Up to 8 inches of rain to threaten flooding in northwestern US

A return of storms to the northwestern United States will deliver drenching rain and wintry travel hazards this week.

“An atmospheric river of moisture originating near the tropics in the West Pacific Ocean will stretch across the Pacific and help fuel heavy rain across the Northwest through Wednesday,” according to AccuWeather Meteorologist Jordan Root.

Weather-related delays to ground and air travel can be anticipated.

Along the coast, there will be the risk of flash and urban flooding, strong winds, mudslides and continuing erosion. Farther inland, travel problems will result from snow, ice and rain.

The track of the storms will bring milder conditions and rain with some exceptions along the Interstate 5 corridor of the Northwest.

Rainfall generally totaling 2 to 4 inches will otherwise soak Portland, Oregon, and the other cities along Interstate 5 including Seattle and Olympia, Washington, and Salem and Eugene, Oregon.

Widespread flash flooding will threaten places along the coast and the western slopes of the northern Cascade Mountains, where rainfall should total 4-8 inches. Some rivers and streams may overflow their banks.

However, many of the rivers in western Washington are running at their lowest levels since the end of the summer, according to the National Weather Service in Seattle.

The rain will be windswept at times. The strength of the winds toward midweek could lead to sporadic tree damage and power outages along the coast. Seas will build as the winds howl, threatening to cause beach erosion.

Snow levels will rise well above the mountain passes early this week, meaning motorists traveling through Snoqualmie Pass along Interstate 90 will face water on roads and reduced visibility from downpours and fog instead of a snow-covered highway.

“This will not be a big snow event for the Cascades except for the highest peaks as snow levels will rise to between 7,000 and 7,500 feet on Tuesday,” AccuWeather Senior Meteorologist Dan Baker said.

Several hours of freezing rain can be enough to cause dangerous travel and power outages from Portland, Oregon, eastward through the Columbia River Gorge along I-84.

Motorists venturing east of the Cascades will face a significant hazard due an extended period of ice in the valleys of northern Oregon and southern Washington.

Travel will be treacherous around Ellensburg and Wenatchee, Washington. Slippery roads and sidewalks may persist for a few hours after the air temperature rises above freezing. In these areas, a significant buildup of ice is likely with extensive power outages.

The risk of ice will extend to Pendleton, Oregon, as well as along Interstate 90 to Spokane, Washington, and into western Montana, before an eventual changeover to rain.

Wintry weather may become more extensive over the Northwest by this weekend.

“The jet stream (a fast-moving river of air along which storms travel) will take a dip across the West during midweek and will help bring colder air farther south,” Root said. “That will lower snow levels and allow snow to pile up across the Cascades later in the week.”

Periods of rain, albeit not as heavy as earlier in the week, will continue to dampen the Interstate 5 corridor late this week.

While dry weather will hold across California early this week, the second half of the week will turn stormy.

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