The Forcing Mechanisms for a Mesoscale Windstorm in the Vicinity of Metro Vancouver, Canada

A destructive windstorm hit Metro Vancouver and the Sunshine Coast of British Columbia, Canada in the late evening of 28 June 2015. The storm knocked down trees and power lines, causing more than 10,000 electrical power outages. The most severe damage was located near the foot of North Shore Mountains of Metro Vancouver. However, strong winds were not observed due to the absence of meteorological stations in this area. Only an automatic weather station in a fjord between Metro Vancouver and the Sunshine Coast (Howe Sound) reported winds that rose to gale force and persisted for eight hours. Detailed reanalysis was made of the nature of this mysterious windstorm. It was found that the pre-storm boundary layer was very dry, and the storm was initiated by the down-valley drainage of air diabatically cooled from precipitation melting and evaporation; the precipitation aloft was caused by a mesoscale vorticity tube moving across the area. As the disturbance brought the precipitation into the interior, the diabatic cooling effect created a meso-high due to cold-air damming on the eastern side of the Coast Mountains. The resulting cross-mountain pressure gradient was favorable to maintain the strong northerly outflow in Howe Sound. The similarity of the outflow to a hydraulic analog was investigated. Comparison of different numerical weather prediction model outputs provides supporting evidence for these dynamic and thermodynamic mechanisms. The potential of using high-resolution models to predict such down-valley mesoscale windstorms was also examined.