Hurricane Andrew Monthly Weather Review Article

Hurricane Andrew
Monthly Weather Review Article

Written by Max Mayfield, Lixion Avila, and Edward N. Rappaport
National Hurricane Center (NHC)

Excerpts courtesy of the
Monthly Weather Review

The complete article can be found in Monthly Weather Review a scientific journal of the American Meteorological Society using this reference:

Mayfield, M, L. A. Avila, and E. N. Rappaport, 1994: Atlantic hurricane season of 1992. Mon. Wea. Rev., 122, 517-538.

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 10 m/s, 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 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 16 August. At 1800 UTC 16 August, both the TSAF unit and SAB calculated a Dvorak T number of 2.0, and 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 16 August, however, the shear diminished. The depression grew stronger and at 1200 UTC 17 August it became Andrew, the first tropical Atlantic tropical storm of the 1992 hurricane season. The tropical cyclone continued moving rapidly on a heading that turned from west to west-northwest. This course was in the general direction of the Lesser Antilles.

Between 17 and 20 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 900 km to the east-southeast of Bermuda and to a trough that extended southward from the low for several hundred kilometers. These currents gradually changed and Andrew decelerated on a course that 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-h duration. Also, the deep convection did not persist. Instead, it was stripped away from the low-level circulation by the strong south-westerly flow at upper levels. Air Force Reserve unit reconnaissance aircraft investigated Andrew and on 20 August found that the cyclone had degenerated to the extent that only a diffuse low-level circulation center remained. Andrew's central pressure rose considerably. Nevertheless, the flight-level data indicated that Andrew retained a vigorous circulation aloft. Wind speeds near 36 m/s were measured at an altitude of 500 m 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 21 m/s surface winds and an astonishingly high central pressure of 1015 mb.

Significant changes in the large-scale environment near and downstream from Andrew began by 21 August. Satellite imagery in a 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 that 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 United States southeast coast. A ridge built eastward from the high into the southwestern North 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 8 m/s, 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 h later, Andrew reached the borderline between a category 4 and 5 hurricane and was at its peak intensity. From 0000 UTC 21 August (when Andrew had a barely perceptible low-level center) to 1800 UTC 23 August the central pressure had fallen by 92 mb, down to 922 mb. A fall of 72 mb occurred during the last 36 h of that period and qualifies as rapid deepening (Holiday and Thompson 1979).

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

Andrew weakened when it passed over the western portion of the Great Bahama Bank, and the central pressure rose to 941 mb. However, the hurricane rapidly re-intensified during the last few hours preceding landfall on Florida as it moved over the warm 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 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°-2°C warmer at 1010 UTC (an hour after the eye made landfall) than it was at the last "fix" about 25-30 km offshore at 0804 UTC. These measurements suggest that the convection in the eyewall and the associated vertical circulation in the eye and the 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 counterclockwise 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 (e.g., Tuleya and Kunhara 1978; Tuleya et al. 1984; Jones 1987). That situation appeared to have occured 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" by approximately 5 km in the vicinity of Homestead Air Force Base, Florida. It is estimated that the central pressure was 922 mb at landfall near Homestead at 0905 UTC 24 August.

The maximum sustained surface wind speed (1-min average at 10 m elevation) during landfall over Florida is estimated at 64 m/s, with gusts at that elevation to at least 77 m/s. The sustained wind speed corresponds to a category 4 hurricane on the Saffir-Simpson hurricane scale. 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-m height. The wind experienced at o ther inland sites was subject to complex interactions of 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.

Andrew moved nearly due west when over land and crossed the extreme southern portion of the Florida peninsula in about 4 h. 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.

As Andrew reached the north-central Gulf of Mexico, the high pressure system to its northeast weakened and a strong midlatitude trough approached the area from the northwest. Steering currents began to change, Andrew turned toward the northwest and its forward speed decreased to about 4 m/s. The hurricane struck a sparsely populated section of the south-central Louisiana coast with category 3 intensity at about 0830 UTC 26 August. the landfall location is about 35 km west-southwest of Morgan City, Louisiana.

Andrew weakened rapidly after landfall, to tropical storm strength in about 10 h and to depression status 12 h 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 250 mm near its track. By midday on 28 August, Andrew had begun to merge with a frontal system over the mid-Atlantic states.

Meteorological Statistics

All data about the storm was derived from either analysis of satellite images performed by the TSAF unit, SAB, and the Air Force Global Weather Central or from reconnaissance flights by the U.S. Air Force Reserve unit based at Keesler Air Force Base, Mississippi. Additional data were collected onboard a National Oceanic and Atmospheric Administration (NOAA) aircraft.

