The 25 May 2008 Hugo, Minnesota EF-3 Tornado: supercell tornadogenesis in the presence of an apparently cold rear flank downdraft

During the afternoon and early evening of 25 May 2008, a long-lived supercell tracked from east central Minnesota into west central Wisconsin. The storm produced several tornadoes along its path, one of which resulted in EF-3 damage in the city of Hugo, MN. Very large hail, from golf ball to softball size in diameter, was observed along the path of the storm for nearly four hours. The intensity of the storm was not surprising given the environment, which was characterized by strong deep layer and low level shear, and abundant instability. However, as the storm matured north of the Twin Cities metro area there were indications in radar and surface mesonet data that the rear flank downdraft (RFD) associated with the storm was markedly cooler than the storm's inflow environment. A distinct boundary and strong wind speeds were observed in the radar data in association with the RFD, and temperature perturbations of 5-8°C were apparent in the surface observations. Several tornadoes were produced by the storm during this time, including that which resulted in EF-3 damage in Hugo.

An overview of the synoptic- and mesoscale environment over the area on 25 May 2008 will be presented to provide some background on the expectations for convective initiation, mode, and evolution. The evolution of the storm, from the perspectives of the Chanhassen 88-D (KMPX) and Minneapolis/St. Paul Terminal Doppler Weather Radar (TDWR) radars, will be shown in detail as it tracked north of the metro area. A review of radar and surface mesonet data shows not only the presence of a cool RFD, but also a west-east oriented warm frontal boundary, which is believed to have played a crucial role in tornadogenesis in this case. Given the presence of a thermodynamically stable RFD, a non-supercell or hybrid mode of tornadogenesis likely occurred. The precise dynamical role played by the west-east frontal boundary is unclear given the available data, but it seems to have been instrumental in leading to tornadogenesis in a situation where it may not have otherwise occurred. Another interesting aspect of this case was that the RFD was observed to be much cooler than expected given common proxies, such as lifted condensation level, used to anticipate the potential thermodynamic character of the RFD. An examination of surface observations and available upper air data will be utilized in an effort to understand why this may have been the case.