the top so lower the opacity in the layers pallet until you have the desired effect.
Finally when you happy flatten layers and your done.
I hope you enjoy trying this and that you like the effect. It takes a while to do but the effect is quite realistic.