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Tornadoes. Scott R. Lillibridge, M. D

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1 Tornadoes. Scott R. Lillibridge, M. D
Tornadoes Scott R. Lillibridge, M.D. Centers for Disease Control & Prevention

2 INTRODUCTION Background and Nature of the Problem Tornadoes are funnel-shaped wind storms that occur when masses of air with differing physical qualities (e.g., density, temperature, humidity and velocity) collide (1). These violent rotating winds converge to form a vortex, which is usually narrow at the base and which gives a tornado its typical funnel-shaped appearance. Air and debris are actively drawn into the base of the vortex as the tornado moves across the ground, resulting in a path of destruction. Tornadoes in the northern hemisphere rotate counter clockwise, while those in the southern hemisphere rotate clockwise (2).

3 Winds associated with tornadoes can reach speeds in excess of 250 miles per hour (mph) (1,3). Because weather conditions that create tornadoes may be present over a large geographic region, tornado outbreaks, defined as six or more tornadoes, may occur within a relatively short period of time (2). For example, in 1974, an outbreak of 148 tornadoes throughout the eastern United States affected 13 states and resulted in approximately 300 deaths and 6,000 injuries (4). The cost of property damage caused by a single outbreak can be in excess of 200 million dollars (5).

4 As shown in Table 11-1, tornadoes are rated by the Fujita-Pearson Tornado Scale (F0 through F5) on the basis of the estimated wind speed of their vortices and the width and length of their paths (3,6). Most tornadoes (60%) are considered weak (F0,F1), with wind speeds less than 113 mph; these have limited potential to cause injury or destroy property. However, 1-2% of all tornadoes are considered violent (F4-F5), with wind speeds greater than 206 mph; these tornadoes are highly destructive and account for more than 50% of all tornado-related deaths in the United States (Table 11-2).

5 Because the force of a tornado is strongly associated with its potential to cause injury and death, the number of violent tornadoes (F4,F5) per area of land mass may provide a more accurate representation of a state's public health risk from tornadoes. States with high concentrations of F4 and F5 tornadoes include Oklahoma, Indiana, Iowa and Kansas (2). Among all states, Florida has the highest concentration of tornadoes (tornadoes per 10,000 square miles) (Table 11-3), although tornadoes in Florida tend to be weak (F0,F1) and thus to have limited public health impact. When the size of a state is not controlled for (number of tornadoes divided by 100,000 sq km), Texas, however, is consistently the most tornado-prone state in terms of the total number of all classes of tornadoes (Table 11-3) and the number of tornado-related deaths per geographical area.

6 In the United States, most tornadoes (59%) travel toward the northeast at an average ground speed of 40 mph (2). However, tornadoes have been noted to move along the ground at velocities up to 75 mph, remain stationary, or even reverse course. The average length of a tornado is 4.4 miles and the average width of a tornado is 128 yards. Most tornadoes last for only a few short minutes; however, maximum strength tornadoes have been known to travel more than 200 miles (Table 11-4), persist for hours, and span 3 miles in width (2,3). In the United States, the peak time of day for the occurrence of tornadoes is between 5 and 6 p.m. (Figure 11-1).

7 The time of year is also an important factor related to the development of tornadoes (Figure 11--2). April, May, and June are the peak months for the occurrence of tornadoes in the United States; however, the months of peak occurrence vary considerably from state to state. For example, the peak season for tornadoes in North Dakota is actually June, July, and August. Although tornadoes occur most frequently in certain geographical regions, at certain times of days, and during certain months, tornadoes have actually been recorded at all hours of the day and in all months of the year. Tornadoes have also been recorded in Alaska and Hawaii.

8 The property destruction that accompanies a tornado is directly related to the strength of its destructive winds. These forces can be sufficient to sweep houses off their foundations and carry debris hundreds of miles. Tornado outbreaks with multiple simultaneous tornadic storms may create substantial and wide-ranging demands on state and local emergency services and health departments (5). The disruption of electrical power and telephone services and the financial losses due to tornadoes can be catastrophic to a community (3). Because of the highly localized path of destruction that accompanies a tornado, the infrastructure of an affected community usually remains intact. However, these storms can result in mass-casualty situations for emergency responders and local medical facilities (7-9).

