Will trees, limbs, and branches break from the snow?
The risk is high when heavy, wet snow accumulates on the tree, the snow has a snow-water equivalent ratio of between 6:1 and 12:1, and weighs in excess of 10 pounds per square foot. This risk is greater when the trees have full leaves or are not adapted to snow. Light powdery stuff is much less likely to cause problems. Source: http://en.wikipedia.org/wiki/Snow