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Jason C. Senkbeil, Meganne S. Rockman, and John B. Mason

Abstract

The enhanced Fujita scale category 4 (EF4) Tuscaloosa, Alabama, tornado on 27 April 2011 produced 64 fatalities along its 130-km track. Hybrid survey/interviews were conducted with a sample of 211 Tuscaloosa-area residents to determine how the 27 April tornado might change future shelter-seeking plans. Despite a history of tornadoes in the area, only 47% of Tuscaloosa residents had shelter plans in place prior to 27 April, but 62% intend to change their shelter plans or have shelters plans for the future. Changes in shelter-seeking plans were divided into four groups and discussed according to commonalities. Logistic regression with demographic variables was then used to predict those likely to have shelter plans before 27 April and those likely to change their shelter plans in the future. Among these variables, residents over age 55 [odds ratio (OR) 8.9, 95%; confidence interval (CI): 2.167–36.352] and those having a bachelor's degree (OR 5.1, CI: 1.342–19.316) were more likely to have had shelter plans before 27 April. The most significant variable indicating a change in future shelter-seeking plans is being Hispanic/Latino (OR 5.2, CI: 1.753–15.465). These results may assist National Weather Service (NWS) personnel, broadcast meteorologists, emergency managers, and city planners with the development of targeted warning communication tactics and safety strategies for a future tornado event.

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Danielle E. Nagele and Joseph E. Trainor

Abstract

In 2007, the National Weather Service (NWS) began using storm-based warnings (SBWs) rather than countywide warnings. Some analysts have examined the effects of this change, but little empirical research has yet to focus on the public response. Using a random digit dialing sample and a computer-assisted telephone interviewing (CATI) system, data were collected that focused on protective action decision making in counties that were affected by a severe storm or tornado warning. Based on those data, the following paper examines the influence of these new storm-based warnings on protective action decision making by the public. While a significant relationship between being inside the warning polygon and taking protective action was not found, the authors were able to conclude that polygon size is an important factor. Given these mixed results, it is suggested that future work on storm-based warnings focus on the warnings’ dissemination and reception, as well as the optimization of the polygons themselves. It is suggested that the complexities associated with communicating with these risk areas complicate the dissemination process and create difficulties in the public understanding of the warning. The possible need for optimization is reinforced by the significance of the track proximity and polygon-sized variables. In addition, a smaller polygon resulted in protective action, in particular, sheltering. With regard to the preparedness and sociodemographic variables, the study’s results agreed with previous findings on the importance of a family emergency plan. Unlike earlier research this study did not find past experience or education level significant within the regression model and showed mixed results of gender.

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Richard W. Dixon and Todd W. Moore

Abstract

Tornado vulnerability depends on the incidence of and societal exposure to tornadoes for a particular location. This study assesses the vulnerability of Texas counties to tornadoes using tornado incidence and societal exposure composite scores. Three different assessment methods are used to quantify tornado vulnerability and a geographical information system is used for visualization. Using multiple assessment methods facilitates different ways of viewing tornado vulnerability. Even though the three tornado vulnerability maps produced in this study are spatially diverse, some counties were repeatedly identified as highly vulnerable. The most highly vulnerable counties were located within the northern and northeastern portions of the state, specifically in the northeastern corner within the Shreveport, Louisiana, county warning area.

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Maria A. F. Silva Dias

Abstract

This paper presents the development of tornado reports in Brazil since the middle of the twentieth century, both for the country as a whole and for the five regions of Brazil: the south, southeast, central-west, northeast, and north. No official tornado registry exists in Brazil so the reports come from various sources. Most of the tornadoes reported are from the south and southeast regions. The low number of reports for the central-west regions suggests, in view of the literature on intense storms and mesoscale convective complexes, that most tornadoes cases go unreported. The increase in tornado reports is compared to the evolution of population density and communications, with the latter represented by the evolution of local television stations and the popularization of the Internet. One particular event is a new Web site for volunteer tornado reports, which has completely changed the trends of tornado reports. Another possible cause for an increase in the number of tornado reports in the south and southern regions could be a shift in climate variability in this region in the 1970s, which has been reported by several authors. However, the increase in rainfall and extreme events reported by these studies point to an increase of as much as 40%, which is not compatible with the observed tenfold increase in tornado reports.

