Sure you can. whether or not it runs well is another question. There's alot of dust and other funk outside that might hinder good running (keeping track clean and in good order inside is work enough), which is not as apparent with the larger scales.
But, if you are up for it, try it. I wouldn't get too elaborate though. Maybe a simple run. See how it fairs through the seasons. If you look at some of the outdoor layouts you'll notice that alot of the owners live in nice year-round climates. However, I have seen "real" snow scenes, but it was in a larger scale.
IMHO, I wouldn't, not in HO. Although, I would like to have a large scale outdoor layout.