Hurricane Andrew - Meteorological History

Meteorological History

A tropical wave moved off the coast of Africa on August 14. Under the influence of a ridge of high pressure to its north, the wave tracked quickly westward. An area of convection developed along the wave axis to the south of the Cape Verde islands, and on August 15, meteorologists began classifying the system with the Dvorak technique. The thunderstorm activity became more concentrated, and narrow spiral rainbands developed around a developing center of circulation. Based on a Dvorak T-number of 2.0, it is estimated Tropical Depression Three developed late on August 16, while about 1,630 miles (2,620 km) east-southeast of Barbados. Embedded within the deep easterlies, the depression tracked west-northwestward at 20 mph (32 km/h). Initially, moderate wind shear prevented strengthening, though a decrease in shear allowed the depression to intensify into Tropical Storm Andrew at 1200 UTC on August 17.

By early August 18, the storm maintained concentrated convection near the center with spiral bands to its west as the winds increased to 50 mph (80 km/h). Shortly thereafter the thunderstorms decreased markedly during the diurnal minimum and as the storm turned to the northwest, increased southwesterly wind shear from an upper-level low prevented Andrew from maintaining deep convection. On August 19, a Hurricane Hunters flight into the storm failed to locate a well-defined center and on the following day, a flight found that the cyclone had degenerated to the extent that only a diffuse low-level circulation center remained; observations indicated the pressure rose to an unusually high 1,015 mbar (30.0 inHg). The flight indicated Andrew maintained a vigorous circulation aloft, with winds of 80 mph (130 km/h) recorded at flight level. Subsequently, the upper-level low weakened and split into a trough, which decreased the wind shear over the storm. Simultaneously, a strong high pressure cell developed over the southeastern United States, which built eastward and caused Andrew to turn to the west. Convection became more organized as upper-level outflow became better established. An eye formed, and Andrew attained hurricane status early on August 22, while located about 650 miles (1,050 km) east-southeast of Nassau, Bahamas. Six hours after becoming a hurricane, Andrew was predicted to make landfall near Jupiter, Florida with winds of 105 mph (169 km/h).

The hurricane accelerated as it tracked due westward into an area of very favorable conditions, and began to rapidly intensifying late on August 22; in a 24 hour period the atmospheric pressure dropped by 47 mbar (47 hPa; 1.4 inHg) to a minimum of 922 mbar (922 hPa; 27.2 inHg). On August 23, the storm attained Category 5 status on the Saffir-Simpson Hurricane Scale, and at 1800 UTC, Andrew reached peak winds of 175 mph (280 km/h) while located a short distance off Eleuthera island in the Bahamas. Operationally, the National Hurricane Center assessed its peak intensity as 150 mph (240 km/h), which was upgraded to 155 mph (249 km/h) in a post-analysis after the season ended; the hurricane was later re-classified as a Category 5 hurricane. Even with winds of 175 mph (280 km/h), Andrew was a small tropical cyclone, with winds of 35 mph (56 km/h) extending out only about 90 miles (140 km) from its center. After reaching that intensity, the hurricane underwent an eyewall replacement cycle. At 2100 UTC on August 23, Andrew made landfall on Eleuthera with winds of 160 mph (260 km/h). The cyclone weakened further while crossing the Bahama Banks, and at 0100 UTC on August 24, Andrew hit the southern Berry Islands of the Bahamas with winds of 150 mph (240 km/h). As it crossed over the warm waters of the Gulf Stream, the hurricane rapidly re-intensified as the eye decreased in size and its eyewall convection deepened. At 0840 UTC on August 24, Andrew struck Elliott Key with winds of 165 mph (266 km/h) and a pressure of 926 mbar (27.3 inHg). About 25 minutes after its first Florida landfall, Andrew hit just northeast of Homestead with a slightly lower pressure of 922 mbar (922 hPa; 27.2 inHg).

As the eye moved onshore Florida, the convection in the eyewall strengthened owing to increased convergence, and Hurricane Hunters reported a warmer eyewall temperature than two hours prior. However, Andrew weakened as it continued further inland, and after crossing southern Florida in four hours, the storm emerged into the Gulf of Mexico with winds of 135 mph (217 km/h). In the Gulf of Mexico, the eye remained well-defined as the hurricane turned to the west-northwest, a change due to the weakening of the ridge to its north. Andrew steadily re-intensified over the Gulf of Mexico, reaching winds of 145 mph (233 km/h) late on August 25. As the high pressure system to its north weakened, a strong mid-latitude trough approached the area from the northwest. This caused the hurricane to decelerate to the northwest, and winds decreased as Andrew approached the Gulf Coast of the United States. At 0830 UTC on August 26, the cyclone made its final landfall in a sparsely populated area of Louisiana about 20 miles (32 km) west-southwest of Morgan City with winds of 115 mph (185 km/h). Hurricane Andrew weakened rapidly as it turned to the north and northeast, falling to tropical storm intensity within ten hours. After entering Mississippi, the cyclone deteriorated to tropical depression status early on August 27. Accelerating northeastward, the depression began merging with the approaching frontal system, and by midday on August 28, Andrew ceased to meet the qualifications of a tropical cyclone while located over the southern Appalachian Mountains. The remnants continued to the northeast and lost its identity within the frontal zone over the Mid-Atlantic states.