The long band of moisture causing heavy rain throughout Northwest Oregon and Southwest Washington is likely to continue through at least Wednesday afternoon.

Flood watches went into effect on Monday. Some waterways, including Washington’s Grays and Naselle rivers, had already hit major flood stage Monday night.

SW Washington rivers flooded, Oregon Coast schools closed as atmospheric river pounds Pacific Northwest

The weather system hitting the Pacific Northwest is known as an “atmospheric river.”

“Think about a river in the sky,” said National Weather Service meteorologist Noah Alviz. ”It can hold as much water as the Mississippi River, but in the sky.

“When you have that much moisture coming into the Pacific Northwest, that’s going to dump a lot of rain into the area.”

He said as the atmospheric river hits the Coast Range and the Cascades in Oregon and Washington, moisture is forced out — and comes down hard and fast, in a relatively contained area.

Periods of heavy rain today through Wednesday will lead to concerns of river flooding, small stream flooding, urban flooding, and landslides beginning this afternoon. Check the latest river forecasts here: https://t.co/8EI5k3DhAd #ORwx #WAwx pic.twitter.com/f8nQ1iGx6M

— NWS Portland (@NWSPortland) December 8, 2025

“Eugene hasn’t been really hit as hard,” Alviz said. ”So it’s a really narrow, distinct band of moisture that’s impacting Western Washington and far Northwest Oregon.”

And it’s not just the direct rainfall causing issues: Runoff from the mountains is also filling local waterways.

“A lot of our rivers right now are rising and some of them have already reached flood stage,” Alviz said.

After more than a day of constant and heavy rain, soils are also saturated. So just a moderate wind can bring down trees or power lines. Thousands of people were without power in Clackamas County Tuesday morning.

“If you do see any downed power lines, don’t get anywhere near them at all,” Alviz said. ”Don’t’ touch them. Don’t get out of your car if you’re around that.”

Drivers are urged to slow down and to avoid standing water. Landslides have also started to cause issues on area roadways, including U.S. Highway 101.

In order to help ease flooding, officials recommend that people check on gutters and storm drains to make sure they’re clear of debris, particularly fallen leaves.

The atmospheric river is supposed to move through the region by late Wednesday, but the flood watches are in effect through Thursday.

Get the latest updates on major traffic impacts and emergency closures at opb.org/closures.

News Source : https://www.opb.org/article/2025/12/09/atmospheric-river-pacific-northwest-meteorologist-explains/