An Updated Severe Hail and Tornado Climatology for Eastern Colorado

Climatological trends and characteristics of tornadoes and severe hail are considered for eastern Colorado, which has one of the most interesting and complex severe weather landscapes in the country due to its proximity to the Rocky Mountains and many smaller topographical features that influence weather patterns. The domain of interest (37-41N, 102-105.3W) is characterized by a highly-populated and expanding urban corridor along the foothills, and a vast area of sparsely-populated agrarian and ranching communities east of there. Tornadoes and hail storms are ubiquitous in this region, and in fact northeastern Colorado is one of the most active locations in the country for these hazards. Recent years have seen a number of major severe hail events with billions of dollars in damage and even human injuries, so it is of worth to evaluate the changing nature of these events. Prior to around 1997, eastern Colorado tornado and severe hail reports are more suspect due to rapid population growth, an expanding storm chasing community, lack of reporting standards, and a plethora of field projects. For example, the number of F0 tornadoes falls to zero for most of the 1980s decade, while at the same time F1 tornadoes are documented in high numbers, likely due to a variety of meteorological and non-meteorological factors. After the implementation of Doppler radar in the mid-1990s, tornado and severe hail data across eastern Colorado are more reliable, albeit with some remaining biases. Severe hail reports particularly suffer from population bias, as reports show distinct maxima near the Front Range cities as well as along the few major thoroughfares across the eastern Plains. For the period 1997-2018, eastern Colorado tornado reports and tornado days show slight decreasing trends, and 96% of all tornadoes in the domain have been rated (E)F0 or (E)F1. The relatively low ratings are due to the lack of appreciable structures to damage in the eastern Plains; however, Weld County ranks as the top county in the U.S. for tornado segments passing through it. Severe hail reports and days both show an increasing trend since 1997 independent of size threshold. There is also an increasing proportion of significant (2”+) hail reports across the domain. That is, over time a greater percentage of all severe hail stones are reported as being in excess of 2”. Despite the increasing severity of hail events across eastern Colorado over time, the severe hail season, defined as the number of days between the first and last severe hail report in the domain each year, is decreasing in length thanks to an earlier end to the season. The length of the eastern Colorado tornado season shows a similar trend towards an earlier end to the season over time. This localized approach is able to capture small-scale climatological features that national climatologies can miss, thus enhancing understanding of the current and historical trends and societal impacts of local severe weather hazards.