An unusually long atmospheric river stretches from the Philippines to the Pacific Northwest

Atmospheric rivers regularly hit New Zealand. A long one will hit the US and Canadian northwest. Herald photo / Michael Craig

A strong-to-extreme atmospheric river will stretch 7000 miles (11,265km) from the Philippines to the Pacific Northwest this week, bringing significant rainfall, high-elevation snow and strong winds to Washington, Oregon, Idaho, Montana and British Columbia starting today.

While atmospheric rivers commonly hit the Northwest during autumn and winter, the length of this one is very unusual.

Spanning the length of the Pacific, this river in the sky is about three times longer than average and among the longest typically observed on the planet.

Hawaii will see little or no impact; the river will span pretty much all open ocean between the Philippines and the United States and Canadian Northwest.

Weather models also predict that the amount of moisture this atmospheric river will carry will break December records in pockets of central and western Washington - raising the risk for flooding rainfall.

Flood watches span the Cascades and coastal plain from northern Washington to central Oregon, where rainfall totals to Friday could exceed 300mm in the hardest-hit areas, with major flooding possible on Thursday due to near-record-high river levels.

The Skagit, Skykomish, Snoqualmie, Puyallup, Nisqually and Cowlitz rivers in Washington are among those predicted to reach major flood stage, the most extreme level of risk that could bring extensive inundation of structures and roads.

This atmospheric river, whose peak intensity is predicted to reach Category four or five - with five being the highest - on the Centre for Western Weather and Water Extremes’ scale, will come in waves.

The strength of an atmospheric river is determined by how much moisture it transports and its longevity.

This atmospheric river will probably be the strongest of the season so far in the Pacific Northwest - stronger than one that caused flooding and localised wind damage in the region during late October.

“People, structures, and roads located below steep slopes, in canyons, and near the mouths of canyons may be at serious risk from rapidly moving landslides,” the National Weather Service in Portland, Oregon, wrote.

The dense moisture plume is being carried into the region by storms in the Gulf of Alaska. High pressure near California - which will also contribute to unseasonable warmth - will deflect the atmospheric river to the north, squeezing it into a relatively narrow corridor of the Northwest.

Flooding rainfall will be the biggest threat with this system. Because of saturated soils, there will be a higher risk for landslides and debris flows on burn scars, the Weather Service in Seattle noted.

The term “atmospheric river” has been around for a while and is rooted in science.

The term is more than 30 years old. They were initially described as long, narrow filaments of water vapour that persist for many days.

As the science advanced, forecasters developed a scale for measuring the intensity of these rivers in the sky. They are now generally predictable days in advance, and people can better understand the important role they play in transporting water to almost all corners of the world.

Interest in atmospheric rivers spiked in November 2021 when several damaging ones hit the Pacific Northwest and has since continued to rise.

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