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Flooding rain, wintry hazards to plunge southward across US West Coast this week

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The river of moisture will once again target the West Coast starting this week, first hitting the northwestern United States before heavy rain and mountain snow plunges back into California late in the week.

“After several dry days in the West, a pattern change will occur this week as a series of storms slam into the region,” AccuWeather Meteorologist Jordan Root said.

Storms will roar back into the Northwest early this week with flooding rain, strong winds and interior snow and ice. More storms will follow at midweek and beyond with impacts also spreading into California and the Rockies.

Millions of residents all along the West Coast can anticipate a renewed threat of flooding, travel disruptions, mudslides and erosion.

Storm train to first slam Northwest with heavy rain, snow and ice

“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 Monday night through Wednesday,” Root said.

“This will likely result in some dangerous flooding and mudslides, especially near the coast.”

NW Mon Jan 15

Unlike last week when Portland, Oregon, endured ice and then one of its snowiest 24-hour periods on record, the track of the storms will bring milder conditions and mostly rain to the I-5 corridor of the Northwest.

Lingering cold should cause the rain to briefly begin as a period of ice and slick travel in Portland on Monday evening.

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

Travel delays to both motorists and airline passengers can be anticipated. Flash flooding can also result, especially in urban and low-lying areas. The combination of the rain, melting snow and clogged storm drains will further heighten the risk of flooding in Portland.

NW Jan 15

More 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.

Rivers and streams may overflow their banks and flood neighboring communities and streams.

The one good news for western Washington is that many of the rivers 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 on I-90’s Snoqualmie Pass 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.

However, motorists venturing eastward out of the Cascades will face greater hazards due a more extended period of ice in the valleys of northern Oregon and Washington.

This includes along I-90 into Ellensburg and northward to Wenatchee. Travel will become very slippery and dangerous, even for a few hours after temperatures rise above freezing, due to the ongoing and persistent cold.

If enough moisture can survive over the mountains and into these areas, the ice could accumulate enough to cause power outages and tree damage.

The risk of ice leading to slippery travel will also expand along I-84 from Portland to Pendleton, Oregon, as well as along I-90 to Spokane, Washington, and into western Montana on Monday night into Tuesday before an eventual changeover to rain.

“The jet stream (a fast-moving river of air along which storms travel) will take a dip across the West during the 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 and eventually the Sierra later in the week.”

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

Flood risk, travel disruptions to be renewed in California at late week

While dry weather will hold across California early this week, that will change for the second half of the week as the storm track plunges southward.

The first round of rain and mountain snow will spread southward through California on Wednesday and Thursday with a second to follow Friday into Saturday.

The excessive rain totals from last week will likely not be matched in many communities.

“However, it will not take as much rainfall as the last batch of storms to bring streams and rivers out of their banks again,” AccuWeather Senior Meteorologist Alex Sosnowski said. “These streams will remain at rather high levels.”

The risk of mudslides will also be renewed, as well as travel disruptions in Sacramento, San Francisco, Los Angeles and San Diego.

Gusty winds accompanying each storm could also down more trees and cause sporadic power outages.

Unlike the Northwest early this week, snow levels will be low enough later this week for mostly snow in the Sierra. The two storms should combine to allow a fresh 1-3 feet to pile up.

Combined with periods of gusty winds, travel will become extremely difficult at times. Motorists should prepare for restrictions and possible road closures, including on I-80’s Donner Pass.

The snow will once again be a boon for the ski industry and for the region’s water supply during the upcoming warmer and drier months.

An end to snow in the Sierra or the storm train into the West will not come on Saturday. Additional storms are likely to follow later in the week and into next week, leading to more issues with flooding rain, strong winds and mountain snow.