A case study of heavy rainfall in western Canada on 1-2 July 2020 and the key role of impact-based thresholds in weather warnings

Abstract:

Canada Day celebrations were a washout for many communities in the province of British Columbia (BC) as a moisture-laden low pressure system delivered significantly heavy rainfall. Initially tapping into a hot and humid air mass from the Gulf of Mexico and furthered fueled by embedded convection, 70 to 140 mm of rain pummeled the northeast corner of BC over 1-2 July 2020. Widespread rainfall amounts of 20 to 65 mm also fell over central and southern parts of the province as the moisture plume split into two tracks.

This event was significant in and of itself, delivering 48 to 96% of July’s normal monthly precipitation to eastern and central BC communities in a mere two days, but also took place within the context of an already vulnerable river system situation. Winter 2020 brought very high (>120% of normal) snow packs to interior BC. The following spring was relatively cold and resulted in a delay of upper-elevation snowpack melt by several weeks, leading to river flows and lake levels high relative to normal in early summer. Additionally, an active weather pattern in June brought wetter than normal (118 to 143%) conditions to the eastern and central regions of the province.

A drastic increase in rainfall amounts forecast two days prior to the event prompted coordination calls between Emergency Management Organizations, the BC River Forecast Centre, and the Meteorological Service of Canada. In conjunction with these partner agencies, hazardous weather alerts were issued to a far broader swath of communities than operational rainfall warning thresholds would have allowed. This presentation will provide a description of the storm and the challenge of issuing warnings based on both impacts and climatologically-driven rainfall thresholds. The outcome of this seasonally significant rainfall will also open a discussion on impact-based considerations for quantifying future weather events.