What caused the mercer county tornado of 1985?

The F5 tornado that struck Mercer County, Pennsylvania was part of the United States-Canadian Outbreak of 1985, which produced 41 known tornadoes. The setup began with unusually hot, humid weather, with temperatures in the mid to upper 80s. Such conditions provide a lot of energy to fuel thunderstorms. However, a cap, or layer of stable air about a mile off the ground, prevented storms from developing. As a result, conditions beneath the cap became increasingly unstable as the energy could not be released. Eventually the cap weakened as a cold front swept though, causing powerful thunderstorms to erupt over parts of Ontario and Ohio and interact with strong upper level winds. The interaction with upper level winds turned these storms into supercells, powerful, rotating thunderstorms that are the most prolific producers of tornadoes. Mechanisms within these storms caused some of the rotation to tighten and intensify into tornadoes. One of most intense supercells of the day produced a tornado that started in Portage County Ohio, and moved east into Trumbull County, striking first in Newton Falls and then causing F5 damage in the Niles area. The tornado then crossed the state line into Pennsylvania and caused additional F5 damage in the Wheatland area before finally dissipating west of Mercer.