 The setup was similar along the long cold front, and the hot, humid air mass led to the damaging derecho, although the stronger derecho that affected areas well north of there formed much earlier that day in Upper Michigan just before noon local time.It had a clearly defined bow echo (squall line) formation, unlike these cells which were more fragmented.Relevant to the atmospheric dynamics at this time is the fact that, according to the NWS forecast discussion from a number of stations, a terrestrial subtropical storm had developed over Iowa and was moving slowly northeast—apparently this was a low pressure area devoid or almost devoid of extratropical characteristics.