Many people living in the South Florida area were unaware when Katrina strengthened from a tropical storm to a hurricane in one day and struck southern Florida on August 25, 2005, near the Miami-Dade - Broward county line. However, National Hurricane Center (NHC) forecasts had correctly predicted the strengthening, however, and hurricane watches and warnings were properly issued nearly 6-8 days, respectively, before hurricane conditions were felt in the area.
Florida Governor Jeb Bush declared a state of emergency on August 24 in advance of Hurricane Katrina's landfall in Florida. Shelters were opened and schools closed in several counties in the southern part of the state. A number of evacuation orders were also issued, mostly voluntary, although a mandatory evacuation was ordered for vulnerable housing in Martin County.
On August 27, after Hurricane Katrina crossed southern Florida and strengthened to a Category 3 storm, President George W. Bush declared a state of emergency in Louisiana, Alabama, and Mississippi two days before the hurricane made landfall.
On August 28, the National Weather Service in Slidell, Louisiana issued a bulletin predicting "devastating" damage rivaling the intensity of Hurricane Camille. Mandatory evacuations were issued for large areas of southeast Louisiana as well as coastal Mississippi and Alabama.
On Sunday, August 28, Canadian National Railway (CN) suspended all rail traffic on its lines south of McComb, Mississippi (lines owned by its subsidiary Illinois Central Railroad that extend into New Orleans), in anticipation of damage from the hurricane. To help ease the resumption of services after the storm passes, CN also issued an embargo with the Association of American Railroads against all deliveries to points south of Osyka, Mississippi. CSX Transportation also suspended service south of Montgomery, Alabama until further notice.
Amtrak, America's rail passenger carrier, announced that the southbound City of New Orleans passenger trains from Chicago, Illinois, from August 29 through September 3, would terminate in Memphis, Tennessee, rather than their usual destination of New Orleans. The corresponding northbound trains would also originate in Memphis. The southbound Crescent from New York City, for the same period, terminated in Atlanta, Georgia, with the corresponding northbound trains originating in Atlanta as well. Amtrak's westbound Sunset Limited originated in San Antonio, Texas, rather than its normal origin point of Orlando, Florida. Amtrak announced that no alternate transportation options would be made available into or out of the affected area.
The Waterford nuclear power plant was also shut down on Sunday, August 28, before Katrina's arrival.
By August 26, the possibility of unprecedented cataclysm was already being considered. Some computer models were putting the city of New Orleans right in the center of their track probabilities, and the chances of a direct hit were forecast at 17% (with strike probability rising to 29% by August 28). This scenario was considered a potential catastrophe because 80% of the New Orleans metropolitan area is below sea level along Lake Pontchartrain. Since the storm surge produced by the hurricane's right-front quadrant (containing the strongest winds) was more than 20 ft (6 m) near Biloxi, emergency management officials in New Orleans feared that the storm surge could go over the tops of levees protecting the city, causing major flooding. This risk of devastation had been known for some time; previous studies by FEMA and the Army Corps of Engineers had warned that a direct hurricane strike on New Orleans could lead to massive flooding, which would lead to thousands of drowning deaths, as well as many more suffering from disease and dehydration, as the flood waters slowly receded from the city.
At a news conference 10:00 AM on August 28, shortly after Katrina was upgraded to a Category 5 storm, New Orleans mayor Ray Nagin ordered the first ever mandatory evacuation of the city, calling Katrina, "a storm that most of us have long feared". To speed up the evacuations, authorities used contraflow lane reversal on Interstate 10 leading west of New Orleans, as well as on Interstate 55 and 59 leading north from the city. The city government also established a "refuge of last resort" for citizens who could not leave the city, at the massive Louisiana Superdome, which housed approximately 26,000 people with food and water for two days as the storm came ashore. The Louisiana National Guard delivered three truckloads of water and seven truckloads of MRE's to the Superdome, enough to supply 15,000 people for three days.
Louisiana's hurricane evacuation plan calls for local governments in areas along and near the coast to call for evacuations in three phases, starting with the immediate coast 50 hours before the start of tropical storm force winds. Persons in areas designated Phase II begin evacuating 40 hours before the onset of tropical storm winds and those in Phase III areas (including New Orleans) evacuate 30 hours before the start of such winds.
However, many parishes were not able to provide sufficient transportation for citizens who did not have private means of evacuation, and many private care-taking facilities who relied on the same bus companies and ambulance services for evacuation were unable to evacuate their charges. Fuel and rental cars were in short supply and many forms of public transportation had been shut down well before the storm arrived. The end result was that hundreds of thousands of residents and tourists were unable to evacuate and remained in the city. Nonetheless, some estimates claimed that 90-92% of the 1.3 million residents of the New Orleans metropolitan region evacuated including 80% of Orleans parish. More than 80,000 people were homeless.