The National Weather Service in Portland has issued a Flood Watch for Southwest Washington and Northwest Oregon. This watch is in effect from 4:00pm Saturday afternoon through 4:00pm Monday afternoon.
Confidence continues to increase that a classic atmospheric river type precipitation event will set up over the Pacific Northwest over the weekend and continue into early Monday. A deep and moist westerly flow is expected to develop Saturday morning and strengthen later Saturday and Sunday. A very long fetch of moisture, with origins west to the South China Sea, will feed into this system bringing the potential for very large rainfall totals across the region. With the flow forecasted to remain perpendicular to the Coast Range and Cascades for an extended period of time, expect strong orographic enhancement to lead to the highest rainfall totals over the mountains. The Coast Range, foothills and Cascades are expected to see in the range of 6-12 inches of rain with locally higher amounts possible. Lower but still significant totals are expected for the lower terrain. Projected rainfall along the coast is forecasted to be around 4-8 inches, with 2-5 inches expected over the interior lowlands.
Given the latest forecasted rainfall totals it appears that there is the strong potential for flooding over the weekend on numerous rivers and tributaries draining the Coast Range and Cascades. A few rivers may reach major flood stage. The current river stage forecasts indicate that the faster responding rivers could reach flood stage as early as Saturday evening. There also appears to be potential for flooding on some of the main-stem rivers early next week. In addition to flooding potential on the main-stem rivers and tributaries, there are likely to be impacts from urban and small stream flooding, and flooding along low lying pasture land.
In addition to the flooding concerns, excessive rainfall has the potential to create landslides, debris flows and excessive runoff in the vicinity of recent wildfire burn scars. Burn scars from two large fires this summer – the 36 Pit Fire in the Clackamas River basin near Estacada and the Deception Complex near Oakridge will be particularly vulnerable.
Current river forecasts are available here. In our area, major flooding is possible along the Mary’s River, which is fed by several streams in the Coast Range. Other Coast Range rivers, including the Siletz, Alsea, and Siuslaw may also see flooding.
A Flood Watch means there is a potential for flooding based on current forecasts. Landslides and debris flows are possible during this flood event. 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.
You should monitor later forecasts and be alert for possible flood warnings. Those living in areas prone to flooding should be prepared to take action should flooding develop.