This tornado caught us somewhat by surprise. Instead of being spawned by a supercell of some sort, it developed near what was then the tail end of a squall line, as the line was "backbuilding" southwestward through constant formation of new cells. This process is called discrete propagation, and if the environment is just right, can spawn tornadoes on rare occasion. The most notorious example of a tornado from discrete propagation or backbuilding was the F5 Jarrell, TX tornado of 27 May 97, which formed that way in a storm that became a supercell as the tornado was in progress. Jarrell was the most extreme example, however. Most discrete propagation events produce no tornadoes at all, and those which do form usually are not even as well developed as this one. Another cell, forming just to its SW, already was dumping rain around the tornado, which disappeared less than a minute later.
3 NW Duncombe IA (11 Jun 4) looking SW.