Severe floods force mass evacuations in US state of Washington

Severe rainfall has triggered major flooding in the US state of Washington, prompting evacuation orders for tens of thousands of residents.

Governor Bob Ferguson on Thursday urged people in threatened areas to leave and follow instructions from local authorities. Up to 100,000 residents are believed to be affected by evacuation orders.

"We're looking at a historic situation, expecting 61 centimetres higher than the record flood level," Ferguson said in a post on X.

Ferguson said more than 30 major highways had been closed to traffic. The governor also declared a statewide emergency to rapidly mobilise funds and additional personnel.

The National Guard has been deployed to assist with rescue operations in the flooded regions. Helicopters and boats have been used to bring people to safety. Dozens of rivers have overflowed their banks, inundating towns and farmland.

The heavy rainfall in the north-west of the United States is being driven by a weather phenomenon known as an atmospheric river, a band of extremely moist air that can deliver intense rainfall for days.