If there is much snow coyotes will prey heavily on adult deer during the winter. If there is no more than a foot or so of snow the deer can often run away from the coyotes.
To a lesser extent, very cold weather affects the situation too. When it is very cold, say -20, deer seem to act stupid and coyotes can get them even on bare ground.
Deer can handle a lot of snow if there are no coyotes to take advantage of the situation. Before coyotes came to the northeast we had deep snow winters but still had a lot of deer. Now we only see a slight rebound in deer populations after a couple of winters with low snow depth.