PECAN: Characteristics of Potentially Severe Wind Producing Nocturnal MCSs

The evolution of transitioning and mesoscale convective systems as the nocturnal stable boundary layer (NSBL) develops is not well understood. These systems can produce severe surface winds, but the mechanisms through which intense-wind-causing downdrafts reach the surface in the presence of a NSBL remain uncertain, complicating the forecastability of severe wind events. Moreover, strong winds often are observed aloft by radar, but predicting when or if those winds will reach the surface remains an operational forecast challenge.

Important to understanding severe surface wind production is the resultant characteristics of the leading edges of these systems and storm-generated cold pools and their relationship to the environment. Hydrometeor type, distribution, and evolution within these mesoscale convective systems (MCSs), as well as the evolving thermodynamic properties of the NSBL and environmental shear likely play important roles in the initiation, maintenance and cessation of intense, surface-reaching downdrafts.

During the PECAN field experiment (1 June – 15 July 2015), several marginally severe wind-producing MCSs were observed, despite high probability forecasts of severe winds. On 12 July 2015, the PECAN armada deployed several mobile radars in front on an MCS in Minnesota, allowing for multi-Doppler synthesis of the evolution of a forecasted severe wind-producing MCS. Using combined multi-Doppler, dual polarization, mesonet, and sounding analyses, we will discuss why only marginally severe winds reached the surface. Comparisons of cold pool characteristics and leading edge storm structure for several other potentially severe wind-producing MCSs observed during PECAN will allow for additional insights to the critical properties necessary to produce severe surface winds in the nocturnal environment.