Of the 831 tropical storms and hurricanes that formed in the Atlantic Basin during the period 1900 to 1996, about 40% made landfall on or grazed the United States coastline. Of these hurricanes, few directly impacted heavily populated areas. But among those few are some of the greatest weather-related tragedies ever recorded. One example is the powerful hurricane that struck the city of Galveston, TX on September 8, 1900. On a long, narrow barrier island without any sea wall to protect it, Galveston is vulnerable to catastrophe. A massive storm surge pummeled the isolated city into rubble, taking over 6000 lives in the process. Nearly a century later, the real dangers of a hurricane are often underestimated and misunderstood by coastal residents. The August 1992 landfall of an intense category 4 hurricane named Andrew in southern Florida served as a sobering reminder of the real threat that hurricanes can pose to society. It also provided an unprecedented laboratory for meteorologists to study a hurricane up-close. Here we tell the story of Andrew, a story from which many lessons can be learned.
During its first few days as a named storm, Andrew struggled for survival as it encountered high-level westerly winds that ripped apart its thunderstorms. On August 21, Andrew's central pressure rose to 1015 mb - a value akin to some summertime high pressure systems. Andrew was on life-support, barely a tropical storm. However, shifting 500-mb patterns in the mid-latitudes rapidly reversed, a remedy for Andrew's declining health. The more uniform easterly wind flow was orchestrated by a strong high pressure system that was establishing a foothold off the southeast coast of the United States. Coaxed by easterly steering winds, Andrew began to move westward. With unidirectional winds aloft, warm seas below, and an engine that was starting to turn over, Andrew roared to life, maturing from a wobbly tropical storm to a category 4 hurricane in only 36 hours. By 18Z on August 23, the storm reached its maximum strength, with a central pressure of 922 mb (see Figure 11.26b). With the 500- mb high pressure system anchored off the southeast coast and in charge of steering, Andrew took direct aim at southern Florida.
Hurricane Andrew moved rapidly across the Bahamas and into southern Florida near Homestead in the pre-dawn hours of August 24. Color Plate 43 is a color-enhanced infrared satellite image of Andrew as it made landfall into the Florida peninsula around 830Z on August 24. Its point of landfall was 37 km (23 mi) south of downtown Miami, just far enough away to spare the city the full fury of the eye wall. However, southern Dade County, FL, in the vicinity of the eye wall, was devastated by sustained winds of 225 km/hr (140 mph) with gusts of 266 km/hr (165 mph) and an accompanying storm surge of over 5 meters (16 ft). The passage of the eye wall also inflicted considerable wind and storm surge damage on the west coast of southern Florida, where sensitive Everglade wetlands experienced storm surges of approximately 2.7 meters (9 ft) south of the location of the exiting eye. However, because of Andrew's fast pace of 30 km/hr (19 mph), only 5 to 10 cm (2 to 4 in) of rain fell over the region.
Once Hurricane Andrew moved into the Gulf of Mexico, the 500-mb pattern in the mid-latitudes shifted again, as the previously dominant high off the southeast coast weakened and an upper-level trough of low pressure moved eastward across the Central Plains states. This change in upper-air regimes weakened the steering of Hurricane Andrew as it turned northward toward the Louisiana coast, causing the storm to slow to 16 km/hr (10 mph). These stalling tactics caused Andrew to weaken as part of its circulation stayed over land. But the slowdown also allowed the hurricane to dump over 25.4 cm (10 in) of rain on portions of eastern Louisiana and southern Mississippi. In the pre-dawn hours of August 26, Andrew made a second landfall over a sparsely populated section of south-central Louisiana. Subsequently, Andrew merged with an approaching upper-level trough, and pivoted northeastward, its decaying but still formidable low-level circulation spawning a swath of gusty winds and some tornadoes from the lower Mississippi Valley into north-central Maryland.
In the aftermath of Hurricane Andrew, analyses of official and public observations over southern Florida revealed that the storm had made landfall with a minimum pressure of 922 mb (see Figure 11.26b). This made Andrew the third most intense hurricane to make landfall in the United States this century (behind the 1935 Labor Day Florida Keys storm (892 mb) and Hurricane Camille in 1969 (909 mb)). Andrew actually intensified slightly in the hour preceding landfall, attributed, in part, to low-level convergence related to enhanced surface friction as the storm neared land. This helped to augment updrafts in eye-wall thunderstorms. In the path of the energized eye wall, some 2000 localized streaks of damage (some well inland) were attributed to these areas of intense thunderstorms.
Color Plate 44 shows the last complete sweep of the National Weather Service's radar, located at the National Hurricane Center (labeled NHC), before the radar was blown off the roof by storm winds. The eye wall is indicated by the doughnut-shaped pink and burgundy region surrounding Sands Key and Elliott Key. The westward projection of this doughnut closely matches the areas that experienced the strongest wind speeds, highest storm surge and greatest damage during the storm.
