You may want to get some sandbags to put against the bottom of the backdoor. Put down a heavy plastic tarp against the bottom of the door and then cover it with sandbags (five or six should do).

I remember Hurricane Donna (1960) and LOTS of trees were uprooted then. It was really sad to see but the amazing thing was the smell of wood as the trees sat in the streets for the first week after the hurricane. I hope that is not repeated with Irene.