High Streamflow Advisory: South Coast, including:
o Lower Mainland and Fraser Valley
o Metro Vancouver
o Howe Sound
• Vancouver Island
Issued: 05January2009 6:30PM
The BC River Forecast Centre is issuing a High Streamflow Advisory for the South Coast, including Metro Vancouver, the Lower Mainland and the Fraser Valley, Howe Sound, and Vancouver Island.
An intense warm and wet frontal system, with a subtropical moisture feed, is approaching the South Coast. It is anticipated to arrive overnight tonight, bringing rain Tuesday and with heavy rain beginning Tuesday afternoon or evening. Current forecasts indicate the heavy rainfall will persist though the day Wednesday and lasting into Thursday morning. Environment Canada has issued a “Special Weather Statement”, and is anticipating releasing Rainfall Warnings in the next few hours.
Rainfall amounts in the current weather forecast from Environment Canada are very significant, with as much as 80-100+ mm of rain for Metro Vancouver and the Lower Mainland over 36-48 hours, and as much as 100-150 mm for the Fraser Valley (Surrey, Langley, Abbotsford, Mission, Chilliwack, Hope, etc.), and into Howe Sound (Squamish, Pemberton). With the south-westerly flow of this system, mountainous areas along the North Shore (West Vancouver, North Vancouver, Coquitlam, Maple Ridge, etc.) may receive heavier rain. Lesser amounts of rain are anticipated for south and east Vancouver Island (Nanaimo, Parksville, Duncan, Victoria).
Along with the rainfall, temperatures will warm significantly and the freezing elevation through the South Coast mountains will rise to about 2000 metres by early Wednesday, generating snowmelt. It is expected to remain elevated until Thursday. The cold weather of the past three weeks has allowed considerable snow to accumulate down to near sea level along the South Coast. This snow will melt rapidly with the arrival of the warm front. Most of the snow up to 800-1000 metres elevation is anticipated to melt by Thursday morning, adding considerable snowmelt water to rivers and streams.
Despite the presence of snow down to sea level in the South Coast, the accumulation of snowpack in the mountains is well below normal. Current snow measurements at the Chilliwack River automated snow pillow is near its record low level of the past 18 years of record. Some of the higher elevation snow along the South Coast may melt as well, add to flows in rivers.
In addition, the prolonged period of cold weather along the South Coast will have resulted in some areas of frozen soil. This reduces the potential for infiltration of rain and snowmelt water into the soil, and may result in more rapid runoff in some areas.
The magnitude of the forecast rainfall combined with the duration of the rain and snowmelt are will result in significant increases in river levels throughout the South Coast. Small creeks and streams in the Lower Mainland, Fraser valley, North Shore mountains, and along the Sea-To-Sky corridor in Howe Sound will begin to rise by later Tuesday or early Wednesday. They will rise until at least early Thursday, depending on when the heavy rain eases. With the heavy snow accumulations down to low elevation, localized high water and flooding is likely.
Rivers on the lowland areas of the Fraser valley (such as the Nicomekl R., Serpentine R., in Langley and Surrey), will rise quickly Wednesday and may possibly reach flood stage. Tidal rivers such as the Nicomekl River will have peaks coincident with their high tides, near noon on Wednesday and approaching the 4AM high tide Thursday.
Larger rivers such as the Chilliwack, Stave, Coquitlam, Seymour, Squamish, etc. will all begin to rise late Tuesday and will peak no earlier than mid-day Thursday. They are all expected to show significant stage increases.
Lesser rainfall amounts are forecast for Vancouver Island. However, the combination of rain and snowmelt will produce rises in river levels and possibly localized high water and ponding, particularly on the south Island (Parksville to Victoria).
This is expected to be a very significant high flow event.
The River Forecast Centre will provide an update by noon Tuesday, or earlier if necessary.
________________________________
Allan Chapman
Head, River Forecast Centre
BC Ministry of Environment
4th Floor, 395 Waterfront Crescent, Victoria BC V8W 9M2
PO Box 9362 STN PROV GOVT
Tel: 250-387-9472, Fax: 250-356-1202
Website:
http://www.env.gov.bc.ca/rfc/ 
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Topic: High Stream Flow Advisory (Read 7349 times)