The mid-valley and much of western Oregon is under a flood watch through 4 a.m. Wednesday, according to the National Weather Service.
Heavy precipitation — from 1 to 4 inches of rainfall — may cause small stream flooding in inland valleys and along the Oregon coast. The rain, plus snowmelt, may create localized flooding issues in the Willamette Valley.
Minor flooding may occur in the Marys River in Benton County, the Luckiamute River in Polk and Benton counties and Johnson Creek in Multnomah and Clackamas counties.
The flood watch includes the following areas: 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, and south Willamette Valley.
The flood watch includes southwest Washington, the greater Vancouver area and the Interstate 5 corridor in Cowlitz County.
Landslides and debris flows are also possible. People, structures and roads located below steep slopes, in canyons and near the mouths of canyons may be at serious risk from rapidly moving landslides.
According to the Oregon Department of Geology and Mineral Industries, 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.
The department offered these hints for people for who live, work or travel 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.
For more landslide and debris flow information: https://www.oregongeology.org/Landslide/debrisflow.htm
Tuesday's weather forecast is calling for a 100 percent chance of rain, clouds and the possibility of a snowy mixture at times. Rainfall of about a half-inch is expected.