President Bush declared Katrina to be a national emergency on August 27, 2005.

http://web.mit.edu/12.000/www/m2010/finalwebsite/katrina/gov...

> Before Hurricane Katrina made landfall, Governor Katherine Babineaux Blanco declared a state of emergency in Louisiana on August 26, 2005, and asked President Bush to do the same at the federal level the next day, a request with which he complied. This authorized FEMA to organize and mobilize resources as it saw fit to help the residents of New Orleans (Office of the Press Secretary 2005).