False Alarms, Tornado Warnings, and Tornado Casualties

Abstract

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.

The Relationship between Tornadic and Nontornadic Convective Wind Fatalities and Warnings

“…Brotzge and Erickson (2010) found that 73.6% of tornadoes in the 2000-04 period were associated with a NWS warning. Simmons and Sutter (2005) established that after the installation of the Weather Surveillance Radar-1988 Doppler (WSR-88D), the percentage of tornadoes warned increased to 60% from 35% prior to installation. A regression analysis of tornado casualties illustrated that expected fatalities were 45% lower for tornadoes occurring after installation of the new radar network.…”

“…A regression analysis of tornado casualties illustrated that expected fatalities were 45% lower for tornadoes occurring after installation of the new radar network. Further investigation by Simmons and Sutter (2008b) determined that tornado warnings do reduce fatalities, and increased lead times for tornado warnings up to approximately 15 min reduced casualties, while longer lead times resulted in increased fatalities compared to no warning. The greatest reduction in fatalities occurs at lead times of 10-15 min, possibly because of the public concluding that the warning is a false alarm if a tornado does not occur soon after the warning is issued (Simmons and Sutter 2006).…”

“…In terms of housing type, the most vulnerable populations are those living in mobile homes (Brooks and Doswell 2002;Sutter 2005, 2008a;Ashley 2007;Sutter and Simmons 2009;Chaney and Weaver 2010). Timing of the storm can increase vulnerability.…”

“…Several studies have found that nocturnal tornadoes are particularly hazardous, as they are more difficult to identify, the public is less likely to receive a warning because people are sleeping, and residents of vulnerable structures are more likely to be home (Simmons and Sutter 2005;Ashley 2007;Ashley et al 2008). Furthermore, tornado fatalities are affected by the day of the week of occurrence, as expected fatalities are about 70% higher on a weekend than a week day, again presumably because residents of vulnerable housing stock are more likely to be home rather than away at work (Simmons and Sutter 2008b). Previous research has also illustrated that the month of tornado occurrence has a large impact on the number of fatalities.…”

“…Previous research has also illustrated that the month of tornado occurrence has a large impact on the number of fatalities. Doswell (2003), Ashley (2007), and Simmons and Sutter (2008b) suggest that off-season tornadoes may be more deadly because people are more aware of the hazard during the traditional severe weather season (i.e., April-June) and less prepared during other times of the year. Finally, the age of victims has also been shown to have an effect on tornado fatalities.…”