Analyses of a Severe MCS and Tornadic Mesovortex Observed by PECAN on 5–6 July 2015

Understanding and forecasting nocturnal thunderstorms and their hazards remain elusive goals. To this end, an expansive array of fixed and mobile observing systems were deployed in the summer of 2015 for the Plains Elevated Convection At Night (PECAN) field experiment to intercept and observe nighttime atmospheric phenomena. During the night of 5 July 2015, an array of eight mobile radars and numerous ground-based surface and upper-air profiling systems directly sampled a severe mesoscale convective system (MCS) as it moved through southeastern South Dakota. The MCS was responsible for several severe wind reports, including one over 80 mph, and produced an EF-0 tornado near Dolton, SD.

In this study, observations of these phenomena from mobile radiosonde vehicles, Doppler radars, and aircraft are assimilated into an ensemble analysis and forecasting system to analyze this event. All ensemble members simulated low-level mesovortices within the MCS in a complex environment very similar to those observed by the PECAN platforms. Forecasts from select members were analyzed to examine the processes leading up to the development of the vortices. In all, a supercell-like mechanism appeared responsible, with vertical vorticity initially developing via tilting in baroclinic downdrafts and then undergoing intense stretching near the surface to form the low-level mesovortex.