At the beginning of mobile Doppler radar observations, just after 0000 UTC 11 December, the band was a solid, intense, LLAP band. The mobile Doppler radar data reveal the presence of a 1-km deep east-west oriented wind shift within the band during this period. This wind shift moved southward and exited the band within the first two hours of Doppler radar observations, implying that this convergence line was unlikely entirely due to the transverse circulation associated with the band. Dual-Doppler analyses reveal the presence of numerous small vortices along this boundary, likely arising from horizontal shear instability. Vertical motion was also enhanced along this boundary, owing to low-level convergence, so it was likely that vortex stretching further intensified these vortices. Additionally, synoptic and mesoscale observations, along with in-situ thermodynamic observations are presented to ascertain characteristics of this boundary, including its likely type.
After this boundary exited the band, the band became broken and dominated by individual convective elements. This was also coincident with a 500-hPa shortwave trough crossing Lake Ontario and the region entering a regime of differential negative vorticity advection, resulting in a slightly less favorable thermodynamic environment. Following this transition, the number of vortices decreased dramatically, but isolated vortices continued to be observed. A series of dual-Doppler wind syntheses are presented that document the life cycle of one of these vortices. Unlike the earlier vortices, this vortex did not form along a significant preexisting wind shift, but instead was located beneath one of the stronger convective cores in the band, again implying that vortex stretching played a role in its intensification and maintenance.
The band resolidified during the afternoon of 11 December as a secondary 500-hPa shortwave trough approached the region, resulting in a higher inversion height and thus a deeper convective boundary layer. The importance of the wind shift early in the period, as well as the influence of the vorticity maxima on the thermodynamic environment will be discussed to explain changes in band morphology.