High Streamflow Advisory
September 23, 2020
The River Forecast Centre is issuing or maintaining a High Streamflow Advisory for:
- Central Vancouver Island
- Eastern Vancouver Island
- West Vancouver Island
- North Vancouver Island
- Sunshine Coast
- Howe Sound and Sea-to-Sky including the Lillooet River
- North Shore Mountains
- Fraser Valley tributaries including the Chilliwack River
A series of storms is expected to impact the British Columbia coast over the next several days. The first system has begun and will continue into Thursday, with following systems passing again later Friday and into the weekend.
Environment and Climate Change Canada has issued a Special Weather Statement for the region, with forecasted rainfall totals for the first storm in the 25-40 mm range for Vancouver Island, with amounts exceeding 100mm over West Vancouver Island, and 30-50 mm for the South Coast with higher amounts in the mountains. Precipitation amounts already observed (to 1400hr Wednesday September 23) have been in the 20-100 mm range on West and North Vancouver Island, 15-55mm in Central (inland) and Eastern Vancouver Island, and 20-75mm in the Sunshine Coast, Howe Sound, North Shore Mountains and Sea-to-Sky corridor.
These storms will mark a significant shift in streamflow runoff conditions which have been dominated by drier summer patterns for the past several weeks. Rivers are expected to rise rapidly in response to rainfall on Wednesday, with peak flows from the first storm expected late-Wednesday or Thursday, with larger rivers expected to reach peak levels later on Thursday or into Friday. Current hydrologic modelling is indicating the potential for streamflow in the 2-year to 5-year flows in some areas which can lead to localized issues and minor flooding.
Additional storm cycles into the weekend will bring additional periods of high streamflow; advisories will be updated to reflect anticipated conditions as we approach the weekend.
The River Forecast Centre continues to monitor the conditions and will provide updates as conditions warrant.
A High Streamflow Advisory means that river levels are rising or expected to rise rapidly, but that no major flooding is expected. Minor flooding in low-lying areas is possible.