Unusually strong Category 5 atmospheric river hits Alaska and British Columbia

A severe Category 5 atmospheric river (AR) — associated with an area of low pressure and a trans-Pacific moisture plume — swept through the Gulf of Alaska last weekend, bringing days-long heavy rainfall to coastal British Columbia, Canada, and southeastern Alaska.

Because of its duration and the concentration of moisture moving across the ocean, experts suspect this was among the most intense atmospheric rivers to transit the northeast Pacific in a satellite-based record going back to 2000.

The AR developed within the trans-Pacific moisture plume south of the intensifying low-pressure system, eventually making landfall over Southeast Alaska and British Columbia on September 22. AR 4 – 5 conditions were observed in southern Southeast Alaska and coastal British Columbia.

The heaviest precipitation was recorded in the Coast and Hazelton Mountains in British Columbia and Glacier Bay National Park, Alaska, with some locations receiving more than 150 mm (6 inches) of rain. This heavy rainfall caused minor riverine flooding in the Telkwa River, British Columbia, where the flow exceeded the 20-year return interval. In the southern region of Southeast Alaska, rainfall ranged between 25 – 75 mm (1 – 3 inches), corresponding to roughly a 1 to 2-year return interval.

In the town of Bella Bella, British Columbia, between 50 and 100 mm (2 and 4 inches) of rain fell each day from September 21 to 24, with up to 100 mm (4 inches) more falling in the following days.

This atmospheric river was powered by the Arctic Oscillation’s intense negative phase, an uncommon September situation that can increase atmospheric river activity in the Pacific Northwest.

Preliminary estimates indicate that this was an unusually strong event, according to scientists from the Center for Western Weather and Water Extremes (CW3E) at the University of California, San Diego. Their initial calculation of integrated water vapor transport (IVT), which measures a combination of wind speed and moisture levels in the atmosphere, revealed that this system had significantly higher values compared to other atmospheric rivers in the North Pacific over the past 23 years.

“The extremity of the Gulf of Alaska atmospheric river IVT is remarkable,” said Bin Guan, an atmospheric scientist at NASA’s Jet Propulsion Laboratory and the University of California, Los Angeles. He said the days leading up to the event saw the development of a strong Arctic Oscillation in its negative phase. Such low values are extremely rare for September but can be conducive to atmospheric rivers in this region. “This could be one of the conditions that potentially contributed to this exceptionally strong atmospheric river event,” Guan said.

The moisture for this event started in Southeast Asia. It was pushed across the Pacific Ocean, transporting it to the North American shore, resulting in ideal conditions for this massive storm.

When the system reached land, it emitted water vapor in heavy rain, resulting in extended rains and hazardous circumstances in coastal areas. 

References:

¹ An Uncommonly Strong Atmospheric River – Earth Observatory – September 27, 2024

² CW3E Event Summary: 22 – 24 September 2024 – CW3E – September 27, 2024

Harsha Borah

Harsha Borah is an experienced content writer with a proven track record in the industry. Harsha has worked with LitSpark Solutions and Whateveryourdose, honing skills in creating engaging content across various platforms. A gold medalist in a state-level writing competition organized by Assam Tourism, Harsha’s travelogue on Tezpur was widely appreciated. Harsha’s article, "The Dark Tale of the Only Judge in India to Be Hanged," ranks second on Google and has garnered over 11 000 views and 8 900 reads on Medium. Outside of writing, Harsha enjoys reading books and solving jigsaw puzzles.

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