Probably the best reason that the warning about the possibility of the New Orleans levees failing was that people hear about new doomsday scenarios all the time. We constantly hear about the possibility of new terrorist attacks within the US. None have taken place since 9/11. Before 9/11, there was the belief that Y2k would make the lights go out. A few ATM's in Britain acted up, and that was it. During the Cold War, we worried about getting nuked.
Global warming is supposed to be an ongoing process, and you can find experts who say it is happening and experts who say it isn't. The planets were supposed to line up and cause the Earth's axis to shift so that one of the poles would be facing the sun and the other would be facing away.
On the other hand, the Earth is supposedly enjoying a period of usually warm weather which is supposed to end soon.
Some disasters are anticitpated in advance, some are predicted but never pan out, and some are prevented by good preperation. Sometimes politicians might claim credit for preventing a disaster that was never really a possibility. Most of the doomsday predictions I hear come from people down on their luck, and they often discribe the chaos to come with a smile on their face, as if they look foward to everyone being brought down to their level, or maybe the opprutunity to loot.
I am going to make a prediction of my own. The next time you turn on the news and you hear a reporter scream "Oh the humanity!", some expert will be on the news later that day saying that he knew that would happen.