Statistical Analysis of Hail-Accumulating Thunderstorm Occurrence along the Colorado Front Range

The Colorado Front Range lies in Hail Alley, the area that receives the greatest frequency of large hail in North America. At the same time, thunderstorms in this region can produce hail accumulations up to 60 cm deep, affecting traffic and people’s safety. Since hail reports and hail research have mainly focused on hail size rather than hail depth, deep hail accumulations have been understudied and reports about hail depth are sparse. To combat this, the Colorado Hail Accumulations from Thunderstorms (CHAT) project began in 2016 and focused on developing a data archive of hail accumulations. Previous research within the project led to the development of a radar-based hail accumulation algorithm which uses radar reflectivity and dual-polarization data to calculate hail depth in real time. Using hail accumulation maps based on operational weather radar in Denver, frequency, spatial and temporal distributions of these events were generated. The frequency of deep hail events was found to be associated with certain months, years, and large-scale locations rather than small-scale locations. While shallow accumulations (< 3 cm) are more frequent, in the Colorado Front Range, deep hail events occur at least 15 times per year. Environmental conditions leading to deep hail accumulations have also been analyzed. The preliminary analysis showed that various environmental conditions change in the direction expected of a supercell storm, but do not change in the magnitude that would be traditionally seen with the formation of such storms. Since deep hail is so frequent in the Front Range, these statistics guide forecasters, first responders, and the general public to be aware of hail’s unpredictability. Hail can occur anywhere in the Front Range and even small accumulations affect local wellbeing.