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Excessive rain, melting snow may cause worst California flooding since 1997

By Alex Sosnowski, AccuWeather senior meteorologist
January 08, 2017, 11:28:44 AM EST

    An onslaught of excessive rain and mountain snow will threaten lives and property in northern and central California into next week.

    "There is the potential for excessive rain, combined with melting snow to trigger the worst flooding in northern California since 1997 and perhaps 1986," according to Senior Vice President of AccuWeather Enterprise Solutions Mike Smith.

    The storm rolling ashore this weekend will not be in a hurry to leave. Another will quickly follow Tuesday into Tuesday night.

    Ca flooding Jan 7

    "The storm this weekend will be the latest in a series of major storms to affect the West Coast this winter," according to AccuWeather Senior Meteorologist Brett Anderson. "The storms are piping a river of moisture into the region from the Pacific Ocean and will go a long way toward drought relief but at a price."

    Storm to unleash flooding, mudslides

    Unlike the storm along the West Coast at midweek, which concentrated over northern California and southern Oregon, the impacts from the storm this weekend will be significant, far-reaching and long-lasting.

    At the very least, the storm this weekend will cause disruptions to travel due to wet conditions on the highways and low cloud ceilings and gusty winds at area airports over much of the Interstate 5 corridor.

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    Rounds of rain, ice and snow will spread northward through Washington and inland over Nevada, Idaho and Utah as the weekend progresses.

    Portland, Oregon, will bear the brunt of the wintry weather in the Northwest with an ice storm threatening to cause power outages and tree damage in and around the city.

    Cities that can expect multiple days of wet weather from the storm include San Francisco, Sacramento and Fresno, California.

    Enough rain will fall to cause urban flooding, as well as raise the risk of mudslides. The risk of mudslides will not be limited to recent wildfire areas.

    A heavy amount of rain will fall and a tremendous amount of water will be released into area streams and rivers.

    ca rain jan 7

    Through Tuesday night, there is the potential for a general 4 to 8 inches of rainfall. However, rainfall of 8 to 16 inches can occur along the west- and southwest-facing lower slopes of the Sierra Nevada and the northern California Coast Ranges.

    Snow levels will be significantly higher during the bulk of the storm this weekend, when compared to recent storms, according to AccuWeather Storm Warning Meteorologist Alexandria Davis.

    Snow levels may rise to near 9,000 feet in the Seirra Nevada.

    "As a result, some of the snow can rapidly melt at intermediate elevations and lead to the potential for flash and stream flooding," Davis said.

    In the worst-case scenario, a significant threat to lives and property can result. Some of the snow on the mountainsides could liquefy and lead to major small stream and river flooding over the Central Valley of California.

    "Periods of moderate rain and mountain snow could linger into midweek in northwestern California and the Sierra," AccuWeather Senior Meteorologist Kristina Pydynowski said.

    Yet another storm may slam into the Pacific coast later next Friday or Saturday.

    "Despite flood control improvements in recent years, it is likely that current flood protection measures will not be able to capture all of the runoff from the storms through next week in northern California," Smith said.

    Once reservoirs are filled from the train of storms, the water will spill downstream where major river flooding can occur.

    The siege of storms has the potential to wipe out or greatly erase the long-term drought conditions in the region.

    Rain will also expand southward into parts of Southern California, including the Los Angeles and San Diego areas, later Sunday night into Monday.

    "Some of the rain will fall heavily at times, leading to travel delays and isolated flooding, especially in the burn areas," Pydynowski said.

    However, unlike that of areas farther north, rainfall will be of rather short duration in most cases.

    Snow to pile up in high country

    While excessive rain pours down in the lower elevations, extreme snowfall will target the mountains of northern and central California.

    Rising snow levels this weekend will push the heavy snow to the highest peaks.

    Ca snow Jan 7

    Snow levels will then fall to pass levels of the Cascades and Sierra Nevada on Sunday night and Monday. A repeat of the high snow levels from this weekend will not be repeated on Tuesday and Tuesday night.

    The highest elevations of the Cascades and Sierra Nevada will pick up several more feet of snow into Tuesday night. This will be on top of the 3-6 feet of snow that fell in some areas through Wednesday, Jan. 4.

    "The extreme snow will be a boon for the ski industry and for the region's water reservoirs in the coming warm months," Pydynowski said.

    "However, officials will likely be forced to halt travel over the mountain passes."

    The varying temperatures and dry versus wet snow can also elevate the avalanche risk in the high country.

    Winds to raise surf, may cut power

    While wind-induced waves along the coast will not be extreme, minor coastal erosion is possible and seas will become dangerous for small craft.

    When enhanced by terrain effects, winds can become strong enough to down trees and power lines. The saturated ground will allow trees to topple that would normally withstand strong winds.

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