The Short and Long-term Variability of F2 or Stronger (Significant) Tornadoes in the Central Plains

An analysis of the interannual and interdecadal variability of significant tornado events that occurred over a four state region in the central plains was performed over a 53 year period (1950 - 2002) using the Storm Prediction Center (SPC) archives and simple statistical techniques. A synoptic composite analysis using the National Centers for Environmental Prediction (NCEP) - National Center for Atmospheric Research (NCAR) re-analyses was performed in order to determine whether there was support for the statistical relationships through the large-scale composite dynamics. The results showed that when the 53 year raw annual tornado occurrences are used, there was no statistically significant El Nino-related variability, and El Nino years produced slightly more tornado occurrences. However, when annual tornado occurrences were examined across different phases of the Pacific Decadal Oscillation, there was a likely tendency for more tornado occurrences in El Nino years during PDO2 and no significant interannual variability during PDO1 years. During the 1950 - 1976 period, other studies have found that significant tornado occurrences were overestimated. When a simple correction factor was applied here and the interannual variability re-examined, the strength of the statistical relationships changed such that there was now a likely tendency for fewer tornado occurrences in El Nino years during the PDO1 period, and no statistically significant relationship for PDO2 years. Finally an examination of the composite dynamics during the bulk of tornado season revealed that, out of five years examined, the large-scale flows were of similar dynamic character for four of them. The season which produced the most tornadoes qualitatively was shown to be slightly more conducive to severe weather production when using simple empirical severe weather indexes