Hurricane Katrina's Genesis August 2005
Published: June 04, 2008
The following narrative summarizes the initial formation period of Tropical Storm Katrina which intensified to a Category 1 hurricane at landfall near Miami, Florida, and became a Category 5 hurricane in the Gulf of Mexico. This is an excerpt from the Tropical Prediction Center’s Tropical Cyclone Report by Knabb et al (2005)
The complex genesis of Katrina involved the interaction of a tropical wave, the middle tropospheric remnants of Tropical Depression Ten, and an upper tropospheric trough. This trough, located over the western Atlantic and the Bahamas, produced strong westerly shear across Tropical Depression Ten, causing it to degenerate on 14 August approximately 825 n mi east of Barbados. The low-level circulation gradually weakened while continuing westward, and it eventually dissipated on 21 August in the vicinity of Cuba. Meanwhile, a middle tropospheric circulation originating from Tropical Depression Ten lagged behind and passed north of the Leeward Islands on 18-19 August. A tropical wave moved through the Leeward Islands and merged with the middle tropospheric remnants of Tropical Depression Ten on 19 August, forming a large area of showers and thunderstorms north of Puerto Rico. This activity continued to move slowly northwestward, passing north of Hispaniola and then consolidating just east of the Turks and Caicos during the afternoon of 22 August. Dvorak satellite classifications from the Tropical Analysis and Forecast Branch (TAFB) of the Tropical Prediction Center (TPC) began at 1800 UTC that day. The upper tropospheric trough weakened as it moved westward toward Florida, and the shear relaxed enough to allow the system to develop into a tropical depression by 1800 UTC 23 August over the southeastern Bahamas about 175 n mi southeast of Nassau. The depression was designated Tropical Depression Twelve rather than “Ten” because a separate tropical wave appeared to be partially responsible for the cyclogenesis, and, more importantly, the low-level circulation of Tropical Depression Ten was clearly not involved.
The two GOES satellite animations illustrate the evolution of Katrina in the formative stages during the four day period of August 21-24, 2005. The water vapor images show the flow associated with the upper trough. The upper vorticity center within the trough is clearly seen, as it tracks along to the west to the north of the pre-Katrina system. The second satellite animation includes visible daytime images along with the 3.9 micrometer nighttime images. The tropical wave and the mid-level remnants of TD10 can be tracked as it undergoes cyclogenesis and becomes Tropical Storm Katrina.
The images suggest that the active deep convection that produced the cold IR areal maximum near 21N 74W at 0815 UTC 23 August played an important role in Katrina’s genesis. The low-level circulation center that became Katrina’s center either originated or abruptly intensified due to this convection. Fig.1 shows the location of this convective maximum on the 0815 UTC image along with the location of the upper level vorticity center. The “best track” analysis (Knabb et al 2005) has the first Tropical Depression designation at 23.1N 75.1W at 1800 UTC 23 August, and first named storm location at 24.5N 76.5W at 1200 UTC 24 August. Fig. 1 shows those locations along with the track of the pre-existing disturbance (dashed) and the best track positions through the end of the animation periods at 2345 UTC 24 August.
Fig.1. GOES-East 3.9 micrometer images at 0815 UTC 23 August 2005. X marks the center location of the convective maximum. C marks the location of the upper level vorticity center seen in the water vapor images. The best track locations during the 4-day period of 21 August through 24 August are shown by the solid line. The tropical storm symbols mark the first tropical storm (35-kt) intensity and the location at the end of the satellite animations with maximu winds of 45-kt. The dashed line is the track of the pre-existing disturbance center.
Knabb, R.D., J.R. Rhome, and D.P. Brown, 2005: Tropical Cyclone Report, Hurricane Katrina, 23-30, August 2005, National Hurricane Center. Online at http://www.nhc.noaa.gov/2005atlan.shtml