Multiple systems threaten to soak B.C. coast with excessive rainfall totals
The first of multiple, moisture-infused systems will usher in fall in B.C. this weekend, bringing up to 100 mm for parts of the coast. Flooding concerns will arise in some areas
Even though fall will just begin this weekend, B.C.'s pattern will be more reflective of late season rather than just the start of it.
The ridge that has helped keep coastal B.C. dry for a few days is finally breaking down, allowing for not one, but several systems to push in to the region over the next week. The setup will include multiple atmospheric rivers. Rainfall totals will be quite high through next week, leading to elevated concerns over flooding in some areas.
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The first bout of the heavy rainfall in the central and northern sections of the coast could lead to accumulations of 100 mm or more through Sunday for the harder-hit areas.
The southern parts of the province, though, will remain dry to start the weekend, but temperatures will be a degree or two on the cool side of seasonal. By Sunday, there's a chance for rain showers to reach the Lower Mainland.
Through Sunday:
Heavy rain pushed into British Columbia late Friday, and will impact much of the central and northern coast through the weekend. The set up will be considered a weak atmospheric river (AR).
Upwards of 100 mm of rainfall is expected for the central and northern coast, and Haida Gwaii, including the communities of Bella Bella and Bella Coola. Rainfall totals over the 24-hour period may exceed 100 mm, a threshold for flooding impacts in the area.
High pressure protects the Lower Mainland and southern Vancouver Island for most of the weekend until Sunday, when there is the chance for a couple of showers.
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 the conditions are.
The 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.
Heavy rain with upwards to 150 mm is expected for the northern coast and Haida Gwaii; with lesser amounts for Bella Bella and Bella Coola compared to the weekend system. Heavier rain is forecast for Prince Rupert as the trajectory shifts north, likely dumping close to 100 mm of rainfall for the community.
High pressure protects the Lower Mainland and southern Vancouver Island until the very end of the atmospheric river, allowing for periods of rainfall to push in on Wednesday.
SEE ALSO: Atmospheric Rivers: Expert explains the good and bad of these extreme storms
Atmospheric rivers are, for the most part, what you might imagine from the name — rivers in the sky. They’re long, narrow bands in the atmosphere that carry large amounts of water vapour, usually from the tropics where water and atmospheric moisture are abundant.
Thursday through Friday:
The active pattern brings another system to coastal B.C. late next week, with higher confidence of the storm track shifting towards the South Coast. That will bring a period of unsettled, and potentially stormy, weather.
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.