Outdoor sessions are great year round but when it comes to a session in the winter there are a couple things to consider.  

Temperature: 

This is probably the most important thing to think about with a winter outdoor session.  Your session might be -3 or it might turn -20 by the time we get to your session date.  It is best to be flexible with your time when planning a winter session.  Once we get to that -15/-20 window it if often not ideal for a portrait session.  It will really depend on the breed of your dog, whether it enjoys winter and if you have any younger children in your family.  Although I keep winter sessions short it is still best to be flexible with your time 

Clothing:

Just like when you are planning for sessions in the summer or fall you will need to plan your clothing.  Whether that is coordinating sweaters or winter coats it is always best plan a look that keeps you warm for the temperature and will look great in your portrait.