9 Scope and Relative Importance of Tornadoes North America is the most tornado-active continent and is known for particularly forceful and destructive tornadoes (2). In North America most of these destructive storms occur within the continental United States (10). Among all natural hazards in the United States, only floods and lightning result in more weather-related deaths (11). Other countries, such as Canada, Russia, Australia, China, India, and Bangladesh, are also tornado-prone and have recorded significant tornado-related disasters (2). The worst European tornado occurred on August 19, 1845, when between 70 and 200 people were killed in Moneuil, France. The world's deadliest tornado occurred on April 26, 1989, in Bangladesh near Dhaka when approximately 1,300 people were killed and more than 12,000 were injured. Worldwide, tornadoes are probably under-reported because many developing countries lack adverse weather reporting systems. Consequently, the global impact of tornadoes on public health is not known.

10 Factors that Contribute to Tornado Disasters Tornadoes pose their greatest danger to public health as they move through heavily populated areas (5,8). Unlike other disasters, such as hurricanes, which are preceded by a lengthy warning period that allows response officials to evacuate the vulnerable population, tornadoes occur suddenly. Therefore, unless disaster mitigation steps are undertaken prior to the tornado, preventing death and injury from these storms may be impossible. Inadequate warning and a lack of suitable sheltering for the population at risk are the main contributing factors to the adverse public health effects of tornadoes. Multiple and even redundant sources of warning dissemination are needed to ensure that everyone in the potential path of a tornado is informed of the danger (12-13). For example, during the Wichita-Andover tornado (1991) in Andover, Kansas, the town's only outdoor tornado siren failed to sound as the storm approached (5). However, nearly 81% of the population whose homes were destroyed were able to receive at least 5 minutes of warning through other sources, such as the media or telephone, thus significantly limiting loss of life.

11 Unfortunately, many community residents do not have access to appropriate storm shelters (5). A suitable storm shelter for people threatened by tornadoes is usually defined as a subgrade (underground) structure such as a basement or a structure specifically designed to provide temporary protection from adverse weather conditions. People in multiple-family dwellings and other tornado-vulnerable communities such as mobile home parks pose a special storm sheltering challenge for disaster management officials, particularly when no community storm shelters are located nearby. In addition, the lack of tornado-shelter drills for tornado-vulnerable communities may significantly contribute to residents' risk from tornadoes.

12 FACTORS AFFECTED PROBLEM OCCURRENCE AND SEVERITY Because of recurrent weather patterns favorable for the formation of tornadoes, certain geographic regions are at higher risk for the development of tornadoes than other areas. For example, from 1953 through 1991, five states (Texas, Oklahoma, Florida, Kansas, and Nebraska) accounted for 11,935 (40%) of the 29,953 tornadoes that occurred in the United States (2) (Figure 11--3). Throughout the world, agricultural areas (e.g., the midwestern United States and farming regions within other countries like Brazil and Argentina) tend to be at higher risk for the development of tornadoes because certain weather patterns that promote good crop yields may also contribute to the development of tornadoes (10). In addition to geographic factors, the time of day and the time of year also affect a population's risk from tornadoes. For example, late afternoon is the time of day when tornadoes are most likely to occur. Seasonal variation associated with the development of tornadoes means that the risk from tornadoes to the population at any given location is not constant throughout the year. In the United States, January is the month when the fewest tornadoes occur, and May is the month when the most tornadoes occur (10). Other natural factors that promote the development of tornadoes include weather phenomena such as hurricanes. Hurricane Beulah, for example, which struck in 1967, is credited with spawning 115 tornadoes in the coastal region of Texas (2).

13 Public Health Impacts: Historical Perspective From 1953 through 1991, the United States averaged 768 tornadoes per year (14). During that period, an average of 93 people were killed each year by tornadoes in the United States. In 1882, in one of the earliest and most comprehensive accounts of U.S. tornadoes, U.S. Army Sergeant J.P. Finley reported on 600 tornadoes that occurred from 1860 through 1880 (15). While stationed in the midwestern region of the United States, he investigated many reports of tornadoes. He traced their paths and recorded their patterns of damage.

14 Sergeant Finley also established in the midwestern United States a tornado surveillance network composed of more than 1,000 people who routinely collected and reported details concerning the occurrence and effects of local tornadoes. Such a program was the first comprehensive and systematic weather reporting system, and it set the stage for the development of a modern national meteorological observational system. Many of Sergeant Finley's original observations and safety recommendations have stood the test of time. For example, he recommended that "persons residing in tornado-prone regions should build underground shelters" to provide protection from these storms (15). However, other observations such as the recommendation that "tornadoes can be safely observed from a distance of 300 yards" were not as prudent. (Current recommendations are that anyone threatened by a tornado should take shelter immediately.)

