All tornadoes are spawned from a parent supercell, but not all supercells produce tornadoes. Damage from the 1999 F5 tornado that struck Moore, Oklahoma. Photo courtesy NOAA. Dr. T. Theodore ...

As looming thunderstorm clouds spit out baseball-sized hail and torrential rain, a narrow whirlwind of air stretches its way ...

It was tracking a tornadic supercell into Mercer ... it was reclassified as an F5. That scale would later become adopted nationwide as the benchmark for tornadoes.