Revisiting U.S. Nocturnal Tornado Vulnerability and its Influence on Tornado Impacts

Abstract

This research examines tornadoes and their fatalities by light condition (i.e., daytime and nighttime) for the United States. The study has two primary objectives: 1) catalog and reassess differences in daytime and nighttime, or nocturnal, tornadoes and their fatalities from spatial and temporal perspectives; 2) employ a spatially explicit Monte Carlo simulation technique to calculate differences in daytime and nocturnal tornado-population impact potential by combining climatological tornado risk data with fine-scale, gridded estimates of day and night population density. Results reveal that nocturnal tornadoes remain a substantial impediment to mitigating tornado casualties despite long-term improvements in detection and warning of these events. Nocturnal tornadoes are nearly twice as deadly as daytime events, with fatalities stemming from overnight (i.e., local midnight to sunrise) tornadoes increasing fourfold since the late 19th century. The proportion of all tornado fatalities that occurred during daytime hours has decreased 20% over the last 140 years, while the nocturnal fatality proportion has increased 20%. The stall, or even slight growth, in U.S. tornado mortality rates over the last 30 years has, at least in part, been driven by increasing nocturnal tornado fatalities. Overall, nocturnal tornadoes affect 13% more people on average compared to daytime tornadoes, revealing the importance of time of day in mitigating tornado-population impacts and disasters. Emergency managers, forecasters, first responders, policy makers, and researchers should continue to focus efforts on understanding nocturnal tornadoes, especially regarding how populations receive warnings and respond to these nocturnal threats.