Atmospheric river continues to unleash drenching rainfall in Pacific Northwest

An atmospheric river now on its way to the Pacific Northwest will unleash new waves of heavy rain in already-soaked areas, forecasters warned Saturday.

With the latest storm, some locations in the northwestern United States and British Columbia, Canada, are projected to finish the month with around a foot of rainfall.

The risk for flooding will rise as the active pattern continues, especially for urban and poor-drainage areas, rivers and streams, some of which have already climbed above flood stage. The persistent flow of moisture soaking the Northwest over the last several days is projected to continue into the start of the new week, forecasters say.

An expansive hydrologic outlook was issued along all of western Washington and Oregon due to the heavy swath of rainfall expected to continue across the region and the ample snowmelt occurring as snow levels rise to 7,000 or 8,000 feet.

Temperatures can reach their peak for the week on Sunday, when cities like Seattle and Portland, Ore., are expected to rise into the lower 60s Fahrenheit. Values to this degree are not exactly typical for Northwest cities in late January, with the historical average for this week ranging from 48-49 degrees for both locations. Even throughout all of last January, the mercury did not rise to the 60-degree mark for either location, for reference.

The saturated soils across the region coupled with melting snow can result in an elevated landslide risk into the start of the upcoming week across the varying terrain of western Washington and western Oregon.

Through Sunday, an additional 1-2 inches of rain can soak portions of western Washington, with locally higher amounts upwards of 3 inches possible in upslope regions of the Olympic Mountains and northern Cascades. Cities like Seattle could collect an additional 1-2 inches of rain during this time, heightening the risk for ponding on streets and highways. Travel disruptions can occur due to standing water and heavy downpours through the end of the weekend along Interstates 5 and 90 in western Washington.

"The Skokomish River in northwest Washington is currently at minor flood stage, but the additional heavy rainfall across the region can cause water levels to rise to moderate flood stage over the weekend," explained AccuWeather Meteorologist Elizabeth Danco.

Moderate flood stage on the Skokomish River is categorized as water levels between 17.5-17.9 feet. After peaking at 17.27 feet late Thursday, the river gauge declined to 16.9 feet by Saturday morning. According to recent data, the last time the Skokomish River reached moderate flood stage was early December. However, during this time, the river quickly surpassed moderate flood levels and rose to major flood levels with a peak reading of 18.01 feet.

Based on the rainfall expected through the end of the weekend, forecasters say that the river levels are not expected to reach major flood stage and repeat what occurred last month, although levels may come dangerously close. Major flood stage is categorized as water levels higher than 18.0 feet, and current projections show that levels could climb upwards of 17.8 feet through the end of the weekend before declining some.

As a frontal boundary inches closer to the coast through the end of the weekend, offshore winds will pick up through the end of the weekend. Boaters should take extreme caution, especially of operating small craft, with hazardous seas and strong winds blowing out of the south. Wind gusts on Sunday can reach speeds of 35-45 mph at times off the coast of Oregon and Washington.

Into early week, a break in the active pattern across the northeastern states can allow for a temporary window of dry weather.

"Residents in Seattle could experience a brief break in the wet weather Sunday night into Monday, although this reprieve may only last for 12-24 hours before showery and cooler conditions return," added Danco.

Rain and showers can return to the region by Monday night, bringing the short-lived hint of dry weather to an end. Daytime temperatures in Seattle and Portland are projected to trend cooler throughout the week, declining to the middle to lower 50s by midweek and 40s by Friday.

Another drenching and more expansive wave of moisture can arrive along the West Coast by late Tuesday into Wednesday, potentially reaching areas from southwest Canada to Central California. This midweek storm can bring heavy rainfall and strong winds to locations farther south than storms over the last few days.

As a cooler air mass spreads across the region, snow levels will gradually fall throughout the week and heavy, high-elevation snowfall can occur as the next system tracks overhead.