Major storm train to raise flood, mudslides threat from Northern California to western Washington
At least four significant storms, loaded with Pacific moisture, will roll ashore along the West Coast of the United States through the end of this week. AccuWeather meteorologists warn that each storm will increase the risk and extent of flooding and mudslides.
While each storm may not be a bomb cyclone, inches of rain from storms from Northern California to western Washington will add up.
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From 1 to 2 feet of rain will pour down, especially along the west-facing slopes of the mountains through the Friday after Christmas, AccuWeather Meteorologist Danielle Ehresman said.
"One or more of the storms are likely to develop an atmospheric river that can funnel tremendous amounts of rain like a giant fire hose or snow like a massive snowmaking machine toward portions of the West Coast," AccuWeather Senior Meteorologist Brett Anderson said.
The storms through this weekend will be a precursor to what lies ahead during the last week of December.
The heaviest rain and high-country snow are forecast with the storms just before and after Christmas.
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Motorists can expect multiple rounds of drenching rain along the Interstate 5 corridor from near San Francisco and Sacramento, California, northward to Eugene and Portland, Oregon, as well as Seattle. The periodic heavy rain will also extend to Vancouver, British Columbia. In areas where the water drains poorly, ponding on roads is likely.
The amount of rain that pours down with each storm can trigger flash flooding of small streams. As runoff increases with each storm, the potential for significant river flooding will increase, especially on the short-run rivers that flow from the Olympics, Cascades, Siskiyous and Coast Ranges.
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The number of mudslides and other debris flows will increase, especially in recent burn scar locations. As the soil becomes saturated and moisture seeps deeper, the increasing weight of the topsoil will create the risk of more significant landslides. Washouts can occur, especially on secondary roads in the hilly terrain.
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Snow levels in the mountains will rise and fall with each storm as they often do.
"Snow levels will tend to be rather high and mainly above the passes for the first couple of storms into the weekend," AccuWeather Chief On-Air Meteorologist Bernie Rayno said. "However, colder storms with lower snow levels in general are coming during the week of Christmas."
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Because wintertime weather conditions can change within the stretch of a single mile, motorists are strongly urged to closely monitor local forecasts along their entire route if venturing through the mountains to reduce the risk of becoming stranded in rapidly accumulating episodes of snow.
While snow levels will tend to spend considerable time above pass level with each storm, there will be periods when temperatures are below freezing and snow is falling and creating wintry travel from Donner Pass, California, along I-80 to Snoqualmie Pass, Washington, along I-90, especially this week as Rayno warned about.
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In the high country, above the passes in the Cascades and northern Sierra Nevada, 5-10 feet of snow can pile up in the pattern. Shifting snow levels and the amount of snow falling over the high country will increase the risk of avalanches with each storm as the rapidly increasing snow cover becomes more unstable.
The majority of moisture associated with each storm will avoid Southern California. Most of the time, conditions will be dry and mild in Los Angeles and San Diego.
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However, anyone heading north to visit friends and relatives over the holidays should be prepared for progressively adverse conditions near and north of I-80.
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