Residents in the Pacific Northwest of the US and Canada are bracing for what could be record setting floods as an atmospheric river dumps heavy rain on already swollen rivers.

The US National Weather Service warns that catastrophic flooding is possible in the states of Oregon and Washington along the Skagit and Snohomish rivers.

In Canada, major highways to Vancouver have been closed because of flooding, debris and the risk of avalanches.

There are evacuation orders in place for thousands of people in the US and Canada, and authorities have that warned more rain is on the way on Thursday.

In the US, the governor of Washington state, Bob Ferguson, declared a statewide emergency on Wednesday and estimated 100,000 residents could soon face evacuation orders.

The emergency declaration warned that continued rain and snow at mountain elevations would exacerbate flooding conditions. Supply chains and transportation could be severely impacted, the declaration added.

Skagit County, a major agricultural area north of Seattle, has issued an immediate evacuation order to residents who live on the floodplain.

Some 75,000 people would be evacuated from low-lying areas on Skagit River, the director of the Washington Military Department's Emergency Management Division, Robert Ezelle, told reporters on Wednesday.

Across the border in Canada's British Columbia, there are evacuation orders in place for the communities of Tulameen and Eastgate, and several other areas.

The City of Abbotsford ordered urgent evacuations for 371 properties at 23:00 local time on Wednesday (07:00 GMT on Thursday).

The entire region, spanning parts of both the US and Canada, has received torrential rain from an atmospheric river, which is a phenomenon where water evaporates into the air and is carried by the wind and forms long currents that surge through the sky like rivers flow on land.

The heaviest rainfall is expected to subside by Thursday afternoon, but the water will continue to work its way into rivers.

Another storm is expected on Sunday.