Snow white - but snow weight? (How much on roof)

Unless the snow is very wet when it's coming down, you should not have a problem. The issues start when that snow is compacted by the next snowfall and then that is compacted by the next, and so forth.... With snow, 1' plus 1' is usually not 2' so estimating how much snow and how much weight you really have up there is difficult. Also, once you determine that you need to get up there and get it off, your added weight can cause the problems you were trying to avoid.