Finally, we want to add a breakup mask that blends down onto our mesh. This gives some organic breakup across the surface and makes the snow buildup feel more natural and. In this case, I am using the AO map, and a power parameter to tighten up the AO and overlay it on the snow mask. I chose to overlay the breakup mask so once the the snow coverage gets to a white value, the breakup map is no longer being applied to the snow mask. This allows me to really push the snow coverage to high values and get a fully covered snowy surface, but while the coverage is low, the breakup mask is applied.

Additional tip:

Sometimes it helps to be able to visualize your snow mask in 3D space in your viewport. I like to add a switch that inputs the black and white snow mask into the albedo slot.