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S. Hoekstra, K. Klockow, R. Riley, J. Brotzge, H. Brooks, and S. Erickson

Abstract

Tornado warnings are currently issued an average of 13 min in advance of a tornado and are based on a warn-on-detection paradigm. However, computer model improvements may allow for a new warning paradigm, warn-on-forecast, to be established in the future. This would mean that tornado warnings could be issued one to two hours in advance, prior to storm initiation. In anticipation of the technological innovation, this study inquires whether the warn-on-forecast paradigm for tornado warnings may be preferred by the public (i.e., individuals and households). The authors sample is drawn from visitors to the National Weather Center in Norman, Oklahoma. During the summer and fall of 2009, surveys were distributed to 320 participants to assess their understanding and perception of weather risks and preferred tornado warning lead time. Responses were analyzed according to several different parameters including age, region of residency, educational level, number of children, and prior tornado experience. A majority of the respondents answered many of the weather risk questions correctly. They seemed to be familiar with tornado seasons; however, they were unaware of the relative number of fatalities caused by tornadoes and several additional weather phenomena each year in the United States. The preferred lead time was 34.3 min according to average survey responses. This suggests that while the general public may currently prefer a longer average lead time than the present system offers, the preference does not extend to the 1–2-h time frame theoretically offered by the warn-on-forecast system. When asked what they would do if given a 1-h lead time, respondents reported that taking shelter was a lesser priority than when given a 15-min lead time, and fleeing the area became a slightly more popular alternative. A majority of respondents also reported the situation would feel less life threatening if given a 1-h lead time. These results suggest that how the public responds to longer lead times may be complex and situationally dependent, and further study must be conducted to ascertain the users for whom the longer lead times would carry the most value. These results form the basis of an informative stated-preference approach to predicting public response to long (>1 h) warning lead times, using public understanding of the risks posed by severe weather events to contextualize lead-time demand.

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Alan W. Black and Walker S. Ashley

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A database of tornado fatalities, nontornadic convective wind fatalities, severe thunderstorm warnings, and tornado warnings was compiled for the period 1986–2007 to assess the spatial and temporal distribution of warned and unwarned fatalities. The time of fatality and location as reported in Storm Data was compared to tornado and severe thunderstorm warnings to determine if a warning was in effect when the fatality occurred. Overall, 23.7% of tornado fatalities were unwarned, while 53.2% of nontornadic convective wind fatalities were unwarned. Most unwarned tornado fatalities occurred prior to the mid-1990s—coinciding with modernization of the National Weather Service—while unwarned nontornadic convective wind fatalities remained at a relatively elevated frequency throughout the study period. Geographic locations with high numbers of unwarned tornado and nontornadic convective wind fatalities were associated with one high-magnitude event that was unwarned rather than a series of smaller unwarned events over the period. There are many factors that contribute to warning response by the public, and the issuance of a severe thunderstorm or tornado warning is an important initial step in the warning process. A better understanding of the characteristics of warned and unwarned fatalities is important to future reduction of unwarned fatalities.

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David M. Schultz, Eve C. Gruntfest, Mary H. Hayden, Charles C. Benight, Sheldon Drobot, and Lindsey R. Barnes

Abstract

One of the goals of the Warning Project is to understand how people receive warnings of hazardous weather and subsequently use this information to make decisions. As part of the project, 519 surveys from Austin, Texas, floodplain residents were collected and analyzed. About 90% of respondents understood that a tornado warning represented a more serious and more likely threat than a tornado watch. Most respondents (86%) were not concerned about a limited number of false alarms or close calls reducing their confidence in future warnings, suggesting no cry-wolf effect. Most respondents reported safe decisions in two hypothetical scenarios: a tornado warning issued while the respondent was home and a tornado visible by the respondent while driving. However, nearly half the respondents indicated that they would seek shelter from a tornado under a highway overpass if they were driving. Despite the limitations of this study, these results suggest that more education is needed on the dangers of highway overpasses as shelter from severe weather.

