Meteorological Synopsis
The first severe weather of the outbreak was reported at 12:30 pm in southeast Montana. Several hours later a supercell thunderstorm produced 23⁄4 inches (7.0 cm) hail across southeast Montana, kicking off the outbreak in earnest. Numerous reports of very large hail were received throughout the outbreak with the largest official report of 3 inches (76 mm) 10 miles (16 km) north of St. Lawrence in east-central South Dakota. The hail itself produced thousands of dollars in damage. Many reports of severe straight-line winds and damage were also reported. Numerous storm chaser reports suggest that significant severe weather events also occurred in the sparsely populated area traversed by the storm.
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 tornado or tornadoes from before 8:04 through 8:45 pm local time (01:04-01:45 UTC) (Alexander and Wurman 2005) and the tornado's passage through Spencer itself from 8:37-8:38 pm (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 12/2006) 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 115 m/s (258 mph) 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 tornado may have a multiple vortex structure as it crossed Spencer.
Read more about this topic: Late-May 1998 Tornado Outbreak And Derecho