Living in Minnesota, i can tell you that not being able to train like you do in the summer, year round is a major let down. I personally try to work on my stamina and strength most of the winter. It doesn't take a lot of room to do sit ups, push ups, et cetera.... Also running just to run is very important, i have been slacking off and can feel the effects of my stamina.
i think it is important to remember when training in a winter climate that your body isn't going to forget how to vault a table or a precision a rail in 6 months. If parkour is really about getting through the landscape as quickly, as efficiently and as safely as possible, then practicing parkour in the winter should look nothing like practicing parkour in the summer. A case in point being the ice, running on ice and icy situations can be extremely difficult, and jumping nigh on impossible. I hope this helps, and have a happy couple days off.