A series of back-to-back atmospheric rivers are set to bring intense gusts and up to ten inches of rain to parts of the West Coast – with unrelenting storms expected until Wednesday. 

More than half a dozen states are under winter weather alerts, as the atmospheric river brings up to 12 inches of snow and rain to parts of Idaho, Wyoming, Colorado, northwestern California and Utah. 

The heaviest rainfall is likely to occur in western Washington State.

Another atmospheric river will add to the 40 inches of snow that has already blanketed mountains in the area – and in Wyoming, some areas are expected to be batting winds of up to 60mph. 

A series of back-to-back atmospheric rivers are set to bring intense gusts and up to ten inches in rain to parts of the West Coast

More than half a dozen states are under winter weather alerts, as the atmospheric river brings up to 12 inches of snow and rain to parts of Idaho , Wyoming , Colorado , northwestern California and Utah

The predicted deluge is due to a weather phenomenon known as an atmospheric river, a band of water vapor that can stretch 1,000 miles long and 350 miles wide. 

As the moisture moves inland from the tropics into higher latitudes, it can bring extreme snow and rainfall, as well as the possibility of flooding. 

National Weather Service office in Portland, Oregon, warned: ‘Flooding caused by excessive rainfall is possible through next week for all locations west of the Cascades. 

‘Those in areas prone to flooding should be prepared to take action should flooding occur. Monitor the latest forecast & be alert for possible flood warnings.’

Meanwhile, areas near Seattle have been warned that avalanches may occur as the storm continues. 

This includes Stevens and Snoqualmie Passes, the West Slopes of the southern Washington Cascades and Mt Hood.

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The National Weather Service Prediction Center said: ‘This will stream milder air into western Oregon and Washington, forcing rain to be the dominant precipitation type.

Snow has already hit parts of Washington, turning the landscape into a winter wonderland

‘The Winter Storm Severity Index shows major Impacts are possible in the highest elevations of the Pacific Northwest and the Rockies, likely causing hazardous to even impossible travel conditions in these impacted areas.

‘Given enhanced rainfall during the weekend, the soils will be more vulnerable and thus the potential for greater run-off and flooding.

‘Snow levels are initially expected to be rather high, and therefore western facing slopes of the Cascades may be subject to an additional risk of flooding due to snow melt.’  

Chad Hecht, meteorologist at the Center for Western Weather and Water Extremes at UC San Diego, told CNN: ‘In scenarios where we see multiple atmospheric rivers move onshore in the form of a family, hydrologic impacts tend to be exacerbated due to the lack of time for rivers and soils to recede back to baseline.’ 

At the beginning of November, the season’s first snows frosted the Rocky Mountains, Great Lakes and New England, resulting in the most extensive early-November snow cover in at least two decades.

Meanwhile, areas near Seattle have been warned that avalanches may occur as the storm continues

Snow was on the ground in 17.9 percent of the contiguous United States, according to the National Oceanic and Atmospheric Administration.

Around the same time last year, snow covered just 3.4 percent of the area. The 20-year average snow cover over the Lower 48 on Nov. 1 is 5.5 percent.

‘There have been 2,700 local storm reports of ice or snow over the last week,’ AccuWeather Meteorologist and Social Media Producer Jesse Ferrell said on Wednesday.

Trick-or-treaters from Minnesota to New York saw snow on Halloween, with enough to shovel in cities like Minneapolis, which saw between two and four inches.