Parts of Washington state, Oregon and British Columbia are again in the crosshairs of an atmospheric river, leading to concerns about flooding and landslides after an autumn that was extremely active in the Pacific Northwest. Ten inches or more of rain could be on the way in the higher elevations of the Cascades, with several inches possible in the lowlands, along with the threat of rivers becoming overwhelmed.

Flood watches are in effect for most of northwest Washington, including downtown Seattle.

Seattle is already running about half a foot above average in rainfall measured since Sept. 1. The season most favorable for repeated atmospheric rivers usually tapers off as the holidays approach, but 2022 appears to have other plans.

The city also had its wettest autumn on record, a period punctuated by an atmospheric river in mid-November that knocked out power to 170,000 customers and left much of Whatcom County, including the city of Bellingham, underwater. Bellingham is under a flood watch again Tuesday.

The atmospheric river now

The core of the atmospheric river stretched from near Seattle to the northern fringes of the Willamette Valley in Oregon as of early Tuesday. An inch of rain had been measured in northwest Pierce County near Gig Harbor, Wash., around 6 a.m., with a pair of observers reporting 1.13 inches. Those values will continue to climb into Thursday morning.

The atmospheric river is the result of a squeeze play of sorts between weak low pressure passing between Vancouver Island and Graham Island and general high pressure to the southeast, like two meshing gears that entrain a filament of moisture between. Some of that juiced-up atmosphere originates from as far away as Hawaii, contributing to the heavy downpours.

The Center for Western Weather and Water Extremes uses a 1 through 5 scale to rank atmospheric river events on the basis of their integrated vapor transport (how much water they carry) and duration. The center’s experts expect the ongoing episode to peak at level 3.

By their estimates, nearly 1,200 pounds of water could pass through every one-meter cross section of the atmospheric river each second, a flow rate comparable to that of some of the world’s largest rivers.

Seattle picked up 2 inches of rain last Wednesday and had tallied 4.48 inches for the month through Monday evening. The city had logged more than half an inch of rain by 5 a.m. Tuesday and is likely to blow past the average monthly rain total of 5.78 inches by the time the event ends Wednesday.

Forecast

Low pressure eventually will slide ashore and flip winds around more out of the northwest Thursday, spelling an end to the deluges. Until then, rain will expand across western Washington into the overnight hours, remaining moderate Wednesday before lifting north as a “dry slot” works through. A swirl of surface low pressure will buffet the coastline of southwest Washington and northwest Oregon during the morning hours Thursday with 50 mph gusts before the rain comes to an end.

The greatest precipitation totals will be found in the coastal range of southwest Washington as well as the Cascades, where up to 10 inches could fall in the highest elevations. That’s because atmospheric rivers carry the bulk of their moisture several thousand feet above the ground; that moisture is forced up the mountains, where it is cooled to the point of condensation and precipitation. A broad 1 to 3 inches is likely in the lowlands and the interior valleys.

Farther inland, cold air entrenched across eastern Washington and northern Idaho in the Columbia River Basin could support a hint of light icing, with a glaze to as much as 0.2 inches of ice accretion possible.

Snowfall is forecast for elevations beginning at 6,500 to 7,500 feet in the Cascades through Wednesday. Above that elevation, amounts may be measured in feet. At lower elevations, the Weather Service forecasts heavy rain with “5 to 10 inches in the Olympics, 3 to 5 inches for the North Cascades and 2 to 4 inches in the Central and Southern Cascades.”

Heavy rain and flooding are also predicted for portions of southwest British Columbia. Environment Canada is predicting 2.5 to 6 inches of rain there, and potentially more in the high terrain.

“Snowmelt over higher elevations as freezing levels climb will contribute to runoff which may result in high stream levels and localized flooding,” the agency wrote. “Heavy downpours can cause flash floods and water pooling on roads. Don’t approach washouts near rivers, creeks and culverts.”

The same ingredients that gave rise to the atmospheric river will contribute to strong winds over the Northern Tier and northern Rockies on Tuesday, with gusts approaching 75 mph in the mountain passes of northwest Montana.

“High winds may move loose debris, damage property and cause power outages,” warned the Weather Service, which had hoisted high wind warnings in the area.

High pressure will build into the region with continued above-average temperatures by the weekend.