Changes in flood drivers in British Columbia, Canada

In British Columbia, floods predominantly occur in the late spring and summer months due to snowmelt. However, larger, damaging floods have recently occurred in the fall and winter months, raising the possibility of flood mechanisms varying seasonally. Fall and winter floods often occur after an atmospheric river event, but the weeks and months leading up to the event may play a role in setting up the flood event. For example, the devastating November 2021 floods occurred after a significant heavy precipitation event, but this event followed an extended anomalously wet period prior to the flooding. This study places that flood and others in their historical context, comparing the space-time dynamics of hydrologic conditions leading up the flood events. To identify the role of extended antecedent conditions, we performed a series of model experiments with precipitation and other meteorological variables held to climatology at different lead times. These experiments isolate the relative contribution of an extended antecedent wet condition and of a single, heavy precipitation event in determining a flood event. Such experiments highlight the climate variables of interest when adapting to a potentially changing flood regime due to climate change.