The anemometer at Harbour Island, near the northern end of Eleuthera Island in the Bahamas, measured a wind speed of 62 m/s for an unknown period shortly after 2100 UTC 23 August. That wind speed was the maximum that could be registered by the instrument. A higher speed may have occurred at a later time.

Neither of the two conventional measures of hurricane intensity, central barometric pressure, and maximum sustained wind speed were observed at official surface weather stations in close proximity to Andrew at landfall in Florida. Homestead Air Force Base and Tamiami Airport discontinued routine meteorological observations prior to receiving direct hits from the hurricane. Miami International Airport was the next closest station but it was outside the eyewall by about 10 km when Andrew's center passed to its south.

To supplement the official information, requests for data were made to the public through the local media. Remarkably, more than 100 quantitative observations have been recieved so far. Many of the reports came from observers who vigilantly took readings through frightening conditions, including --- in several instances --- the moments when their instruments and even their homes were destroyed.

Some of the unofficial observations were dismissed as unrealistic. Others were rendered suspect or eliminated during follow-up inquiries or analyses. The remainder, however, revealed a physically consistent and reasonable pattern.

  1. Minimum pressure over Florida

    The final offshore "fix" by the reconnaissance aircraft came at 0804 UTC and placed the center of the hurricane only 25-30 km, or roughly 1 h of travel time from the mainland. A dropsonde indicated a pressure of 932 mb at that time. The pressure had been falling at the rate of about 2 mb/h but the increasing interaction with land was expected to at least partially offset, if not reverse, that trend. Hence, a landfall pressure within a few millibars of 932 mb seemed reasonable.

    Shortly after Andrew's passage, however, reports of minimum pressures below 930 mb were received from the vicinity of Homestead. Several of the barometers displaying the lowest pressures were subsequently tested in a pressure chamber and calibrated by the Aircraft Operations Center (AOC) of NOAA. The lowest accepted pressure in the AOC tests is 922 mb. Based on the observations and an eastward extrapolation of the pressure pattern to the coastline. Andrew's minimum pressure at landfall is estimated to be 922 mb. This suggests that the trajectory of the dropsonde from the aircraft did not intersect the lowest pressure within the eye.

    In any case, in the United States, this century, only the Labor Day (Keys') storm in 1935 (892 mb) and Hurricane Camille in 1969 (909 mb) had lower landfall central pressures than Andrew (Herbert et al. 1993).

  2. Maximum wind speed over Florida

    The strongest winds associated with Andrew on 24 August likely occurred in the hurricane's northern eyewall. The relatively limited number of observations in that area greatly complicates the task of establishing Andrew's maximum sustained wind speed and peak gust at landfall in Florida. While a universally accepted value for Andrew's wind speed at landfall may prove elusive, there is considerable evidence supporting an estimate of about 64 m/s for the maximum sustained wind speed, with gusts to at least 77 m/s.

    The strongest reported sustained wind near the surface occurred at the Fowey Rocks weather station at 0800 UTC. The station sits about 20 km east of the shorline and, at that time, was within the northwest part of Andrew's eyewall. The 0800 UTC data included a 2-min wind of 63 m/s with a gust to 76 at a platform height of about 40 m. The U.S. National Data Buoy Center (NDBC) used a boundary-layer model to convert the sustained wind to a 2-min wind of 56 m/s at 10 m elevation. The peak 1-min wind during that 2-min period at Fowey Rocks might have been slightly higher than 56 m/s.

    It is unlikely that this point observation was so fortuitously situated that it represents a sampling of the strongest wind. The Fowey Rocks log indicates that the wind speed increased through 0800 UTC. Unfortunately, Fowey Rocks then ceased transmitting data, presumably because even stronger winds disabled the instrumentation. A subsequent visual inspection indicated that the mast supporting the anenometer had become bent (90° from vertical.) Radar reflectivity data suggests that the most intense portion of Andrew's eyewall had not reached Fowey's Rocks by 0800 UTC and that the wind speed could have continued to increase there for another 15-30 min. A similar conclusion can be reached from the pressure analysis, which indicates that the pressure at Fowey Rocks probably fell by about another 20 mb from the 0800 UTC mark of 968 mb.

