Hurricane Andrew

(Courtesy: NOAA)

Andrew was a small and ferocious Cape Verde hurricane that wrought unprecedented economic devastation along a path through the northwestern Bahamas, the southern Florida peninsula, and south-central Louisiana. Damage in the United States is estimated to be near 25 billion, making Andrew, at its time, the most expensive natural disaster in U.S. history. The tropical cyclone struck southern Dade County, Florida, especially hard, with violent winds and storm surges characteristic of a category 4 hurricane (see addendum on upgrade to category 5) on the Saffir/Simpson Hurricane Scale, and with a central pressure (922 mb) that is the third lowest this century for a hurricane at landfall in the United States. In Dade County alone, the forces of Andrew resulted in 15 deaths and up to one-quarter million people left temporarily homeless. An additional 25 lives were lost in Dade County from the indirect effects of Andrew2. The direct loss of life seems remarkably low considering the destruction caused by this hurricane.

Synoptic History

Satellite pictures and upper-air data indicate that Hurricane Andrew formed from a tropical wave that crossed from the west coast of Africa to the tropical North Atlantic Ocean on 14 August 1992. The wave moved westward at about 20 kt, steered by a swift and deep easterly current on the south side of an area of high pressure. The wave passed to the south of the Cape Verde Islands on the following day. At that point, meteorologists at the National Hurricane Center (NHC) Tropical Satellite Analysis and Forecast (TSAF) unit and the Synoptic Analysis Branch (SAB) of the National Environmental Satellite Data and Information Service (NESDIS) found the wave sufficiently well-organized to begin classifying the intensity of the system using the Dvorak (1984) analysis technique.

Convection subsequently became more focused in a region of cyclonic cloud rotation. Narrow spiral-shaped bands of clouds developed around the center of rotation on 16 August. At 1800 UTC on the 16th (UTC precedes EDT by four hours), both the TSAF unit and SAB calculated a Dvorak T-number of 2.0 and the "best track" (Table 1 and Fig. 1 [85K GIF]) shows that the transition from tropical wave to tropical depression took place at that time.

The depression was initially embedded in an environment of easterly vertical wind shear. By midday on the 17th, however, the shear diminished. The depression grew stronger and, at 1200 UTC 17 August, it became Andrew, the first Atlantic tropical storm of the 1992 hurricane season. The tropical cyclone continued moving rapidly on a heading which turned from west to west-northwest. This course was in the general direction of the Lesser Antilles.

Between the 17th and 20th of August, the tropical storm passed south of the center of the high pressure area over the eastern Atlantic. Steering currents carried Andrew closer to a strong upper-level low pressure system centered about 500 n mi to the east-southeast of Bermuda and to a trough that extended southward from the low for a few hundred miles. These currents gradually changed and Andrew decelerated on a course which became northwesterly. This change in heading spared the Lesser Antilles from an encounter with Andrew. The change in track also brought the tropical storm into an environment of strong southwesterly vertical wind shear and quite high surface pressures to its north. Although the estimated maximum wind speed of Andrew varied little then, a rather remarkable evolution occurred.

Satellite images suggest that Andrew produced deep convection only sporadically for several days, mainly in several bursts of about 12 hours duration. Also, the deep convection did not persist. Instead, it was stripped away from the low-level circulation by the strong southwesterly flow at upper levels. Air Force Reserve unit reconnaissance aircraft investigated Andrew and, on the 20th, found that the cyclone had degenerated to the extent that only a diffuse low-level circulation center remained. Andrew's central pressure rose considerably (Fig. 2 [87K GIF]). Nevertheless, the flight-level data indicated that Andrew retained a vigorous circulation aloft. Wind speeds near 70 kt were measured at an altitude of 1500 ft near a convective band lying to the northeast of the low-level center. Hence, Andrew is estimated on 20 August to have been a tropical storm with 40 kt surface winds and an astonishingly high central pressure of 1015 mb (Figs. 2 and 3 [87K GIF]).

Significant changes in the large-scale environment near and downstream from Andrew began by 21 August. Satellite imagery in the water vapor channel indicated that the low aloft to the east-southeast of Bermuda weakened and split. The bulk of the low opened into a trough which retreated northward. That evolution decreased the vertical wind shear over Andrew. The remainder of the low dropped southward to a position just southwest of Andrew where its circulation enhanced the upper-level outflow over the tropical storm. At the same time, a strong and deep high pressure cell formed near the U.S. southeast coast. A ridge built eastward from the high into the southwestern Atlantic with its axis lying just north of Andrew. The associated steering flow over the tropical storm became easterly. Andrew turned toward the west, accelerated to near 16 kt, and quickly intensified.

