The role of boundary interaction and augmented low level shear in the formation of a supercell in an otherwise “hostile” deep layer shear environment: Denver Interntional Airport Supercell of 24 May 2009

On the afternoon of 24 May 2009, operations at Denver International Airport were interrupted by an approaching supercell thunderstorm (hereafter referred to as the Commerce City storm). Tornado warnings were issued for the airport and all runways were cleared. The Commerce City storm was unanticipated in either local NWS forecasts or in SPC outlooks. Although, that day, a weak Denver Convergence and Vorticity Zone (DCVZ) was expected to develop, the deep layer shear environment was weak as were the middle and upper tropospheric winds (<20 knots) and not favorable for supercell convection. By mid-afternoon, moderate buoyancy (e.g., surface based Convective Available Potential Energy values ~1200 J/kg) and little convective inhibition had spread into the eastern and southern portions of the DCVZ, with storm initiation occurring scattered over the area. Post analysis of the event indicated that the Commerce City storm formed near the intersection of an outflow boundary with a pre-existing surface boundary along and near Interstate 70. We believe this boundary interaction had three roles in the formation of the storm and its evolution from multicell to a supercell: first, the intersection provided parcel lift and a focus for storm development; second, the low level wind field associated with the outflow boundary provided storm relative flow favorable for supercell convection; and, third, the horizontal vorticity associated with both boundaries was tilted into the updraft as the storm propagated along the boundary producing substantial cyclonic updraft rotation. Thus, despite the fact the storm developed in an environment that appeared semi-hostile to foster and maintain supercellular development, the boundaries mitigated the effects of the miniscule deep layer shear. This case illustrates the importance of correctly assessing storm motion and low level boundary interaction in forecasting supercell convection.