Fifteen cases were selected for analysis. The NCEP/NARR gridded datasets, along with data from radar, soundings, and surface observations, were used to construct the analyses. These cases were divided into two classes: pure cases where lake- and sea-breeze convergence zones were primarily responsible for initiating severe weather in the apparent absence of synoptic-scale forcing, and mixed cases where synoptic-scale forcing acted in conjunction with mesoscale forcing from the lake and sea breezes to generate severe weather. The 15-case sample includes a null event where the arrival of marine air from a sea breeze suppressed convection.
Pure cases were characterized by the absence of significant synoptic-scale forcing. Typically, pure cases featured: 1) a ridge axis at the surface or aloft, 2) temperatures (dewpoints) of at least 30°C (20°C) in the planetary boundary layer with ample heat and moisture to provide instability, and 3) weak flow with weak unidirectional wind shear. In contrast, mixed cases typically featured: 1) a trough at the surface or aloft, 2) boundary-layer temperatures ranging from 20°C to 30°C with dewpoints approaching but not exceeding 20°C, and 3) stronger flow and stronger wind shear than in the pure cases. Perhaps the most essential general finding for all cases was the prevalence of multiple synoptic and mesoscale boundary intersections. These boundary intersections served as locations where convergence and lift were enhanced to the point where deep convection was initiated.