Weathering the storm – Hydrologic Impacts of Extreme Storms in Coastal British Columbia

Abstract:

Atmospheric rivers (ARs) are strongly associated with extreme precipitation and flooding risk on the West Coast of North America. This includes the west coast of British Columbia, Canada, in which the occurrence of ARs and storms caused by atmospheric moisture transported from tropical latitudes drive the fall and winter flood season. However, the connection between storm intensity (measured by metrics including integrated vapour transport and precipitation rate and duration) and hydrologic effects depends on site and storm-related factors that ultimately determine the severity of impacts such as flooding. This study uses a case-study approach to investigate the connection between storm intensity and hydrologic outcomes within British Columbia. These case studies, selected according to storms that resulted in significant hydrologic impacts, are used to firstly investigate the relative importance of factors that contributed to observed extreme flows, including storm-related factors (storm and wind direction, temperature, and precipitation intensity and duration), spatial factors (antecedent soil moisture conditions, slope and elevation), as well as hydrologic response during the storm (storm precipitation partitioning between rain and snow, and the occurrence of rain-on-snow events). By examining specific storm events known to have produced extreme, flood-relevant hydrologic effects, this study provides an initial examination of critical drivers of extreme hydrology within the context of coastal British Columbia. Additionally, aligning these case studies to nascent storm classification systems (including an ARs scale that utilizes integrated vapour transport and precipitation rate and amount, as well as a SuperStorm extratropical cyclone scale recently developed for southwest Canada), contextualizes such evolving forecasting products according to potential extreme impacts that can threaten human life and property.