It's been about 10 years since I made a seasons.bgl (and that was the FSX SDK mind you) but as I recall (and as mentioned by others in this thread) snow cover is determined by the seasons.bgl file.  There are 5 seasons (not 4), and one of them is called "Hard Winter".  HW has snow.  The seasons.bgl is independent of what the P3D dialog will tell you (it might say "Spring" but the seasons.bgl for your location may still be coded for hard winter at your location). I'm not sure if anything has