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During the warm seasons (May-September) of 1996-99, Weather Surveillance Radar-1988 Doppler (WSR-88D) data and severe wind reports (either gusts Ͼ25 m s Ϫ1 , 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 [ϳ66% 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.

The evolution of 273 bow echoes that occurred over the United States from 1996 to 2002 was examined, especially with regard to the radar reflectivity characteristics during the prebowing stage. It was found that bow echoes develop from the following three primary initial modes: (i) weakly organized (initially noninteracting) cells, (ii) squall lines, and (iii) supercells. Forty-five percent of the observed bow echoes evolved from weakly organized cells, 40% from squall lines, while 15% of the bow echoes were observed to evolve from supercells. Thunderstorm mergers were associated with the formation of bow echoes 50%-55% of the time, with the development of the bow echo proceeding quite rapidly after the merger in these cases. Similarly, it was found that bow echoes formed near, and moved generally along, synoptic-scale or mesoscale boundaries in about half of the cases (where data were available).The observed bow-echo evolutions demonstrated considerable regional variability, with squall line-to-bowecho transitions most frequent over the eastern United States. Conversely, bow echoes typically developed from a group of weakly organized storms over the central United States. Bow-echo life spans were also longest, on average, over the southern plains; however, the modal life span was longest over the eastern United States. Finally, the supercell-to-bow-echo evolution was most common across the northern plains, but the data sample is too small for this result to be considered significant.

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The local and larger-scale environments of 184 long-lived supercell events (containing one or more supercells with lifetimes ≥4 h; see Part I of this paper) are investigated and subsequently compared with those from 137 moderate-lived events (average supercell lifetime 2–4 h) and 119 short-lived events (average supercell lifetime ≤2 h) to better anticipate supercell longevity in the operational setting. Consistent with many previous studies, long-lived supercells occur in environments with much stronger 0–8-km bulk wind shear than what is observed for short-lived supercells; this strong shear leads to significant storm-relative winds in the mid- to upper levels for the longest-lived supercells. Additionally, the bulk Richardson number falls into a relatively narrow range for the longest-lived supercells—ranging mostly from 5 to 45. The mesoscale to synoptic-scale environment can also predispose a supercell to be long or short lived, somewhat independent of the local environment. For example, long-lived supercells may occur when supercells travel within a broad warm sector or else in close proximity to mesoscale or larger-scale boundaries (e.g., along or near a warm front, an old outflow boundary, or a moisture/buoyancy axis), even if the deep-layer shear is suboptimal. By way of contrast, strong atmospheric forcing can result in linear convection (and thus shorter-lived supercells) in a strongly sheared environment that would otherwise favor discrete, long-lived supercells.