Storm history of Hurricane Katrina
Tropical Depression Twelve formed over the southeastern Bahamas at 5:00 p.m. EDT (2100 UTC) on August 23, 2005, some of the tropical depression was from the remains of Tropical Depression Ten, which had dissipated from the effects of a nearby upper tropospheric trough. While the normal standards for numbering tropical depressions in the Atlantic shows that the old name/number is kept when a depression dissipates and regenerates, satellite data indicated that a second tropical wave combined with Tropical Depression Ten north of Puerto Rico to form a new, much more advanced system, which was then named as Tropical Depression Twelve. Simultaneously, the trough in the upper troposphere weakened, causing the wind shear in the area to decrease, also allowing the new tropical depression to develop. In a later re-analysis, it was found that the low-level circulation of TD Ten had completely detached and dissipated, with only the remnant mid-level circulation moving on and merging with the second tropical wave mentioned before. As a result, the criteria for keeping the same name and identity were not met.