(Bloomberg) -- Torrential rains will pound Northern California and Oregon for days as a powerful storm known as an atmospheric river unwinds a stream of moisture from the Pacific Ocean, threatening to unleash floods and landslides.
As much as 10 inches (25 centimeters) of rain may fall through the week starting Tuesday in a region that includes the San Francisco Bay Area, said Marc Chenard, a senior branch forecaster for the US Weather Prediction Center. Areas in the mountains may initially receive snow that could turn to rain as temperatures rise, adding to flood risks.
“It is a several-day event,” Chenard said. “Rainfall totals will end up being pretty significant.”
Atmospheric rivers are capable of delivering an amount of water equivalent to the volume flowing through the mouth of the Mississippi River. The storms are essential to filling California’s reservoirs and aquifers. But they’re often deadly and can inflict billions of dollars in damage, washing out critical roads and rail lines.
Because the impending storm will mostly be a rain event, it won’t add to California’s snowpack, which provides crucial water supplies when it melts in the warmer months. Ranked on a 1 to 5 scale like hurricanes, it’s expected to bring category 3 and 4 conditions from the Oregon border to San Francisco, and less severe category 1 and 2 conditions along the coast of Oregon, according to the Center for Western Weather and Water Extremes.
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