Atmospheric river slams Pacific Northwest, triggering deadly flooding

At least two people have died in Oregon amid ongoing flooding resulting from an intense atmospheric river bombarding the Pacific Northwest. One man was found dead Tuesday in Bronson Creek in Washington County, his death presumed to be a drowning, a day after a man was swept to his death in Johnson Creek in in Portland.

Record river crests spurred substantial flooding in western Washington state, with additional rainfall expected Wednesday. Four rivers across western Washington remained in moderate flood stage early Wednesday, but at least five hit major flood stage Tuesday.

Among them was the Stillaguamish River at Arlington, about 40 miles north of Seattle. Major flood stage begins at 19 feet, but the river peaked at a record 21.34 feet Tuesday. The previous record was 21.16 feet on Dec. 12, 2010. Tuesday’s peak meant water poured into the adjacent river valley, flooding scores of homes and businesses.

The entire town of Silvana in Snohomish County, about 40 miles north of the city, was reachable only by boat after becoming an island because of flooding on the Stillaguamish. Firefighters helped evacuate the town of roughly 200 people. To the southeast, about a dozen homes were flooded in Granite Falls on the south fork of the Stillaguamish.

Four people and a dog were rescued around midday Tuesday in Al Borlin Park in the town of Monroe, some 25 miles northeast of Seattle. The park is a peninsula formed at the confluence of Skykomish River and Woods Creek.

In Rosburg, Wash. - about 100 southwest of Seattle - the Coast Guard rescued five people caught in floodwaters.

The flooding was exacerbated by swift snow melt in the mountains. Atmospheric rivers, which originate in the tropics, pump in mild air. That means a sudden influx of meltwater into area rivers and creeks, leading to a more significant inundation of low-lying areas.

Flood watches continue to blanket areas from the Klamath National Forest in northwest California all the way north to the Canadian border. Eight flood warnings have been issued in Washington, and there are growing concerns that mudslides and landslides could occur.

“Rainfall of 2 to 8 inches over the past 24 hours has increased soil moisture to moderate levels across western Washington,” wrote the National Weather Service in Seattle. “This amount of rain will put extra pressure on soil instability, leading to an increased threat of landslides.”

Several small landslides were reported around Seattle, which received a calendar-day record 2.39 inches of rain Tuesday.

Rainfall totals

The atmospheric river began drenching the Pacific Northwest on Monday after earlier rains over the weekend. Precipitation totals have been very elevation-dependent; the highest totals have been realized in high terrain, where the mountains focus moisture.

In Seattle, rainfall reports ranged from 3.8 to 4.3 inches since Monday, but totals were double that in mountains to the east. The Alpine Meadows Snotel, on the Snohomish-King county line, posted 9.6 inches. Totals between 7 and 9 inches were common along the South Fork of the Snohomish River.

Along the Olympic Peninsula to the west, up to 7.79 inches were reported in Hoodsport, Wash.

High totals populated the map in Oregon, with a maximum of 9.6 inches just northwest of Mount Hood. The heavy rains had yet to dip south into California, save for near its border with Oregon; 3.17 inches fell at Upper Coon Mountain.

Where the atmospheric river is now

As of Wednesday morning, a cold front - ahead of which tropical moisture was being slung up the coast through the atmospheric river - was lurking just about 200 miles offshore. One batch of moderate rainfall was present in northwest Oregon and southern Washington, but the heaviest downpours were in Northern California.

Parts of southwest Oregon and Northern California could receive another 2 to 3 inches of rain. To the north through western Washington, additional rainfall should range between 1 and 2 inches.

What’s next

Even though the atmospheric river was making landfall, more rain, downpours, mountain snow showers and isolated thunderstorms are anticipated beneath a high-altitude disturbance still to come onshore. Cold air aloft will encourage surface air to rise and form pockets of precipitation. These showers will pinwheel ashore late Wednesday and especially Thursday.

As the cold front trailing the atmospheric river comes through, temperatures will drop. That will lower snow levels. Moderate to heavy snows are expected at mountain summits as a result, especially above 8,000 feet, where 2 to 5 feet could fall.

In Washington, the snow may descend to pass levels, hampering travel. A brief lull in activity is expected Friday before a resurgence in moisture heralds the arrival of another atmospheric river Saturday.