Tornadoes in Winter? Assessing the Climatological Trends and Meteorological Environments of Cold-Season Tornadoes

Tornadoes that occur during the cold season, defined here as November – February (NDJF), pose many unique societal risks. For example, people can be surprised by tornadoes happening at a time of year they generally do not expect. The public can also be unsuspecting of significant weather due to the bustle of major holidays like Thanksgiving, Christmas, and New Year’s, when most are concerned with family activities and not thinking about the weather. Cold-season tornadoes also have a propensity to be nocturnal and occur predominantly in the southern and southeastern U.S., where variable terrain, inadequate resources, and a relatively high mobile home density add additional social vulnerabilities. Over the period 1953–2015, within a study domain of (25-42.5°N, 75-100°W), some 937 people lost their lives as a result of NDJF tornadoes. Despite their enhanced societal risk, very little work has addressed cold-season tornadoes in-depth. This study aims to greatly advance the current state of knowledge of NDJF tornadoes by first establishing a climatology of all (E)F1-(E)F5 NDJF tornadoes, spanning the period 1953–2015 and a domain of (25-42.5°N, 75-100°W). Gridded analysis over the domain assesses temporal and spatial changes in frequency. It is found that NDJF tornadoes are increasing at a significant pace over the past 62 seasons, much faster than the annual tornado count. Spatially, a bulls-eye of increase in NDJF tornadoes is noted across central and western Tennessee, with positive trends stretching southward along the Mississippi River. Conversely, decreasing trends in NDJF tornadoes are found across the eastern High Plains. Spectral analysis reveals a cyclic pattern of enhanced NDJF counts every 3-7 years, coincident with the known period for ENSO. Statistical tests for time series of ENSO and also the Arctic Oscillation are performed and indicate that tornadic winter seasons tend to occur during La Niña and positive-phase Arctic Oscillation, which can potentially be useful in establishing a seasonal forecast of NDJF tornado activity. Turning to a meteorological perspective, several variables from the NCEP/NCAR Reanalysis are evaluated to assess favorable environments for tornadic cold seasons. It is found that active seasons are characterized by an anomalous mean trough over the western U.S. and ridge in the eastern U.S.; warm and moist conditions across the Southeast, perhaps due in part to low-level jet transport from a warmer Gulf of Mexico; enhanced 1000-500 hPa wind shear; and more instances of adequate CAPE. Finally, an analysis of convective mode of NDJF tornadoes reveals that both linear and discrete tornadoes are plentiful during the winter, although tornadoes from discrete cells are stronger and much deadlier on average. Harnessing all the results from this study provides a much deeper understanding of NDJF tornadoes, an understanding which serves to improve public awareness and ultimately save lives.