What is an atmospheric river and how long will it soak Western Washington?

by Alfred Charles, KOMONews.com Executive ProducerThursday, November 11th 2021


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Radar Thursday afternoon shows a line of moisture moving into Western Washington that will dump heavy rain on Seattle.https://imasdk.googleapis.com/js/core/bridge3.489.0_en.html#goog_2055062974Volume 90% Radar Thursday afternoon shows a line of moisture moving into Western Washington that will dump heavy rain on Seattle.

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A system that is expected to dump several inches of rain on an already soggy Seattle area is expected to raise the risk of flooding for much of Puget Sound, starting Thursday and lasting well into Friday night.

KOMO News meteorologist Shannon O’Donnell said we can thank an atmospheric river for the additional precipitation, which she said is defined as “a long plume of moisture that carries copious amounts of precipitation from the subtropics toward the mid and upper latitudes. Heavy, warm rain, especially when combined with melting snow, can quickly lead to severe river flooding.”

Medium Pacific Satellite

And there is a specific type of atmospheric river, known as a Pineapple Express.

“It is a specific type of atmospheric river that tends to be rooted close to the Hawaiian Islands,” O’Donnell said, adding that it brings warmer air to the region instead of colder conditions. Low temperatures Thursday night were expected to only drop to the low 50s with highs on Friday expected to reach the upper 50s.

According to O’Donnell, between 4 and 5 inches of rain could fall in the Snoqualmie River Valley and Seattle could see rainfall up to 2 1/2 inches through Friday.

Moderate to major river flooding is expected from the system. More forecast information can be found here.

National Weather Service meteorologist Jacob DeFlitch said there is nothing out of the ordinary with the drenching rains that are in the forecast.

Seattle Rainfall Stats

“November is generally our rainy season,” he said. “It’s kind of usual to see these type of events.”

Because the ground is already soggy from the rainfall earlier in the week, the risk of rapidly rising river levels is enhanced, DeFlitch said.

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