Seasonal Predictability of Severe Thunderstorms based on ENSO: Methodology and Evaluation of the 2015 forecast

Relating hail and tornado occurrence to the climate system is an important step on the road to seasonal and sub-seasonal forecasts of severe thunderstorm activity for the United States (U.S.). Hail and tornado indices for the probability of occurrence as a function of convective parameters have been derived using the National Climatic Data Center's Storm Data and environmental data from the North American Regional Reanalysis (NARR) for the period 1979-2012. The value of using these indices, along with carefully controlled observations, to identify links between the climate system and severe thunderstorms will be illustrated using the El Niņo Southern Oscillation (ENSO). The phase of ENSO has long been hypothesized to influence severe thunderstorm occurrence over the U.S. However, limitations in the severe thunderstorm observation record, combined with large year-to-year variability have made demonstrating such a relationship difficult, particularly during spring, the peak hail and tornado season. We show that fewer hail and tornado events occur over the central United States during El Niņo and conversely more occur during La Niņa.

Using this relation, a simple statistical forecast based on the ENSO state of the previous winter can provide skillful probabilistic forecasts for spring severe thunderstorm activity in the central US. In March 2015, this statistical model was used to make a probabilistic severe weather activity seasonal forecast for the 2015 season of 60% chance of normal conditions and 30% chance of below normal conditions. In this presentation, the relationship between severe thunderstorm activity and ENSO will be discussed, along with an evaluation of the forecast for the 2015 season. These results will be discussed in the context of other climate factors that may contribute to enhancing or suppressing seasonal severe thunderstorm activity.