When I had a separate coop, I weatherized the windows on the sides because the roof has plenty of ventilation. The thing to be most concerned about is drafts in the winter and of snow blowing and getting inside. I suspect being in Springs proper, it should be much easier to keep the massive wind gusts that I get all the time out here on the eastern plain in the fall/winter. I gerry rigged up a heavy duty commercial outdoor fabric curtain that I could encase the coop in during the blizzards. During the day, you might have to find a place for them to hang out as most backyard coops are rather smallish during the wicked blizzards. On the awesome hand though... the bad weather isn't that often.
Just remember, if you have to choose ventilation of warmth, always go for the ventilation, not drafts though. If your good at building on, you could build on a small section to the front where the trap door is with just inexpensive plastic stapled to a cheap frame to help keep the snow from going into the coop and also to provide them an area during the day a little warmer to hang out in.