SEATTLE -- A wet autumn in the Pacific Northwest is about to get even wetter as an atmospheric river is set to bring multiple inches of rain to western Washington and Oregon.
Flood Watches are now in effect for all mountain-fed rivers in western Washington and northern Oregon. In addition, heavy rains may prompt areas of urban flooding around the greater Seattle and Portland areas.
An initial wave of rain will move in Wednesday night, with steadier, heavier rains expected Thursday afternoon and lasting into early Friday.
About 1-2 inches are expected in the Seattle Metro area, with 2-3 inches likely in southwestern Washington and through the Portland Metro area.
"Heavy rain can trigger debris flows and landslides in steep terrain, and the risk is higher in wildfire burn areas," the Oregon Department of Emergency Management said Wednesday. "There is potential for debris flows in the burned areas of the Cascades, including the Riverside, Beachie Creek and Lionshead fire sites. Eagle Creek and Holiday Farm fire areas may also be potentially impacted."
The Cascades, Olympics and Coastal Mountains of Oregon will see even more, with 5-8 inches of rainfall expected. The storm has tapped into warmer, tropical air, which will send snow levels rising to around 8,000-9,000 feet.
That sets up a double whammy of rain falling at higher elevations to run off into rivers instead of sticking as snow; plus, with temperatures rising above freezing, some snowmelt will add to the runoff.
Some major rivers in Western Washington, such as the Snoqualmie, Skagit and Snohomish Rivers, are forecast to reach moderate flood stage and, in the case of the Snoqualmie River, may nudge up close to major flood stage at Carnation.
The Flood Watches remain in effect until Friday afternoon in Oregon but linger into Saturday in Western Washington.
Landslides are a risk as well
Forecasters are warning that the threat of landslides is high around Western Washington, with grounds at or near saturation in the wake of weeks of rainfall.
Some parts of Western Washington have received 8 inches of rain in the past week. Seattle has had rainfall on 19 of the past 21 days, while Portland has had rain on 20 of the past 22 days.
No rest for the dreary
The heavier rains will taper off by later on Friday, but an unsettled pattern remains with rain in the forecast at times for the Pacific Northwest lasting well into next week.