Intense winter sun warms up the tree trunk, especially on the southwest side. The sun warms the bark and apparently the living tissue called cambium is fooled into waking up. When the temperature drops again suddenly, this tissue dies and we notice dry, ridged areas or cracks. While this doesn’t kill the tree, the area is weakened and the tree is more susceptible to disease.
What can you do to prevent sunscald? Purchase pieces of white tree wrap and cover the trunk, especially the lower southwest side. You can do this even now and remove the wrap once the snow is gone.
For more info and photos, check out this link.