Powerful atmospheric river threatens steep rains across B.C.
A high-end atmospheric river will sweep into British Columbia’s North Coast this week
Even though fall just began, B.C.'s looming pattern will be more reminiscent of the end of the season rather than the start.
The potent setup will include multiple atmospheric rivers, including a powerful event that could qualify as a Category 5, the highest on the ranking scale. Rainfall totals will be quite high through this week, leading to elevated concerns over flooding in some areas.
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Rainfall totals could exceed 200 mm this week for hardest-hit areas, courtesy of a plume of moisture from southeast Asia. Areas that experience the heaviest rainfall this week could see landslides and power outages, as well as pooling and ponding of water on roadways.
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.
This next AR is forecast to bring heavy rain to northern and central areas of the coast. This particular AR will be stronger than the previous event, but it will track a bit farther north.
Upwards to 200 mm of rain is expected for the northern coast and Haida Gwaii, with lesser amounts for Bella Bella and Bella Coola compared to the weekend system. 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.
High pressure protects the Lower Mainland and southern Vancouver Island once again 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.
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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.
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.
The wettest city in Canada is Prince Rupert, B.C., which averages more than 2,500 mm of yearly precipitation. British Columbia is a province of weather extremes. Just 800 kilometres southeast of Prince Rupert is Canada’s driest city, Kamloops, which averages under 300 mm of precipitation annually.
In other words, it takes Kamloops roughly eight years to accumulate what Prince Report records in a single year.
Prince Rupert’s wettest September on record occurred back in 1908, tallying 517 mm, while the driest September on record was 1929, amassing just 26.4 mm.