Streamflow runoff conditions are shifting due to the heavy rainfall soaking communities across the south coast.
A high streamflow advisory was issued by the River Forecast Centre on Sept. 23 when the first major rainstorms of the season kicked into high gear.
Localized flooding could be seen in some low-lying areas like Fraser Valley tributaries or the Chilliwack River, by Thursday or Friday, as river levels start to rise rapidly.
“These storms will mark a significant shift in streamflow runoff conditions which have been dominated by drier summer patterns for the past several weeks,” according to the advisory from the B.C. River Forecast Centre.
“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.”
Hydrologic modelling shows the potential for streamflow levels to reach the 2-year to 5-year flows in some areas which could lead to localized issues and minor flooding.
The first major rainstorms brought heavy rain in the 25-40 mm range for Vancouver Island, with amounts exceeding 100 mm over West Vancouver Island, and 30-50 mm for the South Coast with higher amounts in the mountains. Precipitation amounts for Sept. 23 were 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.
“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 is issuing or maintaining a High Streamflow Advisory for:
- North Shore Mountains
- Metro Vancouver
- Fraser Valley tributaries including the Chilliwack River
- Central Vancouver Island
- Eastern Vancouver Island
- West Vancouver Island
- North Vancouver Island
- Sunshine Coast
- Howe Sound and Sea-to-Sky including the Lillooet River
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