The major storm is expected to bring heavy rain and flooding throughout the week, causing river water to rise and heavy snowfall in many areas.
Up to 380mm of rain is forecast for the week, with some areas in the Cascade Mountains alone seeing up to 457mm of rain, affecting many areas that are rarely flooded, weather forecaster Andrew Orrison of the US Weather Center (WPC) said.
Overall, the region will experience several consecutive days of heavy rain, he said.
Atmospheric rivers are long, moist air masses originating from the Pacific Ocean, which can transport water equivalent to the flow at the mouth of the Mississippi River when it makes landfall.
On a 5-point scale, heavy rains this week are forecast to reach level 4, in many coastal areas of Washington and Oregon, according to the CW3E Center of the Scripps Institute of Oceanography, University of California San Diego, USA.
Many areas in the western United States still rely on major Pacific hurricanes, which bring snow to replenish water.
Forecaster Orrison said the storm will make landfall in waves from December 8, pausing on December 9, then heavy rain will return. This causes the soil to be saturated with water, leading to additional rainfall and flooding and landslides.
The Clackamas River near Oregon City is forecast to rise nearly 2.7m over the next few days, the Willamette River in Portland may rise about 1.2m above current high tides; and the Columbia River near Oregon's largest city is forecast to rise about 0.9m, according to the National Weather Service.
Several other areas are forecast to see much stronger water levels: The Cowlitz River in Randle, Washington, could rise nearly 4m while the Snoqualmie River in Carnation, west of Seattle, is forecast to rise more than 3m.
Many areas in the western United States still rely on major storms from the Pacific Ocean to supplement water resources.
In the past few months, the entire western United States has seen many storms from the Pacific Ocean, significantly reducing droughts.
According to data from the US drought monitoring system, California, which is not affected by the current storm, recorded a drought area decreasing from 39.56% in the past 3 months to only 0.15% last week.
In Washington, which was dry almost the entire area three months ago, the drought has now dropped to 79.97%. Oregon also improved by more than 20%, with the remaining drought area being just 31.44%.
Despite heavy rain forecasts, snow in the western mountains is below average, leading to water shortages next year.
The snow layers are considered reservoirs that store water until spring and summer, providing water for residents, agriculture and hydroelectric dams in the area.
While the storm is forming in the western United States, Chicago recorded 10cm of snow, causing about 105 flights at O'Hare International Airport to be canceled.
