On May 22, one of the deadliest tornadoes in U.S. history struck Joplin, Mo., killing 159 people and injuring more than 1,000. A National Weather Service (NWS) Central Region Service Assessment from the U.S. Department of Commerce released in July examined the emergency warnings and response to those warnings during the tornado.
The tornado, rated EF-5 on the Enhanced Fujita Scale, with maximum winds over 200 mph, affected a significant part of a city with a population of more than 50,000 and a population density near 1,500 people per square mile. As a result, the Joplin tornado was the first single tornado in the United States to result in over 100 fatalities since the Flint, Mich., tornado of June 8, 1953.
This was essentially a warned event in that advance notice of the tornado was given, critical information was communicated and received, and most people sought the best shelter available to them. The timely actions of the weather enterprise (NWS, media, emergency management), and the eventual response of local businesses, churches, schools and the general public undoubtedly saved many lives.
Still, to learn what more can be done to help reduce fatalities from strong and violent tornadoes, a NWS assessment team examined relevant issues ranging from internal NWS warning operations to dissemination strategies to public warning response. The assessment focus was on communication, dissemination, community preparedness, and the public warning response leading into and during the event.
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Many of the key findings within the report involved societal aspects of warning response and risk perception. Responding to warnings is not a simple act of stimulus-response; rather it is a non-linear, multi-step, complex process. Relationships between false alarms, public complacency and warning credibility are highly complex as well.
For example, there was a full spectrum of responses on most key interview points during the assessment. The assessment team interviewed over 60 survivors, as well as emergency managers, city officials and members of the media, among others. During the tornado, some people took shelter in appropriate locations, but did not survive. Others mistakenly drove their vehicles into the tornado path but somehow lived to tell of it.
The vast majority of Joplin residents did not immediately take protective action upon receiving a first indication of risk (usually via the local siren system), regardless of the source of the warning. Most chose to further assess their risk by waiting for, actively seeking and filtering additional information.
The reasons for doing so were quite varied, but largely depended on an individual's worldview formed mostly by previous experience with severe weather. Most importantly, the perceived frequency of siren activation in Joplin led the majority of survey participants to become desensitized or complacent to this method of warning. This suggests that initial siren activations in Joplin (and severe weather warnings in general) have lost a degree of credibility for most residents - one of the most valued characteristics for successful risk communication.
Instead, the majority of Joplin residents did not take protective action until processing additional credible confirmation of the threat and its magnitude from a non-routine, extraordinary risk trigger. This was generally achieved in different ways, including physical observation of the tornado, seeing or hearing confirmation, and urgency of the threat on radio or television, and/or hearing a second, non-routine siren alert.