GratefulBear, your best bet is to put out small shelters, hidden as well as possible in the wooded area (you don't want to draw attention to these for the cats' safety). I see Katie already gave you some info on making homemade shelters - here's another: www.neighborhoodcats.org/info/wintershelter.htm
It's almost impossible to put a shelter where it won't be affected by snow. Try to face the shelter away from the north, the direction most storms and wind come from. You need to elevate the shelters with rocks or bricks at least 8" or more off the ground. If you can, put two wooden boards leaning toward each other (like this: ^) to make a peaked shaped "entrance" to the shelter. This helps keep some snow away from the entrance. If you place a board directly in front of the entrance (which of course would be better), unfortunately, the cats won't use the shelter because they need to have an unobstructed view out the entrance. They don't feel safe if they can't see out.
When there's lots of snow, you'll have to go to the shelters and dig out paths for them, and make sure the snow hasn't covered the shelter entrances. Also make a kitty "latrine" area for them by clearing snow away from a patch of ground that's not too close to their shelters.