15 Interestingly, although the annual number of tornadoes reported in the United States generally increased from 1921 through 1990, the annual number of tornado-related deaths actually decreased (Figure 11- 4) (2,14). One reason for the decrease in mortality associated with tornadoes is the increasing effectiveness of the National Weather Service's (NWS's) severe storm warning systems. Since 1952, when the NWS first began broadcasting tornado warnings, the number of tornado-related deaths has been declining steadily (2). Increased reporting of tornadoes is also thought to have been facilitated by the combination of improvements in NWS storm detection technology (e.g., Next Generation Weather Radar or NEXRAD) and the development of a national system of NWS volunteer weather spotters. People are injured or killed by tornadoes when they are struck by energized debris or when their bodies are thrown into stationary objects. Head injuries have been observed to be the most likely cause of death among tornado victims (16-17). For every person who dies as a result of a tornado, an estimated 20 people are injured (8). Immediate public health relief needs of survivors include medical care, shelter, and assistance in collecting dispersed property and disposing of debris.

16 FACTORS INFLUENCING MORBIDITY AND MORTALITY ASSOCIATED WITH TORNADOES Natural factors As shown in Table 11-2, a population's risk for injury and death from a tornado increases with the tornado's strength. Fortunately, violent (F4 and F5) tornadoes are rare (2). Another factor related to the risk from a particular tornado is the length of its path. However, the association between the length of a tornado's path and its destructive power is somewhat confounded by the fact that tornadoes with long paths also tend to be the most powerful and destructive (10). Another factor that influences morbidity and mortality from a tornado is the time of day that it occurs. For example, in 1947, in Compton, Kansas, 78 people were killed by a tornado that struck at night without warning (2). Tornadoes that occur at night, when people are sleeping, are more dangerous because people are less likely to hear any warnings. Another factor that can contribute to morbidity and mortality is adverse weather conditions such as hail or rain that obscure the presence of tornadoes, making their detection by weather spotters more difficult.

17 Human Factors Human factors also contribute to the severity of the adverse public health consequences associated with tornadoes. Epidemiologic studies have identified several risk factors for injury and death during tornado disasters, including the following: 1) residing in mobile homes; 2) being more than 60 years old; 3) remaining in a vehicle; 4) failing to seek shelter during a tornado warning; and 5) being unfamiliar with tornado-warning terminology (2,5,8-9,16,18-19). Although the number of tornado-related deaths in the United States is decreasing, the number of tornado-related deaths among people residing in "mobile homes" may be growing at an alarming rate (5). In addition, people with disabilities such as decreased vision or hearing may not be aware of tornado warnings. Elderly people are also at risk from tornadoes for reasons that may relate to both sensory and physical disabilities (5,8,18). Several myths concerning proper actions to take following a tornado warning may increase people's risk for injury or death. The belief that one can outrun a tornado or accurately judge its direction while driving in an automobile is extremely hazardous. For example, during the 1979 Wichita Falls tornado, 40% of people who died were attempting to flee in automobiles (8). Ironically, many of these vehicular occupants had left areas of relative safety and moved directly into the path of the storm. Another belief that has no scientific basis is that tornadoes will not repeatedly strike the same geographical location. This belief is belied by the experience of Codell, Kansas, which was struck by a tornado for 3 straight years--1916, 1917, and 1918 (2). Although such repeat strikes are rare, they serve to remind us that the direction and path of any given tornado is purely a matter of chance. Yet another dangerous myth is that opening or closing windows may mitigate tornado damage; in fact, such actions only serve to delay a person's seeking safe shelter.

18 As consistently demonstrated in several studies, tornado-related morbidity and mortality rates are higher when no effective storm warnings are issued and when no suitable storm shelters are available. Furthermore, the effectiveness of official warning-dissemination systems is limited if populations are unable to understand the warnings because they are delivered in a culturally inappropriate manner (20). For example, during a tornado disaster in 1987, the unincorporated rural community of Saragosa, Texas (population , largely Hispanic), was struck by a single, violent, maximum-strength tornado (F-5) that killed 30 people and injured 131 more (20). Results of a postdisaster investigation suggested that, in addition to an inadequate number of storm shelters for the town's population, the adverse public health impact of the disaster was exacerbated by the warnings not being disseminated in Spanish as well as in English (21).

