Hail, Tornadoes and the Climate System: Analyzing the impacts of the El Niño Southern Oscillation on Interannual Variability

Each year over the United States, large hail produces in excess of one billion dollars of insured losses. Despite this impact and the risk of locally incredible damage, hail generally receives less attention in severe storms research. While environmental conditions that produce hail are reasonably well understood, these relationships have seen little application to the climatic system. A relationship between environmental characteristics and large hail occurrence has been developed to estimate the monthly frequency of hail occurrence. While thermodynamic sources of energy are essential to promote the strong updrafts that support hail, other factors such as vertical wind shear, interactions on the microphysical scale, moisture loading and structure of the vertical temperature profile can influence the potential for relatively large hail formation. These relationships imply that indices for other severe phenomena (e.g. tornadoes) may not be representative of the appropriate set of ingredients.

Using observations from the National Climatic Data Center's Storm Data, hail and tornado indices that describe the probability of occurrence of events given objective fitting of a Poisson distribution to pre-defined convective variables are derived. These indices describe the monthly climatological likelihood is based on environmental data from the North American Regional Reanalysis (NARR) over the continental United States for the period 1979-2012.

Using these relationships, a link between El Nino Southern Oscillation (ENSO) phase and tornadoes and hail during winter and spring is demonstrated. The phase of ENSO influences United States (US) has long been hypothesized to influence severe thunderstorm occurrence over the United States. 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 tornado season. We show that fewer tornadoes and hail events occur over the central United States during El Niño and conversely more occur during La Niña. These changes in tornado and hail events are reflected in large-scale environmental indices that are correlated with the occurrences of tornadoes and hail climatologically. While the ENSO signal during the spring is weak compared to that in winter, there are changes to the large-scale environmental conditions in both seasons, and strongly statistically significant changes to tornado and hail occurrence. This relationship is used to illustrate how a long-range seasonal prediction of severe weather activity may be possible in seasons preceded by moderate to strong ENSO patterns.