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Atmospheric river with heavy snow, rain to slam Pacific coast

By Alex Sosnowski, Accuweather.com

An atmospheric fire hose will blast areas of the Pacific coast this week and could result in major travel disruptions from heavy snow, flooding and strong winds, AccuWeather meteorologists warn.

A phenomenon, known as an atmospheric river, or a plume of intense moisture, will take aim at part of the West coast and move southward during the middle days of this week, before weakening by Friday. This setup can lead to excessive amounts of rain and snow in a short period of time.

The storm poised to drop southward along the United States Pacific coast from mid- to late week was poised over the Bering Sea on Monday. Image courtesy of AccuWeather Enhanced Real Vue satellite
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The precipitation from the storm is likely to be intense for a period of eight to 12 hours with the worst conditions likely in western Washington from Tuesday night to Wednesday, forecasters say. In western Oregon, the storm will hit hard during the day on Wednesday. From Wednesday night to Thursday, the storm will hammer Northern California as it progresses slowly to the south.

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Intermittent rain and snow will pester the region ahead of the main storm through Tuesday evening in the coastal Northwest. Wet snowflakes are likely to reach down to sea level during this time. A small accumulation is possible around Portland, Ore., and over some of the hills around Seattle.

Travel-snarling snow is likely in the intermediate and high elevations of the west-facing slopes of the Cascades as the main part of the storm and the stream of moisture drops southward. There is a chance that the major passes in the Cascades, including Snoqualmie, could close for a time due to rapidly accumulating snow.

A general 1-2 feet of snow will fall at pass levels in Washington and Oregon with locally higher amounts and several feet of snow likely over the high country. Snow could fall at the rate of several inches per hour for a time.

As is often the case with many storms that move in from the Pacific, a surge of warm air will occur and limit and perhaps prevent much snow from falling right at sea level. However, due to the intensity of the precipitation close to the onset of the storm, a small amount of accumulating snow is likely in Seattle. Elevations near sea level to a few hundred feet in elevation will pick up a coating to an inch or two.

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Snow could quickly pile up to a few inches over the hilltops in the Interstate 5 corridor of Washington.

Warm air is likely to have more time to reach areas farther south at low elevations in southwestern Washington and western Oregon, including the Portland area. There can be a brief period of wet snow or mixed rain and wet snow at the onset and the conclusion of the storm.

Rain will be substantial and could be heavy enough to lead to urban and small-stream flooding. Motorists should be on the lookout for rockslides and other debris flows.

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Strong winds will add to travel difficulties for a time, especially for those flying into and out of the region. Gusts can be strong enough to break tree limbs and lead to sporadic power outages in coastal areas and along the western slopes of the Cascades.

Farther south in Northern California, the storm will pack a punch during the second half of the week.

"While the rain and mountain snow will be beneficial from a drought standpoint, enough can fall to lead to travel delays and disruptions," AccuWeather meteorologist Haley Taylor said.

Long-term drought conditions range from moderate to exceptional based on the latest conditions from the United States Drought Monitor.

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During Wednesday night as rain spreads southward along the coast and over the Sacramento Valley of California, snow will waste no time falling at a heavy rate in the Coast Ranges, Siskiyous and northern Sierra Nevada.

Travel may become slippery and difficult over Siskiyou Summit, along I-5, in southern Oregon by Thursday morning.

The bulk of the rain will fall on the San Francisco and Sacramento areas on Thursday. While the rain may not be as intense as areas farther north along the Pacific coast, it can still lead to significant runoff, slick driving conditions and even ponding on some roads.

Snow will reach Donner Pass, Calif., along I-80 on Thursday midday or afternoon and will continue into Thursday night with the potential for enough to lead to major travel delays and perhaps even a road closure for a time. A general 1-3 feet of snow is likely in the northern and central Sierra Nevada with locally greater amounts over the high country and at some of the ski resorts.

Rain is forecast to reach Los Angeles and San Diego, and the lower south and west-facing slopes of the mountains from Thursday night to Friday evening. While the rain will not be nearly as intense as that of areas in the Northwest and in Northern California, it can still lead to slow commutes due to slick roads, poor visibility and some runoff.

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Snow levels are likely to remain above Cajon and Tejon passes in Southern California, but some of the ridges and peaks in the region could receive a few inches of snow on Friday, Taylor said.

A couple of rain showers may survive the trip over the mountains and reach the deserts in Southern California on Friday afternoon and night as well.

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