All he had to do is say something like the following in one of the many news conferences that he had:
"I am ordering a mandatory evacuation of the city. Our plan calls for us to bus our citizens to Baton Rouge and other cities out of the strike path; however, our Governor has refused to accept them or open shelters for them. Therefore, I am going before national TV like this to request that the Governor change her mind so that we can save as many of our citizens as possible. In the event that we cannot get everyone out, there are several shelters of last resort in the city..."
Well, you should run for mayor of NO,,,,I'm sure the job will be vacant soon ;) Of course there is nothing to be mayor right now,,,,