Associating Hail Occurrence and Large Scale Environment for the Continental United States 1979–2012

Hail can often be thought of as the poor child of severe storms research. Despite posing the risk of locally incredible damage and a higher frequency of large loss swathes than tornadoes, little attention has been given to developing a relationship between environmental characteristics and hail occurrence. While thermodynamic sources of energy are essential to promote the strong updrafts that support hail, the enhancement of these updrafts via changing wind speed and direction with height (vertical wind shear) is a very important contribution to large hail formation. However, other complex interactions likely on the microphysical scale, moisture loading and structure of the vertical temperature profile can influence the potential for relatively large hail, meaning that indices for other severe phenomena (e.g. tornadoes) may not be representative of the appropriate set of ingredients.

Observations of hail, like those of other severe thunderstorm phenomena suffer from a number of non-meteorological temporal and spatial inhomogeneties. Using observations from the National Climatic Data Center's Storm Data, environmental relationships that describe the probability of occurrence of severe hail are determined using an objective fitting of pre-defined convective variables to a Poisson distribution. This allows an extended climatology of hail to be produced via a hail ‘index'. This index for 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. The index has been found to differ notably from that for tornadoes and provides a reasonable approximation to the spatial and temporal distribution of severe hail (greater than 1 inch) over the continental United States. In addition, the index shows improved performance in capturing the spatial distribution of severe hail as compared to environmental relationships that consider all severe thunderstorm phenomena. Here, seasonal and inter-annual variability of hail occurrence will be presented, along with an analysis of the limitations of the index.