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Eastern Canada Flooding 2017 and its Subseasonal Predictions

ABSTRACT Severe damage was caused by a flooding event across eastern Canada during the first week of May 2017. Thousands of residences were affected, and many people were evacuated from their homes in southern Quebec and eastern Ontario. This event was mainly caused by persistent heavy rainfall during that week. In this study, the ability to make a useful prediction of this flooding event about two weeks in advance is assessed for 11 subseasonal-to-seasonal prediction models. It is found that the above-normal precipitation in eastern Canada during the week of 1–7 May was predicted by most of the models a few weeks in advance although the forecast anomaly was, in general, weaker than observed. These models also predicted a high probability of extreme precipitation. Analysis of the atmospheric circulation pattern associated with the flooding event revealed a wave train of 500 hPa geopotential height anomaly along mid-latitudes from the North Pacific across North America to the North Atlantic, which sets up a favourable environment for strong water vapour transport from the Gulf of Mexico and the western Atlantic to eastern Canada. Most models were able to predict this wave train. We found that this flooding event is likely connected to the tropical Madden–Julian Oscillation (MJO) through atmospheric teleconnections. During the week of 24–30 April the MJO was observed to be in phase 7 with enhanced convection in the western-central Pacific. A numerical experiment was conducted using a linear model with specified tropical diabatic heating similar to MJO phase 7. The resulting 500 hPa geopotential height response has many similarities to the observed wave train that was responsible for the flooding event.