A second period of rapid intensification started by 7:00 p.m. CDT on August 27 , and by 12:40 a.m. CDT on August 28 , Katrina was upgraded to a Category 4 hurricane with maximum sustained winds of 145 mph ( 233 km\/h ) .
The storm then began a second period of rapid intensification starting at 7:00 p.m. CDT on August 27 , and by 12:40 a.m. CDT on August 28 , Katrina was upgraded to a Category 4 hurricane with maximum sustained winds of 145 mph ( 233 km\/h ) .