The derecho of July 5, 1999 in Quebec: a rare event

During the night of July 5, 1999, severe thunderstorms affected most of southern Quebec. A derecho was moving at 100 km/hr causing considerable destruction on its path. By definition, a derecho is any rapidly moving extratropical convective system that produces downbursts causing significant and widespread damage. It has some of the features of a mesoscale convective system (MCS), as well as some of the features of a squall line, yielding radar a critical role into identifying a detailed structure of the event.

This case study documents the evolution of the derecho and reviews the dynamic and thermodynamic factors leading to its formation. Furthermore, it gives a deeper insight into the associated shear pattern that is conducive to such a destructive event. Finally, as it is often the case with any significant severe event, it enhances the particular combination of parameters that makes the formation of derechos possible.