19 PREVENTION AND CONTROL MEASURES
PREVENTION AND CONTROL MEASURES Warning One of the most important prevention tools that NWS warning meteorologists use to reduce a population's risk from tornadoes is the NEXRAD (Next Generation Weather Radar) doppler radar weather system (22). This new technology provides tornado detection that is from 30% to 60% more sensitive than conventional radar at distances of more than 200 miles (23). Doppler weather radars are able to detect the movement of tornadoes an average of 20 minutes before touchdown. Unfortunately, in 1990, doppler radar coverage had not yet extended to include the region of Plainfield, Illinois. The Plainfield tornado (F-5) resulted in 28 fatalities and 274 injured people (22). The extremely short time period between the tornado warning and the tornado's arrival in Plainfield and the odd shape of this particular tornado's funnel cloud made it difficult for observers to recognize the approaching storm as a tornado.

20 The resulting tragedy illustrates the importance of expanding the availability of new technology such as doppler radar to all tornado-prone regions In addition to improvements related to tornado-detection, the NWS is undergoing a major nationwide modernization program to improve the dissemination of weather hazard information through radio, television, and telephone. Storm warnings are increasingly being incorporated into the commercial news media (e.g., television's hour "Weather Channel") (24). Continued improvements in tornado detection and warning dissemination are credited with the decrease in the annual number of observed tornado-related deaths in the United States (2).

21 Epidemiology: Assessment and Surveillance Epidemiologists may be required to provide technical assistance in the conduct of emergency health assessments of populations affected by tornado disasters (25-26). Such assessments following tornado disasters are usually conducted by following the path of the storm in a simple door-to-door fashion or by assessing the needs of people who have been displaced from their homes into temporary shelters. Because tornadoes have their greatest adverse effect on public health when they move into populated areas, assessment teams should initially focus on highly urbanized residential areas when resources are limited. . Two important logistical requirements influence the success of a disaster assessment conducted in the wake of a tornado: Highly detailed maps. Such maps are extremely important tools for those organizing geographic-based field surveys in highly urbanized areas. These are best obtained from state and local authorities. Aerial reconnaissance. The earliest and most reliable information concerning the path of the tornado storm is usually obtained from aerial reconnaissance. If the ground-based assessment is not guided by such aerial visual surveys, it will be delayed and its effectiveness reduced.

22 Postdisaster public health surveillance of a population affected by a tornado may be an important component of the emergency public health response (27). Information obtained from such surveillance can be used to detect epidemics of infectious disease, injuries, and environmentally related illness that occur within a population as a result of a tornado disaster (28-29). To be effective, postdisaster surveillance activities for health and medical conditions should be based at clinical sites and at both official and unofficial shelters where displaced persons have collected. In addition, using a standard shelter surveillance form, relief agencies such as the American Red Cross collect data on the status of victims living in shelters (30). Therefore, epidemiologists conducting postdisaster surveillance should coordinate surveillance and other data collection activities with all relief agencies and with state and local public health authorities.

23 Engineering and Legal Controls Residents of "mobile homes" have been identified as being at especially high risk for death and injury from tornadoes (8,18). The most graphic example occurred in Andover, Kansas, in 1991, when an F-5 tornado struck a mobile-home community and destroyed 205 residential units. Of the 38 unsheltered residents who remained within the mobile-home community, 11 (29%) were killed, 17 (45%) were hospitalized, and 9 (24%) sustained injuries that were treated on an outpatient basis. Incredibly, only one unsheltered person escaped injury. In contrast, none of the approximately 150 residents of the mobile-home community who were able to obtain shelter in the community's single underground storm shelter were killed or injured. Unfortunately, at the time of this disaster, 40% of all mobile home parks surveyed in the affected county did not have a community storm shelter. Although tornado-related mortality in the United States has been steadily decreasing from 1985 through 1993, 151 (36.5%) of the 414 tornado deaths that occurred in the United States during that period were among mobile home residents, even though only 7% of the U.S. population lives in mobile homes (31). This population is thus an important one to target for tornado disaster mitigation (32). Apart from enhancements in warning, the most effective prevention and control measure for reducing tornado-related injuries and deaths is to provide adequate sheltering options for this population (23).

