The Anomalously Large 2018 Hail Season of Eastern Colorado: A Local Perspective on a National Trend

The National Center for Environmental Information (NCEI) Storm Events Database contains the most complete documentation of severe weather reports for the United States. However, it is well-known that utilizing the data archive for analysis requires great care due to numerous biases and inhomogeneities, both meteorological and non-meteorological. These issues become even more apparent in regional climatologies, as is illustrated here for eastern Colorado. The Front Range and eastern plains regions of Colorado (with a western boundary of -105.3°W longitude for this study) are among the most tornado-prone regions in the country and are also frequently impacted by damaging hail storms. However, assessing the regional climatology of these hazards is muddied by numerous data artifacts. Eastern Colorado averages around 30 confirmed tornadoes per year, 93% of which are of (E)F0 or (E)F1 intensity. An increasing trend in (E)F0-(E)F5 tornadoes is observed since 1953, although there is essentially no trend in the past 20 years. The tornado record contains an anomalous dearth of (E)F0 tornadoes in the 1980s decade, concurrent with a spike in (E)F1 tornadoes, which may be due to a wealth of field projects investigating the Denver Cyclone feature during this decade. An average of 185 severe hail reports per year since 1955 has been observed over the eastern Colorado domain, with an increasing trend over time for all hail sizes, due in large part to population growth as well as efforts from CoCoRAHS to increase the number of hail observations. The continued increasing trend in hail days in the past 20 years is in contrast to the national trend, which is essentially zero. Spatially, severe hail reports are highly connected to major roadways and cities, revealing a strong population bias. In fact, all but one of the eleven eastern Colorado counties with an increasing trend in population since 2000 have a corresponding increasing trend in either hail reports or hail days over the same period. Tornadoes are more evenly distributed across the domain, with the most tornadoes reported in Weld County, which actually boasts the most tornado segments per year of any county in the U.S. It is found that 43% of tornado reports and 66% of severe hail reports are made by trained spotters, according to the Storm Events database. The general public is responsible for many more severe hail reports (12.4%) than tornado reports (6.4%), whereas storm chasers have a much greater reporting contribution to tornado reports (21.4%) than severe hail reports (2.8%). Understanding this regional severe weather climatology is important when doing any statistical analysis of reports, as results can be highly sensitive to the selection of temporal and/or spatial domains. This work also provides a more complete climatology of eastern Colorado severe hail and tornadoes which can be used in studies that explore any future changes in frequency or intensity of these phenomena.