I don't know that I believe God way trying to purge New Orleans, per se. You could argue that He didn't do a good job if that was the only reason for Katrina, because Mardi Gras and everything else will undoubtedly return once the city is rebuilt.
I do, however, believe that God can and does use natural disasters to bring about repentance and to show his love, mercy and power. The book of Revelation is full of examples of God using earthquakes, hail, etc. to judge the world while at the same time giving people a chance to turn to Himself.
On the one hand, disasters like this are a result of sin, so you could say that they could always be classified as a punishment from God. But on the other hand, I think it might be a mistake to say that all natural disasters are a result of God stepping into nature and deliberately sending bad storms our way.
Does that make sense? As I reread it, I can see how it could be construed as a contradiction, but do you understand what I am trying to say?