Cold-season Tornadoes: Climatological, Meteorological, and Social Perspectives

Tornadoes that occur during the cold season, defined here as November–February (NDJF), pose many societal risks due to their occurrence at a time of year when they are not expected by the general public, their frequency of nighttime occurrence, and their tendency to strike the particularly-vulnerable southeastern United States.   A climatology of all USA (E)F1-(E)F5 cold-season tornadoes within the domain (25-42.5°N, 75-100°W) is developed over the past 62 years (November 1953–February 2015) to investigate their frequency, spatial distribution, and intensity, as well as frequency changes and spatial shifts through time.  Specifically, two consecutive 31-year periods are compared, motivated from a colder temperature regime (Period 1, 1953/54–1983/84) and a warmer temperature regime (Period 2, 1984/85–2014/15).  It is found that Period 2 has seen 444 more NDJF tornadoes than Period 1, with the majority of that increase being attributed to month of November.  Spatially, the largest increase in cold-season tornadoes has occurred across western Tennessee and the largest decrease across eastern Oklahoma.  Spectral analysis reveals a statistically significant cycle of enhanced counts every 3-7 years for almost all months and seasons, which interestingly aligns with the ENSO cycle.  Indeed, La Nina episodes are correlated with cold-season tornado counts, although the relationship is weak at best.  A stronger teleconnection correlation exists with the Arctic Oscillation (AO), which explains 25% of the variance in NDJF tornado counts.  In particular, a positive phase AO is associated with enhanced cold-season counts.  To assess the environmental conditions conducive for cold-season tornado events, several variables from the NCEP/NCAR Reanalysis dataset are investigated.  A composite analysis of the 10 most tornadic seasons/months versus the 10 least active tornado seasons/months is performed, revealing in general a much warmer and more moist environment in the Southeast during most active years, as well as enhanced 1000-500 millibar wind speed shear.  Upper level analysis confirms a favorable pattern for severe weather and tornadoes over the Southeast in the most active seasons, with a large trough in the West and a ridge in the East.  As the key ingredients for potentially destructive cold-season tornadoes continue to be discovered, the hope is for better forecasting and risk communication to help reduce negative impacts.