This animation toggles back and forth between the base-reflectivity and storm-relative velocity images of the Spencer supercell, at 8:33 pm CDT (0133Z). At this time, the tornado is near these farms in extreme eastern Hanson County, about to cross into McCook County and hit Spencer. Note the pronounced hook echo on the SW side of the supercell in the reflectivity frame. In the velocity frame, note the red triangle and tan circle. Those respectively signify the Mesocyclone (MESO) and Tornado Vortex Signature (TVS) alarms in the Sioux Falls WSR-88D. Image courtesy NWS Sioux Falls SD.