The flow of moisture extended about 5,000 miles from Japan to Washington, according to NASA's Earth Observatory site.

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While stretches of weather systems that cross the North Pacific are not uncommon during the fall and winter, the atmospheric river that opened Seattle’s rainy season — dubbed “The Big Dark” by the National Weather Service in Seattle —  was notably long.

At times, the flow of moisture extended about 5,000 miles from Japan to Washington, according to an item posted on NASA’s Earth Observatory site Thursday.

“That is about two to three times the typical length of an atmospheric river,” Bin Guan, a researcher at the Joint Institute for Regional Earth System Science and Engineering, told NASA.

The length and duration of the storm system allowed for what NASA called a “mesmerizing” series of satellite images that showed how the sky looked in mid-October, when the jet stream began to push the line of clouds from Asia. On Oct. 14,  NASA wrote, “a near continuous cloud line stretched all the way to the west coast of North America.”

Satellite imagery shared by the National Weather Service shows clouds, full of moisture, stretching from China to British Columbia. The images were taken Oct. 15 and 16. (NASA)

Bryan Mundhenk, of Colorado State University, told NASA that close observation of the satellite images reveals that the “atmospheric river is really comprised of a series of waves — extratropical cyclones in various life cycle stages — progressing along a semi-stationary storm track.”

While the river of rain was only the opening pitch in the season, it dropped more than 4 inches of rain on the western slopes of the Cascade and Olympic mountains, NASA said, and brought to Seattle some of our rainiest days since February.