Evidence of the raw power of the hurricane was everywhere. Color Plate 45 shows two trees that snapped in the category 4 winds. It is likely that one was blown down by the winds in the forward portion of the eye wall and the other succumbed to winds blowing in the opposite direction in the rear portion of the eye wall. Color Plate 46 dramatically shows the danger of debris driven by 225-km/hr (140 mph) winds.
In the months following Hurricane Andrew, many in southern Florida attributed the massive devastation to poor construction practices. However, according to Dr. Sheets, shoddy construction explained only about 5 to 10% of the damage. Much of the damage occurred because windows were not securely fashioned with special storm covers. This allowed wind and water to penetrate the interior of structures, in many cases leading to their collapse.
The evacuation process along the United States coastline focuses on getting people away from water. The National Hurricane Center has worked closely with local communities in developing evacuation zones for different Saffir-Simpson category hurricanes, using output from a computer model called the Sea, Lake and Overland Surge for Hurricanes (SLOSH). This model provides guidance in predicting which coastal areas will be affected by a storm surge. Because storm surge height depends strongly upon the topography of the coastline and the adjacent ocean bottom, there can be dramatic local variation between storms of equal intensity. For example, the SLOSH model predicts that a storm similar in strength to Hurricane Hugo would produce a storm surge of 2.7 meters (9 ft) on the east coast of southern Florida but more than 9 meters (30 ft) over the Florida panhandle.
Andrew was almost a "nightmare storm" for New Orleans, according to Dr. Sheets. Indeed, New Orleans is the most vulnerable city in the United States to storm surge damage because it is the Death Valley of the Gulf Coast: parts of the city lie below sea level. In addition, New Orleans is virtually surrounded by water, with the mighty Mississippi to the south, Lake Pontchartrain to the north, and Lake Borgne to the east. Scattered marshes, bayous, and smaller lakes complete the lineup. A massive system of levees protects New Orleans, but they do not form a watertight perimeter. A direct hurricane impact on the city would overwhelm the intricate system of levees and pumps that keep the city dry. And, like Galveston, access to the city could easily be severed hours before the eye wall and storm surge strike. During Hurricane Andrew, only an estimated 9% of the population of New Orleans heeded evacuation orders. Had Hurricane Andrew made landfall only 140 km (87 mi) to the east, New Orleans would have been in the eye wall and the number of deaths would likely have soared.
The National Hurricane Center alerts communities about the threat of hurricanes in two ways. The first is a hurricane watch, which is issued for a coastal area when there is a threat of hurricane conditions within 24 to 36 hours. The second is a hurricane warning, which is issued when hurricane conditions are expected over a coastal area in 24 hours or less. Watches and warnings are issued in a similar fashion for tropical storms. When issuing these watches and warnings, the NHC must consider conflicting societal impacts. Enough time must be given to evacuate densely populated areas, especially on barrier islands or other locations where evacuation routes can easily be cut off by rising water. At the same time, evacuations must be carefully targeted, given that an estimated $12.5 million of business losses and hurricane preparation costs is incurred per day for every 160 km (100 mi) of coastline ordered to evacuate. Fortunately, computer models that predict the track of a hurricane are improving. In fact, a new model released by the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in June 1994 is expected to improve forecasts of the eye's location by about 20% over current models.
In the aftermath of Hurricane Andrew, many southern Florida residents have stated that they will evacuate in advance of orders from local officials. While those in designated evacuation zones (and all mobile home owners under a hurricane warning area) should evacuate immediately when ordered by local authorities, unnecessary traffic from those in safe structures outside the evacuation zones ultimately threatens the lives of others. It has been estimated that such unnecessary evacuation would increase evacuation times for the Florida Keys from 34 to 80 hours. With Andrew barely a tropical storm 80 hours prior to landfall (with the possibility of striking nearly anywhere or even fizzling out), it is understandable that the Hurricane Center cannot provide precise lead time for evacuation that far in advance.
Effective evacuation requires the cooperation of residents in allowing those who are truly threatened to evacuate. Because of the possibility of gridlock on the highways, the need for a back-up plan of last-resort refuge is an additional consideration in the evacuation plans of populated areas. Sturdy high-rise buildings could serve as a site for vertical evacuation, allowing individuals to move to higher ground without adding to the threat of gridlock on the highways. Residents and even tourists of coastal areas should be familiar with evacuation procedures in the coastal communities where they are. This information is usually provided in the information sections of telephone books. It is essential to heed the hurricane evacuation orders of local governments - it might save your life, as it did for many in the case of Hurricane Andrew.
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