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A powerful storm is expected to engulf the West Coast and will have the potential to unleash a month's worth of rain and feet of snowfall from the Cascades to the Sierra Nevada into early next week. Heavy rain and snow will be beneficial for drought relief, but it can lead to major problems for travelers and pose risks to lives and property.
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AccuWeather meteorologists have been tracking this storm since Thursday, where the strengthening cyclone moved from the Bering Sea between Alaska and Russia into the open waters of the Pacific Ocean just south of Alaska on Friday.
As of Saturday afternoon, the storm had moved onshore in the Pacific Northwest, bringing heavy rain, mountain snow and strong winds to western Washington and Oregon. Freezing levels are anticipated to drop in portions of Washington to the valley floors through Sunday resulting in the possibility of Seattle seeing a few flakes -- mainly in the morning hours with temperatures in the low to mid-30s.
"This is a large area of low pressure in the northeast Pacific that is going to gradually shift its way south along the West coast through early next week," Accuweather Meteorologist Joseph Bauer noted.
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| AccuWeather radar imagery taken Saturday afternoon depicting heavy rain (yellows and reds) moving inland and snow falling (blue) in the Cascades and into far Northern California. |
While impacting a vast area from southeastern Alaska and coastal British Columbia Friday and Friday night, the storm has begun to blast northwestern Washington, spreading southward and inland over Washington and Oregon with snow falling across the Cascades over the major passes such as Snoqualmie, Stevens and White Pass. Feet of snow will occur before the end of the event in these regions, especially across the highest peaks. Road closures are anticipated in the major mountain passes.
"The storm's excessive rainfall could add insult to injury as the zone from British Columbia to western Washington has weathered storm after storm this fall," Accuweather Senior Meteorologist Alex Sosnowski explained.
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Seattle recorded its wettest meteorological autumn, which spans from September through November, on record with 19.04 inches of rain.
A general 2-4 inches of rain with locally higher amounts are forecast to fall along the Washington and Oregon coasts, as well as the lower, west-facing slopes of the Cascades through Sunday. Slow travel is likely, due to rain and gusty winds along the Interstate-5 corridor.
As the storm meanders south into next week, a river of moisture, know as an atmospheric river, will be unleashed onto much of California and into the West bringing enhanced precipitation as the storm looks to nearly stall for a time or slow down significantly. Rain totals could climb as high as several inches in central and northern parts of California as the slow-moving storm may deposit multiple feet of snow in the Sierra Nevada.
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The amount of rain and mountain snow that is anticipated to fall throughout the duration of this storm will lead to dangerous travel conditions and road closures with the threat for flooding, washouts and mudslides. The inclement weather is likely to add to shipping delays ahead of the holidays as truck, rail and aircraft travel may be hampered.
The worst of the storm is likely to hit the San Francisco and Sacramento areas from late Sunday to Monday night as the storm slows to a crawl. Normal December rainfall for San Francisco and Sacramento are 4.14 inches and 3.43 inches, respectively. In less than 48 hours, both of those locations could pick up the normal monthly rainfall from this storm. Other locations can also receive a month's worth of rain in a couple of days as the storm slowly progresses across the Golden State.
With the potential for several inches of rain to fall, flooding and mudslides may not be limited to recent burn scar locations.
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"Urban flooding is a strong possibility in cities such as San Francisco, Sacramento and perhaps even Los Angeles," Sosnowski said.
The heaviest rain is expected to spread into Southern California and the Los Angeles area Monday night and may linger through much of Tuesday. Some of mountains right outside of Los Angeles could see some snow mixing in with rain. Motorists should expect major delays when traveling through the region.
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Los Angeles can expect 1 to 2 inches of rainfall from this storm, while areas out towards Point Conception in the San Gabriel, Santa Monica, San Bernardino and San Rafael mountains to the north of the Los Angeles basin could see rainfall totals of 2 to 4 inches where 12 inches could occur over the Sierra Nevada.
As the storm sags south, snow picks up over the Sierra Nevada and Donner Pass Sunday and continue through Tuesday. Monday will be the day that travel will nearly be impossible through Donner Pass as snowfall rates can be in excess of 3 inches per hour for a prolonged period of time, along with gusty winds, the interstate may even close.
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Forecasters warn that 4 to 6 feet of snow in the high country in the Sierra Nevada through early next week is likely with the amount of moisture that is available for this storm to tap in to. This situation could turn dangerous and potentially life-threatening for those traveling through the region.
Even though the storm will pose hazards in the short term, there will be some benefits, especially to the ongoing drought. More than 80% of California is in the grips of extreme and exceptional drought, the two highest levels recognized by the U.S. Drought Monitor.
The multi-day storm along the West Coast should prove to be very beneficial to the snowpack across the Cascades and Sierra Nevada initially and later next week for other ranges that make up the Intermountain West. Ski resorts like Mammoth Mountain and Sugar Bowl will welcome the snow in the Sierra Nevada ahead of the busy holiday season.
AccuWeather meteorologists will continue to track this storm as it moves out of California and into the Wasatch Range and Colorado Rockies midweek ahead of yet another storm that could bring more rain and snow to California late this week.
