I haven't had any experiences at GAdv, but this past season at SFNE I did.
Me, my friend and his family took a trip up to the park on a spring day in April I believe, just after huge rainstorms caused severe flooding throughout the northeast. It was overcast and a bit sprinkly, a perfect day to go. We get to the park 10 minutes after opening and there are seven cars in the parking lot! I had heard about the whole DC superheros area down by the Ct River being flooded, so I knew the area would be closed. Sure enough, the area was closed at both stairways, so you couldn't really see down there. We rode scream so we could check it out, and wow! The entire area beneath the spaghetti bowl of S:RoS, including the queue line, had become part of the river! The water level went up to just at the level of the brake run, and ancompassed the entire twister section of track. It was quite the sight, if only I had my camera! Later when we asked an employee about the situation, they said that water was pouring into the first tunnel, and had pumps running to keep the water out. Fortunately the floods subsided and S:RoS was opened soon afterward!
That day the park was empty to, as I said there were SEVEN cars in the lot. They closed the park at 1 due to the low attendance, but in under 3 hours at the park we rode every ride in the park (aside from Superman of course!) at least once, several of them a few times over. And when we left there were ten cars! What a day...