Severe convective windstorms over the northern High Plains of the United States

During the warm seasons (May - September) of 1996 - 99, Weather Surveillance Radar-1988 Doppler (WSR-88D) data and severe wind reports (either gusts >25 m s(-1), or damage-related reports) over the northern High Plains (NHP) of the United States were analyzed in order to document the primary modes of convection responsible for severe winds. It was found that two-thirds of the convectively generated severe wind reports over the NHP were identified as being produced by organized convective structures rather than by isolated downburst or microburst activity. Specifically, at least 29% of all severe wind reports were produced by bow echoes, 20% by squall lines, 9% by supercell thunderstorms, and 7% by other convective systems not organized in a linear fashion. The occurrence of linear convective storm types that typically produce high winds (i.e., squall lines and bow echoes) were also documented over the NHP during the period of study. It was found that 51% of all squall lines and 86% of all bow echoes were associated with severe surface winds. There was a preference for these storms to initiate near the interface of the Rocky Mountains and the plains [similar to66% formed within 120 km (75 miles) of significant topography], and their typical lifetime was 2 - 4 h. Also of interest, bow echoes had 3 times the number of severe wind reports as severe hail reports, while this ratio was 1.6 for squall lines, and only 0.6 for supercells. The results from these analyses indicate that the nature and evolution of squall lines and bow echoes over the NHP illustrate some differences from similar storms over other regions. Trailing areas of stratiform precipitation were observed to be less common with squall lines over the NHP than other areas. Back-building squall lines were observed less frequently over the NHP, when compared with the southern plains. It was found that storm mergers were associated with the initiation of 41% of the bow echoes and that significant severe wind events occasionally occurred without any linear organization.