Assessing the contribution of extratropical cyclones to river floods that caused property damage in Quebec, Canada

Abstract. In the past few decades, the province of Quebec in eastern Canada has experienced widespread and costly flood events, yet the contribution of extratropical cyclones to these floods remains unquantified. This paper presents a methodology for analysing the contribution of extratropical cyclones (ETCs) to 498 fluvial floods that occurred in watersheds in southern Quebec, Canada, between 1991 and 2020, and highlights key characteristics shared by the 200 flood ETCs (storms that contributed to flooding) identified in the study. This analysis combined reconstituted river outputs, government financial aid claims following flooding, ETC tracks, and precipitation data to identify flood events and their relevant ETCs. ETC contribution was defined as the percentage of rainfall during a search window surrounding each flood event that was associated with the relevant ETCs. Most of these floods occurred in the spring (74.7 %). The majority (72.7 %) of flood events had a high (50–75 %) to very high (>75 %) ETC contribution, and only 2.6 % of events (mostly occurring in summer) had a negligible (< 5 %) ETC contribution. Flood ETCs had larger percentages of tracks originating from the Central US & Gulf of Mexico (+30.3 %) and lower percentages of tracks originating from Quebec & Maritimes (–16.7 %) and Western Canada & Pacific Ocean (–12.8 %) compared to non-flood ETCs (storms that did not contribute to flooding but spent at least one hour inside the Baseline Southern Quebec domain, N = 6,027). Flood ETCs also spent on average 18.2 h more in the domain and generally tracked over southern Quebec as opposed to over the Atlantic Ocean along the east coast of North America or over northern Quebec like non-flood ETCs. The percentages of financial aid claims allotted to flood ETCs were highly variable. Just five of the 200 flood ETCs were associated with 51.0 % of all financial aid claims filed during the 30-year period, indicating that a high to very high ETC contribution rarely resulted in widespread flooding. The method developed in this study could be applied to other regions or types of storms analysed (e.g. tropical cyclones and mesoscale convective systems) to further our understanding of the atmospheric causes of flooding and ultimately enhance flood preparedness.