While some regions across the northern United States freeze under layers of increasing layers of ice and snow, storms lining up over the Pacific Northwest will move inland with increasing moisture this week, inducing multiple forms of flooding from urban areas to small streams and some of the rivers in Washington, northern Oregon, northern Idaho and western Montana in the coming days.
As AccuWeather reports, some of the biggest rain producers will be atmospheric rivers, which will bring concentrated heavy rain into Thursday. A plume of moisture, sometimes referred to as the Pineapple Express, will extend from west of Hawaii to the interior northwestern United States through at least midweek.
Some low-lying roads along the streams and rivers may be blocked by high water. In some cases, the flooding and mudslide risk may be extreme enough to prompt evacuations. Motorists could experience hazardous travel conditions due to rounds of heavy rain, blowing spray and ponding. Conditions may deteriorate at Seattle-Tacoma International Airport and cause delays.
Every Small Stream and River in Washington and Oregon Will Experience Flooding
Nearly every small stream and short-run rivers in areas of Washington and Oregon will experience at least minor to moderate flooding. “Multiple rivers in the Washington Cascades are already at moderate to major flood stage,” said AccuWeather Storm Warning Meteorologist Ethan Rogers. “Multiple rivers downstream of Mount Rainier are expected to approach or forecast to break record levels that have not been observed in 75 years for some.”
“The Snohomish River at Snohomish, Washington, is forecast to set a new record height of 35.30 feet on Friday. The gauge at Snohomish has been in service since 1949,” AccuWeather Social Media Producer Jesse Ferrell said. “The Grays River at Covered Bridge near Rosburg, Washington, set a new record during Monday night of 33.36 feet. The gauge near Rosburg has been in service since 2005 with a previous record of 33.15 feet on Dec. 3, 2007. Several locations along other rivers in the region are forecast to crest within a foot of the record.”
A large zone in the Pacific Northwest, including the city of Portland, Oregon, is forecast to receive 4-8 inches of rain from Monday to Wednesday night. Within this zone, there will be pockets where 8-12 inches of rain is in store — mainly along the west-facing slopes of the Pacific Coast ranges and the Cascades in Washington and northern Oregon. The AccuWeather Local StormMax™ for the first half of this week is 22 inches.
Storms Predicted to Deliver Twice the Normal Monthly Rainfall
While December is typically a wet month in the Pacific Northwest, rainfall during the stormy conditions in the weeks ahead may deliver twice the normal monthly rainfall in some cases. Seattle’s historical average is 5.72 inches. Rainfall in Astoria, Oregon, averages 10.68 inches. At Stampede Pass, Washington, the average rainfall and melted snow for December is 12.60 inches.
In addition to the flooding threat, road washouts and mudslides are likely as the topsoil becomes saturated and unstable. Debris flows will be most common, but not limited to, areas where wildfires have occurred.
Risk of Avalanches Will Increase in High Elevations
In the highest elevations of the Cascades, where many feet of snow fall and temperatures fluctuate, the risk of avalanches will increase. Fortunately, for motorists heading over the passes, freezing levels will generally remain high enough so that most of the storms this week will bring rain and not snow.
Hurricane Hunter aircraft from the Air Force Reserve’s 53rd Weather Reconnaissance Squadron shift their focus during the tropical offseason to investigating the atmospheric rivers. Aircraft are scheduled to investigate the storms this week, according to the latest NOAA schedule.
While a bit of a break or northward shift of the sequence of storms is forecast to occur during the latter part of this week, the storm track will shift farther south once again next week, as the storms will grow larger in size and bring a return of heavy rain (and high-country snow) this weekend to next week.
Until then, a northward bulge in the jet stream will prevail, and that will lead to unusual warmth for the middle part of December along much of the West Coast of the U.S. Farther inland, the warmth will reach its peak in portions of Montana and Wyoming Wednesday before a press of colder air begins and locally heavy snow develops Thursday.
Periodic problems due to flooding and mudslides will continue in the coastal Northwest as rounds of moisture move through the region.
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