Forecasters at the National Weather Service office in Seattle expect an atmospheric river to settle over Western Washington on Friday, Nov. 13, and Saturday, Nov. 14. Their discussion put the moisture-transport values at 800 to 1,000 kg per meter per second. A forecaster described those numbers as among the higher ones seen so far this season.
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Washington’s coast has just been through a punishing run of surf, and the next system arrives before the rivers have fully drained from the last one. The forecast leans on wind, seas and one particular river in Mason County more than on any basin-wide flood threat.
An IVT of 800 to 1,000 and the Category 3 band
The Seattle office’s area forecast discussion, issued at 2:50 a.m. PST on Nov. 8, said flow aloft would consolidate offshore on Friday and set an atmospheric river up over Western Washington. The forecaster wrote that the pattern had the potential to pivot the plume directly over the region and keep rain going from Friday night into Saturday. The moisture-transport measure quoted there, integrated vapor transport or IVT, was in the 800 to 1,000 range, and the same passage called those some of the higher numbers of the season.
The range sits inside a scale built by a team that includes the National Weather Service’s Jonathan Rutz and F. Martin Ralph, director of the Center for Western Water and Weather Extremes at the University of California, San Diego. Their published atmospheric river scale ranks events from 1 to 5 using the peak three-hour IVT, with thresholds at 250, 500, 750, 1,000 and 1,250, and 750 to 1,000 falls in Category 3, described as strong.
A summary of the scale says Category 3 events carry benefits and hazards in about equal measure, and that duration shifts the rating: an event shorter than 24 hours drops one category, while one lasting longer than 48 hours rises one. A Friday-through-Saturday window straddles that line, so the final category depends on how long the plume actually stalls.
The Skokomish River and a seven-day river outlook
The earlier discussion said river flooding was not expected over the following seven days, while noting that repeated rain would saturate soils and raise base flows. It singled out the Skokomish River, which the forecaster called an old friend of the office, as the one to watch if the plume stalled late in the week.
The discussion issued at 2:47 p.m. PST on Nov. 10 made that concern concrete. It said the Skokomish in Mason County could climb back above flood stage from Friday’s rain and that a flood watch was in effect for the county, while no flooding was expected on other local rivers over the next seven days. Reading the two issuances together, the moisture story stayed constant and the river picture sharpened around a single basin.
Any flooded road near that river is a driving hazard first. The weather service’s own flood safety guidance says it is never safe to drive or walk into flood waters, and that about 12 inches of moving water can carry away most cars.
Wind, surf and seas above 20 feet
The Nov. 10 discussion framed the system as a strong Pacific storm arriving Thursday night into Friday with lowland rain, high-elevation mountain snow and gusty wind. It gave a 70 to 80 percent chance of 50 mph gusts along the coast and 30 to 40 mph in the interior, and said the storm would weaken moving inland on Saturday ahead of a second system on Sunday.
Seas are the second concern. Guidance in that discussion showed waves above 20 feet from Friday afternoon, a 20 to 30 percent chance of seas above 25 feet from late Friday into early Saturday, and a possibility of 30 feet in the southern waters. Minor flooding around high tides in the Salish Sea was also listed as possible.
The Nov. 8 discussion explains why forecasters were wary of the coast. A High Surf Warning covered the Grays Harbor and Northern Washington coasts until 10 p.m. PST that day, with seas peaking at 20 to 25 feet in the afternoon, easing to 12 to 15 feet on Monday, rebuilding to 16 to 18 feet on Tuesday and staying above 13 feet through the end of the week. Dangerous beach conditions persisted even after the winds eased.
The same system is drawing attention to the south. The Portland office’s discussion issued at 5:30 p.m. PST on Nov. 10 called Friday another atmospheric river event and gave a 60 to 70 percent chance that vapor transport would exceed 500 along the Oregon coast.
The open question is how long the plume lingers over Puget Sound. Seattle’s forecasters tied their flood concern to a stall late in the week, and the Skokomish watch in Mason County is the first place a slower-moving river would show up.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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