The Oregon coast has a 60-70% chance of vapor transport above 500 from Friday’s atmospheric river

Long-range ensemble guidance cited by the National Weather Service in Portland gives a 60 to 70 percent chance that vapor transport will top 500 kg per meter per second somewhere along the Oregon coast on Friday, Nov. 13. The figure applies to the leading edge of a deep trough over the northeast Pacific. Forecasters wrote that the setup was shaping up as another atmospheric river event.

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A wet Tuesday came first. Rain fell mainly north of a line from Newport to The Dalles, the Olympic Peninsula collected the heaviest totals, and drier weather was expected on Wednesday and Thursday before the next push.

The Tuesday rain gives a sense of the baseline. Areas to the north picked up roughly a quarter inch to 1 inch in 12 hours, and the forecaster expected a 20 to 40 percent chance of showers on Wednesday in Pacific County and along the coast north of Tillamook, with similar showers possible on Thursday along the north Oregon and south Washington coast. Those quieter days are a pause between systems, and the discussion treats Friday as the start of a longer, wetter stretch rather than a one-day burst.

Vapor transport above 500 and the scale it belongs to

The Portland office’s forecast discussion, issued at 5:30 p.m. PST on Nov. 10, ties the probabilities to a specific yardstick. The sentence reads that ensembles indicate a 60 to 70 percent chance for vapor transport to exceed 500 kg per meter per second at the leading edge of the trough Friday, with similar odds that values stay above 250 into Sunday. Those are the thresholds that matter in the published scale for these storms.

That scale, written by a group including the National Weather Service’s Jonathan Rutz and F. Martin Ralph of the Center for Western Water and Weather Extremes, is described in the U.S. Geological Survey’s publication record. Thresholds run at 250, 500, 750, 1,000 and 1,250, and a three-hour peak at or above 500 moves an event out of the weakest rung. A water-sector summary labels the 500 to 750 band as moderate and more beneficial than hazardous, so the Oregon odds describe a storm that starts at the lower edge of meaningful strength.

Duration can raise or lower the final rating, since events shorter than a day drop one category and those longer than two days rise one. Portland’s wording that 250-plus transport could persist into Sunday points to a long-lived event, which is what makes the lower threshold as relevant as the 500 mark.

Isolated flooding and a watch on saturated soils

Rain amounts were left uncertain in the discussion, and the forecaster wrote only that it would be wet, with repeated frontal systems bringing widespread rain and breezy winds from Friday through the weekend. The flood language is measured. Isolated flooding cannot be ruled out because the soils have been soaked by successive systems, yet the chance of widespread river flooding is below 5 percent.

Urban and small-stream trouble is the likelier form, in low-lying or poorly drained areas if the heaviest bands line up, and the discussion advises residents to clear gutters and storm drains ahead of time. The weather service’s flood safety page adds that about 6 inches of fast-moving water can knock an adult off their feet.

Snow enters the picture too. The same text gives a 30 percent chance of a total of 1 inch or more along Santiam and Willamette Passes, a reminder that the system pairs coastal rain with Cascade snow.

Gale warnings and a front with 70 to 80 percent gale odds

Marine hazards are already active. Gale Warnings ran through 4 p.m. PST on Wednesday for several coastal zones, with Hazardous Seas Warnings continuing as late as 7 a.m. Thursday. The discussion expects a strong front late Thursday and into Friday, and it puts the chance of gales at 70 to 80 percent and storm-force winds at 10 to 30 percent, mainly in outer waters.

Seas could exceed 20 feet by Friday afternoon, with a 30 to 50 percent chance of topping 25 feet and a 10 to 15 percent chance of reaching 30 feet in the outer waters. Another strong front is expected during the latter half of Saturday, extending the marine threat past the first day.

Seattle’s forecasters see the same system from the north. In an earlier-in-the-day Western Washington discussion, they described a strong storm arriving Thursday night into Friday with a 70 to 80 percent chance of 50 mph gusts on the coast.

Portland’s discussion leaves one number unresolved: exactly how much rain falls along the coast. Amounts and peak gusts were both labeled uncertain, and the next forecast cycles will say whether the 60 to 70 percent odds firm up, drift toward the 250 threshold, or climb into the 750 band that the scale calls strong.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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