Portland’s Heavy Rain Was Caused by an “Atmospheric River” That Led to Flooding and Landslides Across the State

If it felt like a river just descended on Portland, you're totally right. For the past few days, most of Western Oregon and Washington have seen a downpour of rain caused by an "atmospheric river."

An atmospheric river is a system of abundantly moist air blown up from the tropics. This particular system, which originated near Hawaii and made it up to Oregon over the weekend, caused landslides, floods and Portland's combined sewers to overflow into the Willamette. (The city was just 11 days away from setting a record for "the first calendar year in modern Portland history with zero incidents" in which sewage spilled into the river, says the Portland Bureau of Environmental Services. Can nothing good come out of 2020?)

It also created an impressive display at Multnomah Falls:

But according to National Weather Service meteorologist Dave Elson, the worst is over. The atmospheric river is on its way out, and a colder, drier system is about to move in.

Despite the floods and sewage spill, this atmospheric river wasn't too bad.

"It certainly wasn't an extreme event," says Elson, adding that many of the rivers that flooded in the Willamette Valley usually flood every year anyway.

A rain gauge at Portland International Airport recorded just under 2 inches of rainfall in the previous 48 hours. Other areas of the state, like the Coast Range and Mount Hood, saw much heavier rainfall, 4 and 6 inches, respectively.