Climatological, Meteorological, and Societal Impacts of Cold-season Tornadoes

Analysis of U.S. tornadoes occurring during the cold season, defined in this study as November–February (NDJF), is of great value due to the enhanced societal risks associated with tornadoes during a time of year when they are generally not expected.  All (E)F1-(E)F5 tornadoes in NDJF from 1953–2015 form a climatology for this study over a spatial domain encompassing roughly the eastern two-thirds of the U.S. (25-42.5°N, 75-100°W), where almost all of the cold-season tornadoes occur.  Two subsequent 31-year periods are selected to assess changes in frequency, intensity, and spatial distribution of cold-season tornadoes.  The early period falls in a relatively cold climate regime (Period I, 1953–1984), and the latter in a warm climate regime (Period II, 1984–2015).  Overall, Period II has seen 444 more NDJF tornadoes than Period I, although individual monthly counts reveal that November and January have become more tornadic, December has seen a decrease in tornadoes, and February counts have remained relatively constant.  Spectral analysis of tornado count data shows a cycle of enhanced counts on the order of 3-7 years for both individual months and seasons.  While this period is similar to the ENSO period, correlations between tornado counts and Oceanic Nino Index (ONI) are weak at best, with La Nina being associated loosely with enhanced cold-season tornado counts.  Other teleconnections such as the Arctic Oscillation (AO) explain more of the variability in the count data.  Spatially, there has been a northeastward expansion of the highest cold-season tornado counts from Period I to Period II, with the greatest increase in counts across Tennessee and the greatest decrease in eastern Oklahoma.  It is also of worth to investigate environmental conditions associated with cold-season tornadoes, especially to compare with the known favorable tornadic conditions during spring and summer months.  Using the NCEP/NCAR reanalysis to compare the most tornadic and least tornadic cold seasons, it is found that active seasons are characterized by a large trough in the west; warm and moist conditions across the Southeast, likely due to an enhanced low-level jet transport from the western Gulf of Mexico; and enhanced 1000-500 mb wind speed shear.  With continued research into the meteorological parameters and patterns responsible for destructive cold-season tornado conditions, communication of risk to the general public can be improved and negative impacts from such tornadoes mitigated.