Why was the tornado in Goderich so bad?

The Goderich, Ontario tornado of August 21, 2011 was notably bad because of its intensity. Most tornadoes achieve an intensity no higher than F1 (on a scale ranging from F0 to F5), capable of causing severe roof damage and destroying some weak structures. The Goderich tornado was an F3, unusually strong for Canada. An F3 tornado is capable of ripping roofs and walls away from sturdy structures and tossing car-sized objects like toys.