I went back the preserve this evening with my scope so I could better assess the status of the Great Horned Owl nest. As I had thought, it looked as if 80% of the nest was missing. I decided to take a look around the base of the nest tree to see if I could find anything and sure enough, I quickly found a dead chick on the ground along with the rest of the nest.
Judging from the size of the dead chick, I doubt that there were any others. It certainly wasn't big enough to move around. Any other chicks certainly wouldn't have been old enough to survive outside of the nest.
My guess is that the storm from the day before knocked the nest out of the tree. It was quite windy at my house, which is three miles away. I sure it was probably the same there. Also, looking at the nest material that was on the ground, I think the nest was quite old. Many of the sticks were decayed and very weak. I think it was just a structural failure in adverse weather.
It's a sad ending for these owls this season. Since Great Horned Owls only produce one brood a year, these adults are free to roam. I'll certainly miss seeing them on my daily walks.