(Bloomberg) -- A strong Pacific storm will begin to uncoil across Oregon, Washington and Northern California through the weekend, promising heavy rain and snow that will replenish water supplies but could also unleash destruction.
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Known as an atmospheric river, the storm will begin dropping rain across the Pacific Northwest Thursday and then head south into Northern California, where it will remain anchored through the weekend into next week, said Frank Pereira, a senior branch forecaster at the US Weather Prediction Center. As much as 7 inches (18 centimeters) of rain will fall, with up to 12 inches of snow or more across the Cascade and Sierra Nevada ranges.
“For the next four to five days, it will be a pretty significant rainfall event for at least Northern California,” Pereira said.
The flooding rains arrive after weeks of bone-dry winds fanned wildfires that killed at least 29 people and burned more than 16,000 structures around Los Angeles. Atmospheric rivers are long, narrow streams of moisture that can carry enough water vapor to match the average flow from the mouth of the Mississippi River.
The system set to hit the US West will be beneficial in some areas and harmful in others, according to the Center for Western Weather and Water Extremes. In Oregon, Washington and parts of California, the storm is expected to reach only Category 1 and 2 levels — usually considered a benefit — on a scale devised by the Scripps Institution of Oceanography at the University of California San Diego. However, around San Francisco Bay it may hit Category 3 and 4 levels, which are considered destructive.
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