I believe that road is open until the National Park boundary, then it's gated off for the winter. I've been up there in the winter and eventually you hit snow deep enough that there's only footprints onward. I meet some guy up there a few years ago and we agreed to pull each other out if we got stuck, we made it like a quarter mile before we both just turned around. It's still fun and worth it, but you're not getting to Mowich Lake unless you do it on foot.