Potential for 300 mm of rain, flooding in parts of B.C. amid atmospheric river
A high-end atmospheric river sweeping into British Columbia’s North Coast this week will bring extreme rainfall totals, increasing the threat of landslides and localized flooding.
A multi-day, excessive rainfall affair is underway along parts of the coastline of B.C. as a stream of moisture stalls across the northern section, thanks to a stubborn ridge of high pressure situated across southern areas of the province.
The atmospheric river could end up being classified 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 northern and central areas of the coast, when all is said and done this week.
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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.
Through Wednesday:
Moisture from southeastern Asia is the source region for the 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. A total of 50-100+ mm of rain has already fallen in the North Coast, but this is only half of the event as Tuesday will also be rather wet.
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 of rainfall an hour falls over a 24-hour period, that could add up to 240 mm quickly.
High pressure protects the Lower Mainland and southern Vancouver Island from the heaviest precipitation until the very end of the atmospheric river, which will bring periods of rainfall Tuesday overnight and into Wednesday.
Wednesday could feature embedded thunderstorms, especially as a cold front moves into the Interior by the afternoon.
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.
Cooler temperatures are expected with the late-week system, 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.