24 Medical Treatment and Rehabilitation Because tornadoes create destruction along paths that may span many miles, quickly determining where the majority of casualties occurred can be difficult. Following a major tornado, communication systems are frequently disrupted and highways are often blocked with debris. Many patients with minor injuries may suddenly converge on local hospital emergency departments and overwhelm available staff and material resources. This may hinder efforts by the hospital staff to prepare for more severely injured patients who may arrive later because of their need for extrication, field stabilization, and ambulance transport. Effective hospital disaster planning should ensure that casualties are effectively triaged and distributed throughout the local medical system (33). Studies have shown that tornado-related injuries tend to be severe soft-tissue injuries in the form of contusions, complex lacerations, and multiple fractures, particularly of long bones (7,8,34). Because people with these tornado-related soft-tissue injuries also tend to develop wound infections caused by gram-negative bacilli commonly found in soil (7,35), tornado-related wounds should be considered highly contaminated and handled appropriately. Wound management may include debridement, secondary closure, and appropriate tetanus prophylaxis (e.g., passive or active immunization depending on a patient's immunization status). Antibiotic treatment for tornado-related wound infections should include coverage for gram-negative bacterial organisms (36). In addition to infectious disease problems associated with tornadoes, mental health problems among both responders and victims may be a significant public health issue and should be considered during disaster planning (see chapter on the Mental Health Consequences of Disasters [Chapter 7]).

25 Public Awareness and Education Tornado watches and warnings issued by the NWS to the media and to community organizations are the primary means of alerting the public about an approaching tornado (4). An NWS Tornado Watch means that weather conditions are conducive to the development of tornadoes. An NWS Tornado Warning means that a tornado has been sighted by ground observers or has been detected by advanced technology. One limitation to this nomenclature is that the local population may not understand the difference in meaning between the two terms. For example, after the North/South Carolina tornado outbreak of 1986, a field survey of people in the path of the tornadoes revealed that only 40% of them understood the difference between a "watch" and "warning" (9). Community programs such as maintaining a volunteer system of weather spotters may increase rates of local tornado detection and improve warning dissemination. Because of the strong association between the strength of tornadoes and their potential to cause death or serious injury, tornado-related disaster preparedness should concentrate on those parts of the country that have high numbers of violent tornadoes (Fujita classes F4 and F5). Another important component of disaster preparedness is to educate the public to seek appropriate shelter, particularly in public buildings such as schools. Improved hazardous weather awareness can also be promoted by encouraging households located in tornado-prone parts of the country to have National Oceanic and Atmospheric Administration (NOAA) "Weather Radios," which provide 24-hour information to the public concerning adverse weather conditions.

26 CRITICAL KNOWLEDGE GAPS Given the frequency of tornadoes within the United States, far too little investigation has been done to determine risk factors for injury and death in order to develop more effective strategies for protecting the public from the hazardous effects of tornadoes. Further epidemiologic studies are clearly needed. Numerator data describing the age, sex, and race of people injured or killed and denominator data describing the population at risk are necessary to develop rates of tornado-related morbidity and mortality (37). Epidemiologists' attempts to locate victims (case finding) in the wake of a tornado can be greatly facilitated by their working with county health officials, local hospitals, law enforcement agencies, the American Red Cross and disaster management officials. Several sources of data on tornado-related injuries may be available. Local emergency medical service (EMS) officials may have information on ambulance transport and field medical activities. Patient records from physician offices, hospital emergency departments, and medical records departments may be an excellent source of injury data. County medical examiners, coroners, law enforcement officials, and the media may also provide additional information on tornado-related deaths. Local census information can be extremely useful in developing an estimate of the size of the affected population.