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Philip L. Chaney and Greg S. Weaver

Abstract

Mobile home residents are known to be highly vulnerable to tornadoes and account for a considerable portion of tornado-related fatalities. The problem is partially related to the limited protection provided by the structure; however, shortcomings in preparedness and response to warnings may also play a role. This study investigated mobile home resident preparedness and responses to warnings for identifying areas where they might be more vulnerable than permanent home residents (brick and wood-frame houses). The study site was Macon County, Tennessee, which reported the highest number of fatalities during the 2008 Super Tuesday tornado outbreak. A post-disaster survey was conducted within days of the disaster, and the study group included 127 local residents: 35% mobile home (MH) residents, 61% permanent home (PH) residents, and 4% other. An unconditional exact test was used to test for statistical significance (0.05 level) because the sample was nonrandom. The MH residents were less prepared than the PH residents in all six categories evaluated. The difference was significant in having participated in a tornado drill, having a tornado-resistant shelter on the premises, and having an emergency response plan for seeking shelter. The MH residents were also less likely to follow the plan, and the difference was significant. Furthermore, the MH residents were much less likely to take shelter in a safe location. Preparedness factors that promoted higher evacuation rates among MH residents included having participated in a tornado drill, understanding the definition of a tornado warning, and having a plan for seeking shelter.

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Daniel Sutter and Somer Erickson

Abstract

The authors examine the cost of time spent under tornado warnings issued annually by the National Weather Service (NWS). County-based tornado warnings imposed substantial costs on the nation: an average of 234 million person-hours spent under warnings annually between 1996 and 2004, with a value of $2.7 billion (U.S. dollars) per year. Counties are large relative to tornado damage areas; therefore, county-based warnings overwarned for tornadoes, warning many persons a safe distance from the storm and not in immediate danger. In October 2007 the NWS introduced storm-based warnings (SBW) for tornadoes, which are expected to reduce the area warned by 70%–75%. SBW consequently will reduce the time spent under warnings by over 160 million person-hours per year, with a value of $1.9 billion. The time spent under warnings does not measure the full cost to society because many people do not respond to the warnings. Adjusting for warning response, this study estimates that SBW might save 66 million person-hours actually spent sheltering a year with a value of $750 million. Sensitivity analysis indicates that the value of time spent sheltering saved by SBW exceeds $100 million per year with a probability of 0.95.

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Kevin M. Simmons and Daniel Sutter

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This paper extends prior research on the societal value of tornado warnings to the impact of false alarms. Intuition and theory suggest that false alarms will reduce the response to warnings, yet little evidence of a “false alarm effect” has been unearthed. This paper exploits differences in the false-alarm ratio across the United States to test for a false-alarm effect in a regression model of tornado casualties from 1986 to 2004. A statistically significant and large false-alarm effect is found: tornadoes that occur in an area with a higher false-alarm ratio kill and injure more people, everything else being constant. The effect is consistent across false-alarm ratios defined over different geographies and time intervals. A one-standard-deviation increase in the false-alarm ratio increases expected fatalities by between 12% and 29% and increases expected injuries by between 14% and 32%. The reduction in the national tornado false-alarm ratio over the period reduced fatalities by 4%–11% and injuries by 4%–13%. The casualty effects of false alarms and warning lead times are approximately equal in magnitude, suggesting that the National Weather Service could not reduce casualties by trading off a higher probability of detection for a higher false-alarm ratio, or vice versa.

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