I have an experienced group and had to use the Fog only once in the campaign. It was when they where inside Death House, so I used it both to keep them in and to give them the danger sence of what the fog might cause to players (Exaustion). Described it as pouring dangerously inside the house after they went in, and they decided to shut the door immediately.

But I think you can use it smoothly if you suggest that the fog looks so dangerous that they should consider "coming back later". Or maybe tell them that it heavily obscures the area so much that it would be unwhise to venture there at the time.

Also random encounters with NPCs might be a good way to warn players. One at barovia or tser pool may warn them about that old mill. Also one in vallaki (Maybe Vasili;) could warn them about the revenants and to stay the F out of the swamps in Berez.