While that is true when it first made landfall in Florida, however, once it was in the Gulf it quickly intensified back to a Cat. 5 before retreating back to a category 3 prior to landfall in La. & then again in Miss.
Katrina weakened to a Category 3 before making landfall along the northern Gulf Coast, first in southeast Louisiana (sustained winds: 125mph) and then made landfall once more along the Mississippi Gulf Coast (sustained winds: 120mph). Katrina finally weakened below hurricane intensity late on August 29th over east central Mississippi.
But remember hurricane Andrew did make landfall as a Cat 5. One of only 3 to ever hit the U.S. since 1900.
And what Category would you rate Katrina at if you based it on Pressure?
Hint, Hint...
“Its minimum pressure at its second landfall was 920 mbar (27 inHg), making Katrina the third-strongest hurricane on record to make landfall on the United States, behind Hurricane Camille’s 900-millibar (27 inHg) reading in 1969, and the 1935 Labor Day Hurricane’s 892-millibar (26.3 inHg) record.”
“Katrina made its second landfall at 6:10 a.m. CDT on August 29 as a Category 3 hurricane with sustained winds of 125 miles per hour (201 km/h) near Buras-Triumph, Louisiana. Because Katrina had just weakened from Category 4 and due to the shape of the coastline, sustained Category 4 winds likely existed on land while the eye was over water. At landfall, hurricane-force winds extended 120 miles (190 kilometres) from the center, the storm’s pressure was 920 millibars (27 inches of mercury), and its forward speed was 15 mph (24 km/h). As it made its way up the eastern Louisiana coastline, most communities in Plaquemines, St. Bernard Parish, and Slidell in St. Tammany Parish were severely damaged by storm surge and the strong winds of the eyewall, which also grazed eastern New Orleans, causing in excess of $1 billion worth of damage to the city (see Effect of Hurricane Katrina on New Orleans).”