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This loop of GOES-8 10.7 micron InfraRed imagery shows the development of convection which spawned the multiple vortex F4 tornado that killed 6 people in Spencer, South Dakota around 01:44 UTC on 31 May 1998. Counties in southeastern South Dakota with preliminary tornado reports (from west to east: Davison, Hanson, McCook, and Turner) are outlined in black, and Spencer is located at the filled black square in extreme western McCook county.
An enhanced-V / warm wake cloud top signature was first evident just north of Davison county on the 00:55 UTC image (about 20 minutes prior to the first tornado in Hanson county, and about 50 minutes prior to the Spencer tornado). Another enhanced-V / warm wake signature was apparent on the 23:32 UTC image in northeastern South Dakota, associated with a storm which produced 1.75 inch diameter hail around 23:30 UTC.
Note how the coldest cloud tops (orange to red enhancement, -60 to -70 C) over northern portions of Davison and Hanson counties appear to build back to the southwest after about 01:30 UTC -- this was a result of a cell merger between the original Spencer tornadic supercell and the new convection which was building just to the southwest. Following this cell merger, the enhanced-V / warm wake cloud top signature became larger and more well defined after 02:00 UTC.