The rain was already falling Friday afternoon in western Washington, soaking the shoreline and even showering Seattle with occasional bursts. After a brief lull Friday afternoon, repeated rounds of downpours can be expected through the night.

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Atmospheric rivers, or strips of moisture-rich air with a tropical connection originate as far away as Hawaii. As undulations in the jet stream scoop the juicy air northward, the humidity is transformed into rain and downpours as air is forced up the Cascades and northern Rockies.

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That leads to persistent rains along the western “upslope” regime of mountain ranges. In this case, western Oregon is in the bull’s eye.

“We’ve got atmospheric river conditions heading to the Pacific Northwest over the next couple of days, on and off with breaks in between,” said Martin Ralph, director of the Center for Western Weather and Water Extremes, at the Scripps Institution of Oceanography at the University of California at San Diego.

“We measure an atmospheric river’s strength in integrated vapor transport, the amount [of water vapor] being transported horizontally,” Ralph explained. On a 1-to-5 scale, he pegs this one as a two or three.

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Despite summer being warmer and on average featuring a much more humid atmosphere, most atmospheric rivers form in the wintertime. Why?

“The winds are stronger,” Ralph said.

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“You can have a lot” of moisture in the summertime, but “the stronger the winds, the stronger the water vapor transport horizontally.”

Ralph says roughly 75 percent of all the moisture that rides in on the winds is within the lowest 10,000 feet of the atmosphere.

“It’s the wind in the lowest 10,000 feet that really determines [atmospheric river strength],” Ralph said. That is why the forecast is going to be a soggy one.

“We’re looking at anywhere between two to three inches of rain in the Crystal mountains through the weekend” said Brad Schaaf, a meteorologist at the National Weather Service in Medfrod, Ore. “And we could get more … maybe another one [or] two inches through midweek.”

Storm total rainfall could locally exceed four inches. There is an outside chance a few areas could receive a half-foot.

“It’s pretty normal to see rainfall like this with systems that come through in the wintertime,” Schaaf said. He says his office is only anticipating “small hazards,” including rises in some streams and creeks, and ponding of water on area roadways.

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Snowfall will remain above 7,000 feet, so the copious high-elevation snow forecast should not disrupt any mountain passes. That snow will also absorb much of the rain that falls, preventing it from collecting downstream and spawning any significant flooding.

There will be a break late Sunday between the current atmospheric river’s effects and the next one down the pipeline. That one could be stronger, Ralph said.

“There’s a strong atmospheric river forming between Hawaii and the Pacific Northwest,” Ralph said. “It’s a strong system.” He estimates it could fall into the category 3 or 4 ranking of atmospheric rivers, although it may not be so potent when it makes “landfall” in the Pacific Northwest.

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It is worth noting the term landfall in the context of an atmospheric river only means the moisture is reaching the coast.

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Meanwhile, Ralph has spearheaded an airborne effort designed to probe these atmospheric rivers.

“The Air Force flew [Thursday] night,” Ralph said. “One out of Hawaii, and one out of Travis Air Force Base near San Francisco.”

The two aircraft released a combined 40 dropsondes — instrument probes that collect a vertical profile of the atmosphere — which then transmitted back data that was ingested into global weather models, improving forecasts.

The group is planning another flight Monday or Tuesday.

Atmospheric rivers are not exclusive to the West Coast. They can happen across the globe so long as the criterion are met. Studies have long since been underway to investigate their role in the eastern United States, where they are sourced from both the Gulf of Mexico and Atlantic. One atmospheric river helped contribute to disastrous flooding in the Nashville metro area back in 2010.