Looks like he was warned three days earlier of the threat. From the New York Times:
"The White House was told in the hours before Hurricane Katrina hit New Orleans that the city would probably soon be inundated with floodwater, forcing the long-term relocation of hundreds of thousands of people, documents to be released Tuesday by Senate investigators show."
Documents show that a Homeland Security Department report landed at the White House shortly before 2:00 am, August 29. It read in part:
"Any storm rated Category 4 or greater will likely lead to severe flooding and/or levee breaching."
The Times reports other documents scheduled for release today show DHS leaders warned the huricane's impact would be worse than "a doomsday-like emergency planning exercise conducted in Louisiana in July 2004."