ePodunk has a neat map that shows where Katrina survivors went just before and after the storm:
Hurricane Katrina blew down or flooded the homes of about 3.2 million people along the central Gulf Coast, including 1.3 million in metropolitan New Orleans and 250,000 in Gulfport and Biloxi. A mandatory evacuation order was issued for New Orleans, and authorities scrambled to provide emergency shelter for about 500,000 Americans. Perhaps an equal or greater number were staying with relatives or friends. Katrina caused the biggest mass migration in U.S. history, surpassing the 1927 flood of the Mississippi River. In terms of numbers permanently displaced, the only event that might have been bigger than Katrina is the Civil War.