    Reconnaissance aircraft provided wind data at a flight level of about 3000 m. The maximum wind speed along 10 s of flight track (often used by the NHC to represent a 1-min wind speed at flight level) on the last pass prior to landfall was 83 m/s, with an instantaneous wind speed of 87 m/s observed. The 83 m/s wind occurred at 0810 UTC in the eyewall region about 19 km to the north of the center of the eye. Like the observations from Fowey Rocks, the aircraft provided a series of point observations (i.e., no lateral extent). Almost assuredly, somewhat higher wind speeds occurred elsewhere in the northern eyewall, a little to the left and/or to the right of the flight track. A wind speed at 3000 m is usually reduced to obtain a surface wind estimate.

    Based on past operational procedures, the 83 m/s flight level wind is compatible with maximum sustained surface winds of 64 m/s.

    One of the most important wind speed reports came from Tamiami Airport, located about 17 km west of the shoreline. As mentioned earlier, routine weather observations ended at the airport before the full force of Andrew's (northern) eyewall winds arrived.

    However, the official weather observer there, S. Morrison, remianed on station and contined to watch the wind speed dial. Morrison notes that at approximately 0848 UTC the wind speed indicator "pegged" at a position a little beyond the dial's highest marking of 51 m/s, at a point that he estimates corresponds to about 57 m/s. He recounts that the needle was essentially fixed at that spot for 3-5 min and then fell back to 0 when the anemometer failed. Morrison's observations have been closely corroborated by two other people. He has also noted that the weather conditions deteriorated even further after that time and were at their worst no sooner than 15 min later. This information suggests that, in all likelihood, the maximum sustained wind speed at Tamiami Airport significantly exceeded 57 m/s.

    A number of the wind speeds reported by the public could not be substantiated and have therefore been excluded. The reliability of some of the others suffer from problems that include nonstandard averaging periods and instrument exposures, and equipment failure prior to the arrival of the strongest winds.

    The only measurement of a sustained wind in the southern eyewall came from an anemometer on the mast of a sailboat. For at least 13 min the anemometer there showed 51 m/s, which was the maximum that the readout could display. A small downward adjustment of the speed should probably be applied because the instrument was sitting 17 m above the surface rather than at the standard height of 10 m. On the other hand, the highest 1-min wind speed during that 13-min period could have been quite a bit stronger than 51 m/s. Again, there may have been stronger winds elsewhere in the southern eyewall. Bear in mind that (to a first approximation for a west-ward moving hurricane) the wind speed in the northern eyewall usually exceeds the wind speed in the southern eyewall by about twice the forward speed of the hurricane (Dunn and Miller 1964). In the case of Andrew, that difference is about 16 m/s stronger than 67 m/s in the northern eyewall.

    Several indirect measures of the sustained wind speed are of interest. First, a standard empirical relationship between central pressure and wind speed (Kraft 1961) applied to 922 mb yields around 69 m/s. Second, the Dvorak technique classification performed by the NHC TSAF unit using a 0900 UTC satellite image gives 65 m/s. Also, an analysis of the pressure pattern gives a maximum gradient wind of around 72 m/s.

    The strongest, credible wind gust reported from near the surface occurred in the northern eyewall about 2 km from the shoreline at the home of R. Fairbank. He observed a gust of 95 m/s on a digital readout a few moments before a portion of a wall of his home collapsed, which prevented further observation. The hurricane also destroyed the anemometer. An inspection was made of the anemometer height and exposure, which verified that the instrument was at the 10-m level and it had an open exposure to the east, which was the direction of the wind at the time of failure. To evaluate the accuracy of the instrument, three anemometers of the type used by Fairbank were tested in a wind tunnel at Virginia Polytechnic Institute and State University. Although the turbulent nature of the hurricane winds could not be replicated, the results of the wind tunnel tests suggest that the actual wind at the time Fairbanks read 95 m/s on his digital readout was 79 m/s. Of course, stronger gusts may have occured there at a later time.

    Strong winds also occurred outside of the eyewall, especially in association with convective bands. A peak gust to 72 m/s was observed at a home near the northern end of Dade County on an anemometer of the brand used by Fairbank. Applying the reduction suggested by the wind tunnel tests to 72 m/s yields an estimate of 60 m/s. This is nearly identical to the 59 m/s peak gust (a 5-s average) registered on a National Ocean Survey anemometer located not far to the east, at the coast.

  3. Storm surge

    Andrew came onshore in the northwest Bahamas and southeast Florida near high tide (+0.6-0.8 m) and was accompanied by a locally huge storm surge. The surge at The Current (a settlement near the northern end of Eleuthera Island) reached a phenomenal 7 m.

