Why just practice over the summer?
The winter is usually wet, and materials are hard to come by.. Yet, this is truely the time to practice. Learn where you can find dry materials. What works for a base, and what doesn't? The coal even ACTS differently in a colder environment.
My suggestion would be to practice like mad NOW. If you can successfully use a bow drill in the winter, I dare say you can do it just about anywhere/time.
One caveat: I must have made 100 attempts before I started one fire. Be prepared for many failures. During this time, pay attention to your base, and how it's made. There in lies the key!
Here is an image I picked up somewhere along the way..