Forecast models show that the strong atmospheric river, nearly 11,000km long from the Philippines across the entire Pacific Ocean and entering the states of Washington, Oregon, Idaho, Montana (USA) and British Columbia ( Canada) from December 8, will bring heavy rain and high risk of flooding to the northwest Pacific this week.
Atmospheric rivers are a common phenomenon in the northwest of the United States in the fall and winter, and the length of this phenomenon is considered very rare.
Extending almost the entire length of the Pacific Ocean, this moist air mass is 3 times longer than average, among the longest groups ever recorded on Earth.
Hawaii (USA) is barely affected because this atmospheric river spans most of the waters between the Philippines and North America.
The moisture is forecast to break December records in many areas of central and western Washington, increasing the risk of heavy rain and flooding.
A widespread flood warning has been issued for the Cascade Mountains and coastal areas from northern Washington state to central Oregon.
Total rainfall as of December 11 in severely affected areas could exceed 300mm, with a risk of major flooding on December 10 due to river water levels rising near record levels.
The Skagit, Skykomish, Snoqualmie, Puyallup, Nisqually and cowlitz rivers in Washington are forecast to reach severe flood thresholds, the highest risk level, which could cause widespread flooding and paralyze many roads.
This atmospheric river is forecast to reach level 4-5 on a 5-level scale of the Central Weather and Extreme Waterway (CW3E).
This atmospheric river flows in two waves: the first on December 8 and the second, which is stronger from the night of December 9 to December 10.
This atmospheric river is considered the strongest since the beginning of the season in the northwestern United States, surpassing the entire period in late October that caused flooding and gusts of wind in the area.
Heavy rain is the most fearsome threat. Typhoons and waterlogging have caused a high risk of landslides, landslides and mudflow over old forest fires, according to the National Meteorological Agency in Seattle.
The moderate rain is expected to start on December 9 before strengthening from the night of December 9 to 10, in Washington, northern Idaho and British Columbia. Despite the decline in intensity on December 11, heavy rain is still possible in Washington, Idaho and Montana.
A large amount of snow is forecast due to the high humidity content. In the area near the top of Baker Mountain, Washington, where the heaviest snowfall in the US since the beginning of the season has occurred, the snow could exceed 1.5m. Heavy snow is also expected in parts of British Columbia, Alberta and Saskatchewan this week.
The term " atmospheric river" appeared in 1992, used to describe long, narrow, moist air streaks that have existed for many days in the convective layer. These rivers in the sky play an important role in water transportation globally.
Interest in atmospheric rivers has increased sharply since November 2021, when many consecutive strong waves caused great damage in the northwest of the United States - Canada.
