Great question Pete. Obviously, our seasonal forecast of impacts won't just suddenly stop at the Canadian or Mexican border in real life and the models we use are global in nature. However, the mission of the Climate Prediction Center is to provide forecasts for the United States and thus our forecasts follow the borders of the US. 

If you are curious as to seasonal forecasts for Canada, I urge you to check out Environment Canada's (our counterparts to the north) forecasts located here.