Atmospheric river causes flooding in the Pacific Northwest

Due to heavy snowfall and the risk of avalanches, Snoqualmie and Steve passes, along major freeways east of Seattle, were closed in both directions.

Atmospheric rivers, originating in the tropical Pacific Ocean, are narrow jets of moisture that produce a substantial portion of the precipitation that falls between fall and spring in the western United States and Canada. Several atmospheric river events between October and December resulted in devastating flooding in Washington State and British Columbia.

This particular atmospheric river is rated at Level 4 out of 5, which means its effects are expected to be “extreme” or mostly dangerous, primarily due to potential flooding and avalanches. But it can be beneficial in some ways since its rain and snow increase water reserves.

Rainfall totals of 2 to 3 inches are forecast for the lower elevations and up to 3 to 6 inches in the mountains. The highest totals are expected in the mountains since atmospheric rivers carry most of their moisture several thousand feet above the ground.

Due to its origins in the tropics, the Atmospheric River draws in mild air at unusually high elevations, bringing rain rather than snow up to 6,000 to 8,000 feet, increasing the avalanche threat up to to Monday evening.

“Heavy rain and snow along with warming temperatures and strong winds will create very dangerous avalanche conditions,” states the avalanche warning for the Cascades. “Avalanches can become very large and destructive, and run to the bottom of valleys.”

Due to the potential for heavy rain, flood watches cover much of western Washington and northwestern Oregon through Tuesday afternoon, including Bellingham, Seattle and Olympia. “Excessive runoff can cause rivers, creeks, streams, and other low, flood-prone locations to flood,” the weather service writes.

Flooding has already started along parts of the Skokomish River in northwest Washington. At its peak level expected on Monday afternoon, moderate flooding is forecast “with deep, rapid floodwaters inundating some residential areas, many roads and much of the farmland in the Skokomish Valley,” the weather service writes. .

Flooding of additional rivers in western Washington is possible late Monday through Tuesday morning, the Seattle Weather Service warns.

To the south, a flood warning is also in effect for the Grays River in southwestern Washington with rainfall rates of up to half an inch per hour.

Some of the heaviest showers from this event are expected in southwestern Washington and northwestern Oregon, where the Weather Service declared a “moderate risk” of excessive precipitation. Localized rainfall totals greater than 6 inches are possible, especially in hilly terrain.

In addition to heavy rainfall, the weather system is also bringing wind gusts of more than 50 mph to coastal areas of central and northern Oregon and Washington state as well as “chaotic surf,” according to the National Weather Service. High wind warnings are in effect for much of the coastal area from central Oregon to southern Washington state through 4 p.m. Monday.

The heaviest rains and strongest winds are expected to ease by Tuesday morning, but wet and choppy weather could continue until later in the week.

The atmospheric river is directed towards the coast by an area of ​​low pressure several hundred miles offshore. It draws tropical humidity from northern Hawaii along a conveyor belt thousands of miles long.

In a warming world, strengthening atmospheric rivers are expected to boost heavy precipitation events. By the end of the century, these narrow jets of moisture are expected to strike more frequently at higher intensity (category 4-5), going from “mostly or mainly beneficial” to “mostly or mainly harmful”, according to a published study. in 2020.

Another study found that atmospheric rivers will be about 25% wider and longer, meaning their heavy rains and strong winds will increase by about 50%. The frequency of the most intense atmospheric rivers is expected to nearly double by 2100.

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