A powerful storm is expected to bring several days of heavy rain, strong winds and mountain snow to parts of the Western United States this week.
The storm is the result of an atmospheric river, a long, narrow band in the atmosphere that transports water vapor sometimes hundreds of miles. Sometimes called rivers in the sky, when they make landfall along the West Coast of the United States, they often release this moisture as rain or snow.
The atmospheric river season typically runs from October through March, and is responsible for up to half of California’s annual precipitation. While these systems are vital to replenishing water supplies, they can also cause flooding when they combine with other weather systems, bringing heavy rainfall.
The plume of moisture from this atmospheric river is expected to arrive over the Pacific Northwest late on Thursday before moving south into Northern California by Friday night.
Marc Chenard, a meteorologist at the Weather Prediction Center, said the storm was then expected to stall and weaken before another surge of moisture moved in through the weekend.
“An area of low pressure develops, that could enhance the rainfall a second time over the Northwest,” he said. “By Saturday we should start to see that happen.”
As the low pressure system moves in, temperatures will drop, leading to mountain snow of up to two feet through the weekend. Strong winds are also expected.
Here’s what you need to know:
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Parts of Western Washington and Northern California are at risk of significant rainfall, including the Shasta and Siskiyou areas. Between two and four inches of rain is expected, with some spots, particularly across the Olympic Peninsula and the Northern Cascade Mountains, potentially receiving five inches or more. Much of this rain is expected to fall within 12 hours.
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The Weather Prediction Center has issued a level 1 out of 4 risk for flash flooding starting Friday and continuing through the weekend. The area of risk stretches from near Eureka, Calif., to the coastal ranges and into the Cascades in Washington.
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Heavy mountain snow were expected across parts of mountain ranges like the Cascades, the Northern Sierra Nevada and the Northern Rockies. The Weather Prediction Center said totals of one to two feet of snow are becoming “more likely” at higher elevations.
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Strong winds are expected from Western Washington to Northern California, with gusts over 40 miles per hour likely along ridges and coastal areas. Strong winds are also possible farther inland across parts of Montana, Wyoming and the Four Corners region, where Arizona, Colorado, New Mexico and Utah meet.
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In addition, the forecasters said small stream flooding was possible, particularly over burn scars and in more urban and poor-drainage areas.
Forecasters said this unsettled weather pattern could continue through the end of October, with the potential for additional atmospheric rivers over the region.
Nazaneen Ghaffar is a Times reporter on the Weather team.
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