in southeastern South Dakota produced a series of tornadoes. The family of tornadoes that crossed the Spencer area was observed by a Doppler On Wheels (DOW) radar (Wurman et al. 1997, Wurman 2001). The DOW observed the tornadoes from before 8:04 through 8:45 pm local time (01:04–01:45 UTC)(Alexander and Wurman 2005) and the passage of a destructive F4 tornado through Spencer itself from 8:37-8:38 (01:37-01:38 UTC). DOW measurements of tornadic winds over the largely destroyed southern portion of Spencer have permitted the first (and only as of December 2006) direct comparison of measured winds with F (or EF) Scale damage ratings as reported in the above referenced articles. Peak observed Doppler winds of near corresponded well with the documented F4 damage. The DOW observations showed that the list of tornadoes derived from damage surveys alone, and the F-scale rating of that damage, may be incomplete and underestimate actual tornado intensity (Wurman and Alexander 2005). Single tornadoes may be mis-characterized as multiple tornadoes due to breaks in the observed damage. DOW measurements suggest that the F4 tornado may have a multiple-vortex structure as it struck Spencer. Confirmed tornadoes May 30 event (South Dakota) May 30 event (Derecho) Spencer, South Dakota The Spencer, South Dakota F4 tornado was the most destructive and second deadliest tornado in the history of the state. It was also the fifth deadliest tornado of the year. It began as a large, dust-cloaked tornado northwest of Farmer, South Dakota in Hanson County, concurrent with the demise of the "Fulton" tornado. Continuing toward the east-southeast, it struck several farmsteads before crossing the Hanson/McCook County line a half mile west-northwest of Spencer. At this time, the tornado was being observed by University of Oklahoma's Doppler on Wheels crew, whose mobile Doppler radar data showed up to 220 mph winds in the tornado only a few decameters above the ground. This tornado carved a wide path directly across the town of Spencer between 8:38 and 8:44 pm CDT, destroying or damaging all but a few houses on the Northeast side of town, and blowing over the water tower. Several homes in town were swept from their foundations by the tornado. The tornado killed six people, injured 150 (which is more than one-third of the town residents), and destroyed most of the town's 190 buildings. Many trees and power lines were downed, and vehicles were destroyed as well. 5 of the 6