Atmospheric river to unleash flooding rain, mudslide and avalanche risk in Northwest

One of the biggest storms of the winter will take aim at the northwestern United States this weekend and will pack an atmospheric river capable of triggering flooding, mudslides and avalanches.

As a series of Pacific storms continues in the northwestern United States and in southwestern Canada, the biggest storm of the bunch will include an atmospheric river in portions of Washington and Oregon that can have dangerous and damaging consequences, AccuWeather meteorologists warn.

Storms this week have generally brought 0.50 of an inch to 2 inches of rain to lower elevations of western Washington and Oregon prior to Friday.

A storm from Friday to Friday night focused its moisture across British Columbia and western Washington with a general 0.50-2.00 inches of rain. The rain into Friday night produced locally heavy downpours and resulted in slick travel. In the mountain passes, a combination of rain and snow also brought slippery travel over the summits with locally dense fog and wind being issues as well.

An even bigger storm will take aim this weekend and will feature a long plume of intense moisture, called an atmospheric river, that will be unleashed as a fire hose of rain even well up into the higher elevations of the Olympics and west-facing slopes of the Cascades.

The Center For Western Weather and Water Extremes at Scripps Institution of Oceanography forecasts a moderate to strong atmospheric river with this system a 3-4 on their scale ranging from 1 to 5 with 5 being the most intense.

This storm will be one of the biggest of the winter season for the area and is likely to boost precipitation to or above the historical average in many cases, including east of the Cascades. From Dec. 1 through Feb. 21, Seattle has received 8.95 inches of rain compared to a historical average of 14.38 inches. Vancouver, British Columbia, has picked up 11.82 inches versus 14.32 inches for the same period. Meanwhile, Portland, Oregon, has tallied 11.42 inches compared to an average to date of 13.59 inches.

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Rainfall from the weekend storm alone will bring 1-4 inches along the Interstate 5 corridor with 6-12 inches along the west-facing slopes of the Olympics, Coast Ranges and Cascades of Washington and Oregon. In these upsloping locations, there is an AccuWeather Local StormMax™ of 14 inches. More storms will follow into the middle of next week, which will unleash more rounds of rain and mountain snow.

"In Seattle, the bulk of the rain will fall from later Saturday night to Sunday afternoon, while Portland can expect the most frequent downpours to occur between Saturday evening and Sunday evening, with the heaviest rain to fall on Eugene, Oregon, from Saturday evening to Sunday afternoon," AccuWeather Senior Meteorologist Heather Zehr said.

Freezing levels (the elevation at which there can be a mixture of rain and wet snow) will rise substantially into the high country with 5,800 feet in southern British Columbia to 8,500 feet in the southern Oregon Cascades from Saturday night to early Sunday, before falling.

Because rain will fall very high up into the mountains, melting snow at elevations below combined with the heavy rain will lead to quick flooding of small streams and the short-run rivers. People living along or traveling over these waterways should be aware of the danger.

As the thaw reaches higher into the mountains, the risk of avalanches, mudslides and other debris flows and slides will increase in the hilly terrain, including the well-traveled passes that remain open in the winter months.

As the biggest storm in the bunch pushes inland, additional rounds of moisture from somewhat weaker storms will be accompanied by lowering snow levels.

By Monday in the southern British Columbia mountains and during Monday night and Tuesday farther south in the Cascades of Washington, the air will get cold enough for snow to fall and accumulate over the passes, including Snoqualmie Pass, Washington, along I-90.

The storm from Monday to Tuesday can still pack a punch in terms of drenching rain at the lower elevations, but mainly from northwestern Washington to southwestern British Columbia, where ponding on the roads and highways can slow commutes in Seattle and Vancouver.

The storms will bring bouts of gusty winds to the coast. Winds can be strong enough to knock over some trees and trigger sporadic power outages in western Washington and Oregon. Coastal waters are often rough in the winter, and this stretch into Tuesday will be no exception.

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