One dead as atmospheric rivers bring ‘pineapple express’ rain and floods to US Northwest

Daily rainfall records were broken across Washington state and Oregon

A house inundated by flood waters in Granite Falls, Washington, on Tuesday 5 December 2023 (Kira Mascorella via AP)

Heavy rains, floods, and extra snow will continue to buffet the Pacific Northwest for the next 24 hours, US weather forecasters say.

Daily rainfall records were broken across the states of Washington and Oregon on Monday as a powerful atmospheric river dumped tropical moisture onto the Cascade Mountains.

Terrestrial rivers overflowed their banks in northwestern Oregon and throughout Washington, according to the US National Weather Service (NWS), with flood multiple flood warnings still in place as of Tuesday evening.

One man in Oregon was killed after he was swept away in a flooded creek in Multnomah County, CNN reported, while parts of US Highway 101 and various school districts were closed in both states due to flooding.

The NWS said that the deluge is set to continue throughout Wednesday, warning of "significant rises on many area rivers and streams this week”.

An atmospheric river is a moving stream of air that brings warm, moist air out of the tropics and into more temperate regions.

When that air hits land it is forced up into the colder upper regions of the atmosphere, causing the moisture to condense and fall from the sky as rain.

An atmospheric river that comes from the ocean around Hawaii is often called a "pineapple express", due to Hawaii's historic prominence in global pineapple cultivation.

This week’s river is being pulled up from Hawaii by a large storm brewing in the Gulf of Alaska.

Aside from the rain, it has brought warmer temperatures and extra snow falling on mountaintops.