Meteorologists use the Fujita scale, sometimes called the F-scale, to rank tornado intensity and damage. The scale runs from F0 to F5, with F0 being the weakest tornado and F5 the strongest. Winds in an F5 exceed 260 mph, capable of debarking trees and sending car-sized objects flying 100 yards or more.

In the recorded weather history of Delaware, New Jersey, and southeastern Pennsylvania, there has never been an F4 tornado, let alone an F5. However, one F5 tornado did hit elsewhere in Pennsylvania, on this date in 1985. The tornado first touched down in eastern Ohio around 630 in the evening, and then moved into extreme western Pennsylvania about 50 miles north of Pittsburgh. The tornado was on the ground for an hour on a path that was a quarter-mile wide, with maximum winds estimated at 300 mph.

Twenty other tornadoes struck Pennsylvania that day, all in the western and northern parts of the state. All told, they caused 65 fatalities, more than 700 injuries, and almost 400 million dollars in damage. By far, May 31, 1985, was the worst tornado day in Pennsylvania history.