Wet, Muddy and Slippery Weather Outlook for Oregon

Heavy rain could lead to river and stream flooding for some portion of southwest Washington and Northwest Oregon

(PORTLAND, Ore.) - A prolonged atmospheric river event will bring moderate to heavy rain rates beginning Wednesday night potentially continuing through Saturday.

Liquid equivalent precipitation amounts during this period could range from 2" to 4" for the interior lowlands with 3" to 8" or more along the Coast Range and Cascades.

The National Weather Service has issued flood watches for portions of Northwest Oregon and Southwest Washington, including the following areas, in Northwest Oregon, Central Coast Range of Western Oregon, Central Oregon Coast, Central Willamette Valley, Coast Range of Northwest Oregon, Greater Portland Metro Area, Lower Columbia, North Oregon Coast, Northern Oregon Cascade Foothills, South Willamette Valley, and Western Columbia River Gorge.

In Southwest Washington, Greater Vancouver Area, I-5 Corridor in Cowlitz County, South Washington Cascade Foothills, South Washington Coast, Western Columbia River Gorge, and Willapa Hills for Thursday evening, December 19, through Sunday morning, December 22.

Heavy rain can trigger landslides and debris flows in steep terrain, and the risk is higher in burn areas.

Find the latest information here: https://alerts.weather.gov/cap/or.php?x=1

Debris flows are rapidly moving, extremely destructive landslides. They can contain boulders and logs transported in a fast-moving soil and water slurry down steep hillsides and through narrow canyons. They can easily travel a mile or more. A debris flow moves faster than a person can run. People, structures and roads located below steep slopes in canyons and near the mouths of canyons may be at serious risk.

If your home, work, or route is in a watch area:

• Stay alert. Track the flood watch by radio, TV, weather radio or online. If told to evacuate, do so immediately.
• Listen. Unusual sounds might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of falling mud or debris may precede larger landslides. If you think there is danger of a landslide, leave immediately.
• Watch the water. If water in a stream or creek suddenly turns muddy or the amount of water flowing suddenly decreases or increases, this is a warning that the flow has been affected upstream. You should immediately leave the area because a debris flow may soon be coming downstream.
• Travel with extreme caution. Assume roads are not safe. Be alert when driving, especially at night. Embankments along roadsides may fail, sending rock and debris onto the road.

Source: National Weather Service; Oregon Dept. of Geology and Mineral Industries

At this time, the North Oregon Coast Range and the South Washington Cascades appear to receive the focus of precipitation. However, these atmospheric river events are notoriously difficult to precisely predict more than 24 to 36 hours in advance.

Further complicating the issue, Cascade snow levels will rise from about 3500 feet Wednesday night to above 6000 feet by Friday evening.

Areas that do receive a prolonged period of moderate and heavy rainfall will likely experience significant rises on area rivers and streams. The greatest concern will be on rivers and tributaries that do not have flood control reservoirs.

The National Weather Service will continue to monitor the rivers closely and issue watches and warnings if needed. People living near southwest Washington and northwest Oregon rivers should monitor the latest weather and river conditions for the next week. Listen to NOAA weather radio or check https://weather.gov/portland for further updates.

For more landslide and debris flow information: https://www.oregongeology.org/Landslide/debrisflow.htm