    The 5.2-m storm tide that headed inland from Biscayne Bay is a record maximum for the southeast Florida peninsula. The maximum storm tide on the Florida southwest coast is estimated at 1.5-2.1 m. Storm tides in Louisiana were at least 2.4 m and caused flooding from Lake Borgne through Vermillion Bay.

  4. Tornadoes

    There have been no confirmed reports of tornadoes associated with Andrew over the Bahamas or Florida. However, funnel sightings, some unconfirmed, were reported in the Florida counties of Glades, Collier, and Highlands, where Andrew crossed in daylight.

    In Louisiana, one tornado occurred in the city of Laplace several hours prior to Andrew's landfall. That tornado killed two people and injured 32 others. Tornadoes in the Ascension, Iberville, Baton Rouge, Pointe Coupee, and Avoyelles parishes of Louisiana reportedly did not result in casualties. Numerous reports of funnel clouds were received by officials in Mississippi, and tornadoes were suspected to have caused damage in several Mississippi counties. In Alabama, the occurrence of two damaging tornadoes has been confirmed over the mainland, while another tornado may have hit Dauphin Island. As Andrew and its remnants moved northeastward over the eastern states, it continued to produce severe weather. For example, several damaging tornadoes in Georgia late on 27 August were attributed to Andrew.

  5. Rainfall

    Andrew dropped sufficient rain to cause local floods even though the hurricane was relatively small and generally moved rather fast. Rainfall totals in excess of 175 mm were recorded in southeast Florida, Louisiana, and Mississippi. Rainfall amounts near 125 mm occurred in several neighboring states. Hammond, Louisiana reported the highest total, near 303 mm.

Casualty and Damage Statistics

The number of deaths directly attributed to Andrew is 26. The additional indirect loss of life brought the death toll to 62 as of 7 September 1992. A combination of good hurricane preparedness and evacuation programs likely helped minimize the loss of life. Nevertheless, the fact that no lives were lost in the United States due to storm surge is viewed as a fortunate aberration.

Unfortunately, the count of indirect deaths has increased since 7 September. The Miami Herald reported on 31 January 1993 that it could relate at least 43 additional (indirect) deaths in Dade County to Hurricane Andrew.

Damage is estimated at $20-$25 billion. Andrew's impact on southern Dade County was extreme from the Kendall district southward through Homestead and Florida City to near Key Largo. Andrew reportedly destroyed 25 524 homes and damaged 101 241 others. The Dade County Grand Jury reported that 90% of all mobile homes in south Dade County were totally destroyed. In Homestead, more than 99% (1167 of 1176) of all mobile homes were completely destroyed. The Miami Herald reported $0.5 billion in losses to boats in southeast Florida. The damage to Louisiana is estimated at $1 billion. Damage in the Bahamas has been estimated at $0.25 billion.

Andrew whipped up powerful seas that extensively damaged many offshore structures, including the artificial reef system of southeast Florida. For example, the Belzona Barge is a 66-m, 300 000-kg barge that, prior to Andrew, was sitting in 21 m of water on the ocean floor. About 900 000 kg of concrete from the Card Sound bridge lay on the deck. The hurricane moved the barge about 200 m to the west (50 000-100 000 kg of concrete remain on deck) and removed several large sections of steel plate sidings.

Damage in the Gulf of Mexico is preliminarily estimated at $0.5 billion. Ocean Oil reported the following in the Gulf of Mexico: 13 toppled platforms, 5 leaning platforms, 21 toppled satellite structures, 23 leaning satellites, 104 incidents of structural damage, 7 incidents of pollution, 2 fires, and 5 drilling wells blown off location.

The most devastated areas correspond closely in location to the region overspread by Andrew's eyewall and its accompanying core of destructive winds and, near the coastline, decimating storm surges. Flight-level data about an hour prior to landfall place the radius of maximum wind at the surface was likely little less than 20 km. Areas of southern Florida well outside the eyewall experienced less severe damage and fewer casualties. More than one-half of the fatalities were indirect. Many of the indirect deaths occurred during the "recovery phase" following Andrew's passage.

Hurricanes are notoriously capricious. Andrew was a compact system. A little larger system, or one making landfall just a few kilometers farther to the north, would have been catastrophic for heavily populated, highly commercialized, and no less vulnerable areas to the north. That area includes downtown Miami, Miami Beach, Key Biscayne, and Fort Lauderdale. Andrew also left the highly vulnerable New Orleans region relatively unscathed.