It was an extremely wet November in parts of the Pacific Northwest, causing major flooding on both sides of the U.S.-Canada border.
The latest storm system to hit the region triggered new evacuation orders and at least one mudslide in flood-ravaged British Columbia, Canada, late Sunday.
On the same day, flood sirens sounded in Washington as the Nooksack River overflowed. Henry Braun, mayor of Abbotsford, British Columbia, told reporters the water flow was headed toward the Canadian border city later Sunday. “There’s nothing to stop it,” he said.
Flooding has disrupted the Port of Vancouver’s rail services and sections of several major British Columbia highways remain closed due to high water.
Just over the border in northern Washington, sections of Interstate 5 and U.S. Highway 101 have been periodically closed due to high water or landslides. Bellingham received daily record rain on three days during Thanksgiving weekend, ranging from 1.17 to 1.63 inches. Sumas is another town that has been halfway underwater at times.
As of Monday, Seattle had enough rain to make last month its fifth-wettest November on record, with 10.14 inches.
So what’s creating all this mayhem? The short answer: atmospheric rivers.
Atmospheric rivers are relatively long, narrow regions in the atmosphere, like rivers in the sky, that transport most of the water vapor outside of the tropics. They move with the weather, carrying an amount of water vapor roughly equivalent to the average flow of water at the mouth of the Mississippi River (nearly 600,000 cubic feet per second, according to the National Park Service). When atmospheric rivers make landfall, they often release this water vapor in the form of rain or snow.
Although atmospheric rivers come in many shapes and sizes, those that contain the largest amounts of water vapor and the strongest winds can create extreme rainfall and floods, often by stalling over watersheds vulnerable to flooding. These events can disrupt travel, induce mudslides and landslides due to excessive runoff and snowmelt, and cause catastrophic damage to life and property — everything that’s been happening in the Pacific Northwest over the past few weeks.
Perhaps the most well-known example of an atmospheric river to Americans is the “Pineapple Express,” which is capable of bringing abundant moisture from the tropics near Hawaii to the U.S. West Coast.
While this latest atmospheric river to hit the Pacific Northwest has been persistent and strong, the National Oceanic and Atmospheric Administration says most are weak systems that often provide beneficial precipitation that is crucial to the water supply. Atmospheric rivers are a key feature in the global water cycle and are closely tied to both water supply and flood risks, particularly in the western United States.
Atmospheric rivers are part of the bigger picture of global weather patterns. For the second year in a row, a La Nina is setting up in late fall, expected to last at least through the winter.
During La Nina, strong trade winds push more warm Pacific water toward Asia. This produces upwelling off the west coast of the Americas, sending cooler water to the surface. These cooler waters push the jet stream, and hence more storms, northward.
In its winter outlook issued in late October, NOAA predicted wetter-than-average conditions in the Pacific Northwest over the December-through-January time period. If this happens, recovery from potential winter floods could take months or upward of a year.
In the short term, more rain Tuesday and Wednesday could make flooding issues worse before the atmospheric river over the region fades for a few days. However, it could return this weekend or early next week.
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