May 27, 2012 was an unusually hot and sticky spring day. A severe thunderstorm rumbled over London and south across Lake Erie toward Ohio just after 1 p.m.

The mercury topped out at 31C and skies quickly cleared. Beaches near Cleveland came alive with sun-seekers and swimmers. It was the Memorial Day long weekend.

But by late afternoon the weather office was fielding urgent telephone calls from alarmed citizens.