The BC River Forecast Centre has issued a high streamflow advisory for parts of the South Coast, including Howe Sound.
An atmospheric river is expected to impact the BC Coast beginning on Friday of this week, with an extended period of heavy rainfall expected throughout the region.
The heaviest rainfall is expected on the outer coast of Vancouver Island and along the North Shore Mountains and north side of the Fraser Valley through to Howe Sound and the Sunshine Coast.
Storm total rainfall amounts in the wettest areas may exceed 150-200 mm, with lower elevation and more protected areas expecting amounts in the 50-100 mm range. The source of the atmospheric river is from the sub-tropics, and in addition to precipitation, temperatures are expected to warm, with freezing levels pushing above 2000 m by late-Friday (January 31, 2020).
Rivers are expected rise through Friday and into Saturday in response to this rainfall. Current hydrologic modelling is indicating the potential for flows to reach or exceed 5-year flows; this advisory may be upgraded to a flood watch on Friday if there is more certainty of reaching these river levels.
While some snowmelt is expected during this event, snowpack at higher elevations is deep enough that it will provide some absorption of the added rainfall and buffer the impact of the rainfall on runoff. At mid-elevations (300-800m) some recent snow accumulation is expected to melt with the “rain-on-snow” and may add a modest addition of runoff.
While rainfall amounts are forecast to be lower in inland areas of Vancouver Island, persistent wet weather over the past few lakes has led to high lake levels on Cowichan Lake and Sproat Lake; outflow from these lakes is expected to exceed levels that have be experienced so far this season, and high streamflow is expected downstream in the Sproat and Somass Rivers and the Cowichan River.
The public is advised to stay clear of the fast-flowing rivers and potentially unstable riverbanks during the high-river-level period.
The River Forecast Centre will continue to monitor and provide updates as conditions warrant.