Snow forms should be taken with a grain of salt so to speak. Pressure and flow cause erosion. In an open truck box, for example, reverse flow of the detached eddy bubble carves just as easily. With snow piled on a flat tonneau or flatbed trailer I've seen carving from turbulent wake as well. Just because snow is missing doesn't make it best for that region to be vacuous. However, in most (all?) places where snow remains "filled in" it seems a good bet that material wants to be there. Of course this is just my opinion. Maybe I'll learn something from someone who disagrees with better backed science.