27 Current severe-weather morbidity and mortality surveillance systems (e
Current severe-weather morbidity and mortality surveillance systems (e.g., the National Climatic Data Center) collect only limited information on the circumstances associated with tornado-related injuries or deaths. Medical records and coroner reports are usually not examined by those collecting information on the effects of adverse weather events in order to obtain further data on the mechanism of injury or the cause of death. Little information is collected on the shelter-seeking behavior of victims at the time of injury or death or on whether victims had been adequately warned that a tornado was approaching. In part because of this lack of data, most epidemiologic studies have concentrated on tornado-related deaths, which are more easily measured and less numerous than tornado-related injuries. Morbidity studies following tornadoes have been limited because victims seek medical care in many diverse locations and because patient-care documentation during a disaster may not be a priority for emergency responders (38). In addition, very little research has been done to relate the physical characteristics of a given tornado (e.g., wind speed, width, duration, etc.) with the tornado's public health effects. Studies to determine optimal tornado-warning dissemination strategies are lacking, and the potential for new media technologies such as satellite transmission and cable television to disseminate storm warnings effectively has not been thoroughly evaluated.

28 METHODOLOGIC PROBLEMS OF EPIDEMIOLOGIC STUDIES Like most natural disasters, tornadic storms create situations in which the size of the affected population is difficult to determine. Estimating the size of populations affected by tornadoes is especially difficult because tornadoes often take meandering paths of destruction across a community and, thus, the entire community may not be uniformly affected. Another key element of any epidemiologic investigation of a community affected by a tornado is determining the amount of warning time that population had prior to the tornado's impact. However, estimating precise times is difficult in retrospect (e.g., estimates may be based only on a person's recall), and estimates are therefore subject to considerable bias. In addition, it is impossible to establish the exact mechanism of death for many victims because autopsies are not performed on most people killed by tornadoes. Locating or following up on survivors is also difficult because many victims relocate to temporary shelters or the homes of relatives.

29 Another difficult task in comparing injuries and deaths associated with any specific tornado is determining the extent to which various building types (e.g., woodframe, brick) tend to resist wind damage. Highly resistant structures would be expected to protect their occupants better. In addition, the windspeed gradient associated with a tornado may differ significantly at distances of only a few meters from the funnel cloud. Therefore, the extent to which a person's injuries reflect the full effect of the winds associated with a particular tornado is often unclear. As mentioned above, the NWS conducts nationwide surveillance for tornado-related injuries and deaths, but this system collects very little additional information on personal safety-seeking behavior, the degree of warning that occurred, or people's access to shelter. Although some case-control studies of risk factors associated with tornado-related death and injury have been done, most have lacked the type of multidisciplinary approach needed for good epidemiologic studies, with few incorporating contributions from epidemiology, wind engineering, and meteorology.

30 RESEARCH RECOMMENDATIONS Conduct research to define the optimum use of new warning technology. Although the NWS has developed improved tornado prediction and detection methods (2,22), the communication of this information to the public can still be improved. Automatic telephone warning systems and NWS storm warning radios show great promise. In addition, research to develop storm-warning technology that is specific for both indoor and outdoor populations is required if communities are to be fully protected. Develop methods to protect our increasingly aging population from tornadoes. Because of hearing or vision loss, elderly people may be unable to benefit from new technological developments in warning systems that depend on one's being able to see or hear well. Research is required to determine the best methods to warn and shelter this vulnerable segment of the population. Standardize the collection of data by medical examiners or coroners following natural disasters. Develop standard definitions for tornado-related injury and death. Ensure that teams investigating the effects of tornadoes contain multidisciplinary components. Because tornado disasters involve the correlation of physical and meteorological information with epidemiologic data, multidisciplinary research teams are necessary to optimally study the health effects of tornadoes. Conduct further research into the best use of the new technology and modern media resources for disseminating weather warnings. Concerns have arisen that frequent "false positive" tornado watches might make people less likely to seek shelter when warned in the future.

31 SUMMARY Tornadoes remain the most frequent adverse weather event likely to result in a disaster in the United States. Unfortunately, preventing tornadoes is currently beyond our technical capabilities. Research results have illustrated the life-saving importance of providing adequate warning and shelter to populations at risk. These prevention measures are particularly important in states with large numbers of violent tornadoes (e.g., Kansas, Iowa, Illinois, Indiana, and Oklahoma) and in locations where many people live in mobile homes (5,39). The National Weather Service continues to modernize its nationwide adverse weather warning system. As a result, communities are relying less on warning systems that depend solely on outdoor warning sirens and more on a variety of technologically advanced warning systems, including weather scanners, cable television override, and automatic telephone alerting. Tornado-related morbidity and mortality should continue to decrease in the United States if efforts to warn and shelter vulnerable populations such as the elderly and residents of mobile homes become part of routine community emergency preparedness.

32 REFERENCES


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