Torrential Rains and Flash Flooding in Québec's Outaouais Region on June 23-24th 2011

A series of moisture charged thunderstorms affected a number of municipalities along the Ottawa River valley, including the City of Gatineau, Chelsea, Cantley and Pontiac. The first line of thunderstorms happened in the evening of June 23rd and the second line of thunderstorms hit the same region in the afternoon and evening of June 24th 2011. Approximately 250 mm fell over certain sectors causing damaging flash floods. In the Québec SPC we normally follow a forecast guide for torrential rain in the warm sector. This guide looks at factors including the speed of the weather system, the precipitable water moisture ridge, the convective energy profile, the George (K) index, the wind shear, the low and upper level jets speed and direction, and convergence zones. For this case, the results showed that the region should have expected torrential rain events of 25-40 mm for each day, which greatly underestimates the reality of the event. We believe that local effects, including convergence along the Ottawa River valley, a barrier jet effect as well as upslope against the Gatineau hills played a role in enhancing the rain amounts.