Katrina was a Cat 3 when it hit New Orleans, and Isaac could be up to a 2 by the time it hits. And that is only the wind -- rain is another big piece that can be very damaging.
We'll know after tomorrow...
Edit: Isaac is slow moving (~10mph) and the rain could be up to 20 inches!
http://www.latimes.com/news/nation/n...,2423652.story
That is a scary amount of rain, even if it takes 2 days...