Projecting the End-of-Century Severe Hail and Tornado Landscape across Eastern Colorado Using Synthetic Reports and Pseudo-Global Warming Approaches

Tornadoes and severe hail storms are the two primary severe weather hazards across the Front Range and eastern Plains of Colorado, causing substantial damage and crop losses each year. Understanding how these events may change in frequency and spatial distribution across eastern Colorado is of great worth to the public, insurers, and meteorologists alike. Here, projected changes in tornado and severe hail events are calculated by creating synthetic reports using proxies such as updraft helicity, upward vertical velocity, column-integrated graupel, and Air Force Weather Agency tornado and hail parameters. Gridded values of these proxies over the eastern Colorado are gathered from dynamically-downscaled GFDL-CM3 data by the Weather and Forecasting (WRF) model for control (1971-2000) and future (2071-2100) climate scenarios. Thresholds are computed using the historical tornado and severe hail day climatologies for the control period, and the difference in the number of days in which a respective threshold is exceeded between the periods is calculated. Proxies for each hazard are then averaged, and a gridded analysis gives an idea as to where within the domain tornado and severe hail activity may increase the most. Using this approach, up to 3 additional days of severe hail events and one more day of tornado events are projected across much of northeastern Colorado. Given that approximately six hail events and three tornadoes take place on severe hail and tornado days, respectively, some grid boxes across northeastern Colorado could become noticeably more active by the end of the century. These results are supplemented by an analysis of convective storm populations from high-resolution, 13-year pseudo-global warming (PGW) simulations over the U.S. in which the synoptic conditions are assumed to be unchanging. Occurrences of high-end reflectivity values are expected to increase in the future across eastern Colorado. This is due in part to more concurrent occasions of higher CAPE and higher CIN, which allows for more explosive thunderstorm growth when the inhibition is overcome. For example, for days in the control period in which severe hail was reported, the mean MLCAPE and MLCIN increased by approximately 160 J kg^-1 and 18 J kg^-1 respectively in the PGW runs under the same synoptic conditions. These more “intense” thunderstorms would have a higher potential of producing severe weather hazards such as tornadoes and severe hail. On the other hand, lower-end reflectivities are less prevalent in the PGW simulation. Where projected increases in tornado and/or severe hail events overlap with projected population increases, human risk may rise. While eastern Colorado is selected for its unique topographical influences and being a maximum in both tornado and severe hail events, the approach described can be applied to other localized areas across the country.