Harry and Snape are snowed in together.  Or iced in, or it's so windy that it would be dangerous to go out becuase tree limbs are falling, etc.

I imagine they're in a tiny shack somewhere maybe not too far into the Forbidden Forest.  They could be somewhere else.  How and why they ended up out there is up to you.  The point is, they can't leave for a few hours, or if it was a big snowstorm lets say, perhaps a few days.  They're stuck together in a tiny shack or room with no place to go to get some space from each other.