The Short-and Long-Term Variability of Selected Midwestern Tornadoes

Due to several factors, including geographical location and a wide range of terrain types, the state of Missouri presents many predictive challenges for the modern-day operational forecaster. Many recent studies have shown that Missouri's weather regimes exhibit interannual variability that can be related to the El Nino and Southern Oscillation (ENSO). This study will focus on the long-term trends in the occurrence of, and the interannual variability in, significant tornadoes throughout Missouri and neighboring states. The climatology of significant tornadoes and tornado days were compiled for these regions from 1950 - 1999 in order to examine the issues outlined above. Initial results show that there was a statistically significant downward trend in the number of tornado days. This trend counters the upward trend in tornado activity nationally, which is due in part to better observational methodologies. Also, as was found for other characteristics of the climate of Missouri, significant ENSO related variability in tornado occurrences was found. It was found that there were significantly more tornadoes and tornado days during La Nina years, than during El Nino or Neutral years. In examining this issue further, it was determined that the more active La Nina years are a function of an increase in the number of tornado outbreaks rather than the occurrence of larger outbreaks. Finally, the issue of longer-term interannual variability is examined in order to determine if ENSO-related variability is modified by interdecadal variability such as that related to, for example, the Pacific Decadal Oscillation.