Tennessee tornado frequency, vulnerability, and relation to a large-scale climate variability

This work explores the climatologies of isolated tornadoes and tornado outbreaks across the state of Tennessee, a state that in some years experiences more tornadoes than states in the heart of Tornado Alley. Part one assesses tornado frequency characteristics and fatality statistics within 100 km of three major Tennessee cities (Nashville, Memphis, and Knoxville) between 1950 and 2013. Nashville reported the most tornadoes, (426) but Memphis reported the most fatalities. Knoxville and Nashville tornadoes occurred on fewer days, while Memphis tornadoes were spread across more tornado days. Spring was the most active season for tornadoes, but Memphis still experienced approximately 25% of its total tornadoes in the winter, a season prone to nocturnal tornadoes. There was no statistically significant difference between the seasonality of tornadoes for each of the cities, which is surprising given the longitudinal expanse of the state. Regional-scale analyses of this type provide insight on how tornado risk and vulnerability may vary considerably across a single state.

Part two analyzes tornado outbreak characteristics (1980–2014) from a climatological perspective and assesses how a large-scale climate oscillation may affect tornado and tornado-outbreak frequencies across Tennessee. Results indicate that 72.5% of all tornadoes in Tennessee occur in outbreaks, when an outbreak is subjectively defined as any 24-hour period with four or more tornadoes within the state. Winter, defined as Dec/Jan/Feb, had the second-highest tornado-outbreak frequency. This provides a possible explanation for the high frequency of tornado-related fatalities in Tennessee, as the winter is a time of reduced daylight and is when nocturnal tornadoes, which are twice as likely to kill, are most prevalent. The Multivariate ENSO Index (MEI) was investigated using generalized linear models with a Quasi-Poisson distribution to determine if a relationship existed between tornado activity and a large-scale climate oscillation. Results indicate that above (below) average values of MEI, or El Niño (La Niña) events, are related to times of decreased (increased) tornado activity across Tennessee, and are supported by meteorological considerations. Offering future estimations of tornado activity on a seasonal or monthly scale can aid in reducing susceptibility to these dangerous events.