Rain warnings in B.C. as strong atmospheric river heightens flood threat
Even though fall just began on Sunday, B.C.'s pattern for this week will be more reminiscent of the end of the season rather than the start
A high-end atmospheric river will sweep into British Columbia’s North Coast this week, prompting rainfall warnings with the threat of localized flooding.
The powerful event could qualify as a Category 5, the highest on the ranking scale. Between 150-300 mm of rain is expected in some of the harder-hit areas of the north and central coast, with the heaviest of the rainfall rates likely through the day on Monday.
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Areas that experience the heaviest rainfall this week could see landslides and power outages, as well as pooling and ponding of water on roadways. It will be important to stay up-to-date on the latest warnings in your area.
Monday through Wednesday:
Moisture from southeastern Asia is the source region for the developing atmospheric river, and anyone who has travelled to Japan during the summer months knows how oppressive weather conditions are there.
Atmospheric rivers (ARs) are, for the most part, what you might imagine from the name — rivers of moisture in the sky. They’re long, narrow bands in the atmosphere that carry large amounts of water vapour, usually from the tropics where atmospheric moisture is abundant.
Heavy persistent rain is expected for the central and northern coast, where 150-300 mm of rain is expected. Cities like Bella Bella and Bella Coola are forecast to see the lower end of that, but places in the mountains could see upwards to 300 mm. Over the weekend Bella Bella had already picked up 155.5 mm.
It’s a 48-hour affair as the stream of moisture stalls across the north, thanks to a stubborn ridge of high pressure situated across southern B.C.
Heavier rain is forecast for Prince Rupert as the trajectory shifts north, likely dumping more than 100 mm of rainfall on the community.
Precipitation rates are expected to reach and even sometimes exceed 10 mm/h. That may not seem impressive for the region, but it's the length and duration of the system that matters. If 10 mm/h falls over a 24 hour period, that could add up to 240 mm quick.
High pressure protects the Lower Mainland and southern Vancouver Island from the heavy rain until the very end of the atmospheric river. It will bring periods of rainfall on Wednesday and more unsettled weather into Thursday as the storm track slumps south.
In regions of the heaviest rain, landslides and power outages are threats while pooling and ponding of water on roadways are possible this week.
SEE ALSO: Atmospheric Rivers: Expert explains the good and bad of these extreme storms
Forest fire burn scars are sensitive to the risk for flooding, increasing the threat of soil failure and impacts.
Thursday through Friday:
The active pattern brings another system to coastal B.C. late this week, with higher confidence of the storm track shifting toward the South Coast with a period of unsettled and potentially stormy weather.
This storm will feature a deeper area of low pressure, increasing the threat of strong winds along the immediate coastline of the province.
Cooler temperatures are expected with the late week system, as well, which could bring heavy snow to the peaks of coastal mountains.
Overall, however, a drier-than-normal pattern will dominate for the South Coast region through the end of September and into the beginning of October, as well.