Supercell tornadoes are much stronger and wider than damage-based ratings indicate

Tornadoes cause damage, injury, and death when intense winds impact structures. Quantifying the strength and extent of such winds is critical to characterizing tornado hazards. Ratings of intensity and size are based nearly entirely on postevent damage surveys [R. Edwards et al., Bull. Am. Meteorol. Soc. 94, 641–653 (2013)]. It has long been suspected that these suffer low bias [C. A. Doswell, D. W. Burgess, Mon. Weather Rev. 116, 495–501 (1988)]. Here, using mapping of low-level tornado winds in 120 tornadoes, we prove that supercell tornadoes are typically much stronger and wider than damage surveys indicate. Our results permit an accurate assessment of the distribution of tornado intensities and sizes and tornado wind hazards, based on actual wind-speed observations, and meaningful comparisons of the distribution of tornado intensities and sizes with theoretical predictions. We analyze data from Doppler On Wheels (DOW) radar measurements of 120 tornadoes at the time of peak measured intensity. In striking contrast to conventional damage-based climatologies, median tornado peak wind speeds are ∼60 m⋅s−1, capable of causing significant, Enhanced Fujita Scale (EF)-2 to -3, damage, and 20% are capable of the most intense EF-4/EF-5 damage. National Weather Service (NWS) EF/wind speed ratings are 1.2 to 1.5 categories (∼20 m⋅s−1) lower than DOW observations for tornadoes documented by both the NWS and DOWs. Median tornado diameter is 250 to 500 m, with 10 to 15% >1 km. Wind engineering tornado-hazard-model predictions and building wind resistance standards may require upward adjustment due to the increased wind-damage risk documented here.

A Tornado Watch Scale to Improve Public Response*

A tornado refuge rubric was revised into a six-level, hierarchical Tornado Watch Scale (TWS) from level 0 to level 5 based on the likelihood of high or low-impact tornadic events. Levels correspond to an estimate of the maximum potential tornado intensity for a given day and include refuge/shelter categories of “adequate,” “questionable,” or “inadequate,” which encompass a range of refuge/shelter locations taken from the Enhanced Fujita scale. Ratings are based on a conservative estimate of damage indicators in high winds and the safety of a person taking refuge inside buildings of varying structural design. Audio recordings similar to those used in current NOAA weather radio communications were developed for each TWS intensity level. Recordings representing an existing tornado watch, existing particularly dangerous situation (PDS) tornado watch, and three proposed levels from the TWS were then used in interviews with Alabama residents to determine how changes to the information contained in the watch statements would affect each participant’s tornado safety actions and risk perception. Participants were also questioned about their knowledge and past experience with tornado hazards and their preference between the existing NWS tornado watches and the TWS. Results indicate a strong preference for the TWS when compared to existing products. The TWS was favored for providing additional information, containing descriptions of expected severity, and being easy to understand. The TWS also elicits more adequate safety decisions and more appropriate risk perception when compared to existing products, and these increases in safety were statistically significant.

Household Preparedness for Tornado Hazards: The 2011 Disaster in DeKalb County, Alabama

This paper contributes to existing knowledge on factors that influence adoption of hazards adjustments for tornadoes. The Protective Action Decision Model provides the theoretical basis for the study, which was conducted after the 2011 disaster in DeKalb County, Alabama. Most of the 124 survey participants had received public safety information on how to prepare for a tornado, understood the definition of a tornado warning, had participated in a tornado drill, and had a plan for seeking shelter. Few owned a NOAA weather radio or had a tornado-resistant shelter on the premises. Demographic analysis found that older residents (60+ yr) and households without children were significantly less likely to have participated in a tornado drill, lower income residents were significantly less likely to have a tornado-resistant shelter on the premises or a plan for seeking shelter, and mobile home residents were significantly less likely to have a plan for seeking shelter. Locus of control and past experience were not significantly associated with adoption of hazards adjustments, but suspected reasons for these results are discussed. Many plans that involved evacuating to another location included excessively long travel distances, and several mobile home residents planned to seek shelter inside their residence. Failure to adopt effective preparedness actions in each of these areas could serve as a situational impediment to making an appropriate protective action decision when a tornado threatens the household. The results identify aspects of household preparedness where there is opportunity for improvement, which would reduce vulnerability and enhance community resilience.

Tornado Risk Analysis: Is Dixie Alley an Extension of Tornado Alley?

The term “Tornado Alley” is a gross approximation of the most tornado-prone region in the United States. Depending on calculation methods, Tornado Alley can vary dramatically across the area between the Rocky and Appalachian Mountains. There is some evidence that multiple alleys of peak tornado activity exist around the country, including “Dixie Alley” in the Southeast. Therefore, we assess the spatial tornado risk and seek any regions of elevated tornado risk that are distinctly separate from the traditional Tornado Alley of the Great Plains. Results show there are no tornado risk areas statistically separate from Tornado Alley, but there are large portions of the Southeast that experience more tornadoes than the rest of the country. It appears that Tornado Alley and Dixie Alley are part of a single large region of high tornado risk with a relative minimum near the middle due to the Ozark and Ouachita Mountains. Placement of the maximum tornado density in Mississippi, along with other regions of relative maxima across the Southeast, may warrant modification of the traditional tornado risk map that focuses only on the Great Plains. Understanding such patterns is important for preparing the public and mitigating tornado hazards.