sentences:  Meteorologists knew rain showers and perhaps a thundershower was a good possibility, but were unaware that the helicity, lifted index, and upper level winds were being guided by a strong jet stream with a probable negative-tilt that would create favorable conditions for the development of tornadoes.Just before the noon hour, severe thunderstorms began forming west of downtown Chicago.The first storm started to spawn killer tornadoes in DeKalb and then Kane Counties starting at 12