Andrew reached hurricane strength on the morning of 22 August, thereby becoming the first Atlantic hurricane to form from a tropical wave in nearly two years. An eye formed that morning and the rate of strengthening increased. Just 36 hours later, Andrew reached the borderline between a category 4 and 5 hurricane (see addendum on upgrade to category 5) and was at its peak intensity (Table 1). From 0000 UTC on the 21st (when Andrew had a barely perceptible low-level center) to 1800 UTC on the 23rd the central pressure had fallen by 92 mb, down to 922 mb. A fall of 72 mb occurred during the last 36 hours of that period and qualifies as rapid deepening (Holliday and Thompson, 1979).

The region of high pressure held steady and drove Andrew nearly due west for two and a half days beginning on the 22nd. Andrew was a category 4 hurricane when its eye passed over northern Eleuthera Island in the Bahamas late on the 23rd and then over the southern Berry Islands in the Bahamas early on the 24th. After leaving the Bahamas, Andrew continued moving westward toward southeast Florida.

Andrew weakened when it passed over the western portion of the Great Bahama Bank and the pressure rose to 941 mb. However, the hurricane rapidly reintensified during the last few hours preceding landfall when it moved over the Straits of Florida. During that period, radar, aircraft and satellite data showed a decreasing eye diameter and strengthening "eyewall" convection. Aircraft and inland surface data Fig. 4 [121K GIF]) suggest that the deepening trend continued up to and slightly inland of the coast. For example, the eye temperature measured by the reconnaissance aircraft was at least 1-2C warmer at 1010 UTC (an hour after the eye made landfall) than it was in the last "fix" about 15 n mi offshore at 0804 UTC. These measurements suggest that the convection in the eyewall, and the associated vertical circulation in the eye and eyewall, became more vigorous as the storm moved onshore. The radar data indicated that the convection in the northern eyewall became enhanced with some strong convective elements rotating around the eyewall in a counter-clockwise fashion as the storm made landfall. Numerical models suggest that some enhancement of convection can occur at landfall due to increased boundary-layer convergence in the eyewall region. That situation appeared to have occurred in Andrew. The enhanced convection in the north eyewall probably resulted in strong subsidence in the eye on the inside edge of the north eyewall. This likely contributed to a displacement of the lowest surface pressure to the north of the geometric center of the "radar eye" (cf., Fig. 4 and 6 [107K JPEG]). It is estimated that the central pressure was 922 mb at landfall near Homestead AFB, Florida at 0905 UTC (5:05 A.M. EDT) 24 August (Fig. 4).

The maximum sustained surface wind speed (1-min average at 10 meters [about 33 ft] elevation) during landfall over Florida is estimated at 125 kt (about 145 mph), with gusts at that elevation to at least 150 kt (about 175 mph). The sustained wind speed corresponds to a category 4 hurricane on the Saffir/Simpson Hurricane Scale (see addendum on upgrade to category 5). It should be noted that these wind speeds are what is estimated to have occurred within the (primarily northern) eyewall in an open environment such as at an airport, at the standard 10-meter height. The wind experienced at other inland sites was subject to complex interactions of the airflow with trees, buildings, and other obstacles in its path. These obstructions create a turbulent, frictional drag that generally reduces the wind speed. However, they can also produce brief, local accelerations of the wind immediately adjacent to the structures. Hence, the wind speed experienced at a given location, such as at a house in the core region of the hurricane, can vary significantly around the structure, and cannot be specified with certainty. The landfall intensity is discussed further in Section b.

Andrew moved nearly due westward when over land and crossed the extreme southern portion of the Florida peninsula in about four hours. Although the hurricane weakened about one category on the Saffir/Simpson Hurricane Scale during the transit over land, and the pressure rose to about 950 mb, Andrew was still a major hurricane when its eyewall passed over the extreme southwestern Florida coast.

The first of two cycles of modest intensification commenced when the eye reached the Gulf of Mexico. Also, the hurricane continued to move at a relatively fast pace while its track gradually turned toward the west-northwest.

When Andrew reached the north-central Gulf of Mexico, the high pressure system to its northeast weakened and a strong mid-latitude trough approached the area from the northwest. Steering currents began to change. Andrew turned toward the northwest and its forward speed decreased to about 8 kt. The hurricane struck a sparsely populated section of the south-central Louisiana coast with category 3 intensity at about 0830 UTC on the 26th. The landfall location is about 20 n mi west-southwest of Morgan City.

Andrew weakened rapidly after landfall, to tropical storm strength in about 10 hours and to depression status 12 hours later. During this weakening phase, the cyclone moved northward and then accelerated northeastward. Andrew and its remnants continued to produce heavy rain that locally exceeded 10 inches near its track (Table 2b). By midday on the 28th, Andrew had begun to merge with a frontal system over the mid-Atlantic states.

-- Provided by NOAA