Collision of a Pineapple Express with an arctic outbreak over complex terrains of British Columbia, Canada -- Forecast challenges and lessons learned

The interaction of a moisture-laden Pacific southwesterly flow, known as “Pineapple Express”, with very cold air from a strong arctic outbreak during 1 – 5 December 2007 produced a record number of high impact weather events across British Columbia of Canada. Among the weather hazards are bitterly cold wind chills, heavy snow, freezing rain, heavy rain, and strong winds. The unusual collision of two contrasting air masses caused unprecedented forecast challenges for both the Global Environmental Multiscale (GEM) models and meteorologists at the Pacific Storm Prediction Centre (PSPC) of Environment Canada. In this study, the evolution of the weather systems and the observed severe weather events during this storm cycle are analyzed. Weather forecasts by the GEM models and PSPC meteorologists are compared with the observed high impact weather events during the storm. It is demonstrated that meteorologists at PSPC greatly improved the model forecasts by considering various local effects of the complex terrain in British Columbia that the GEM models cannot resolve well. In particular, with warm advection aloft, it is acknowledged that the GEM reginal model tends to underestimate the cold air entrenched in some narrow valleys. The model guidance was therefore doomed to failure at forecasting the widespread and prolonged freezing rain in the midst of the storm.