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By Paul Rogers
progers@mercurynews.com
Posted: 02/07/2014 05:19:57 PM PST
Updated: 02/08/2014 06:36:50 AM PST

The storms expected to finally bring Northern California a desperately needed deepdrenching this weekend after the driest year in state history aren’t just random showers.

They are the result of a developing situation that scientists call “an atmospheric river,” and recent research has shown that they have played a significant role in breaking droughts in the past.

Sometimes known as “the Pineapple Express,” these rivers of rain are long, narrow bands of highly-concentrated moisture that are formed in the Pacific Ocean and barrel eastward until they hit land, bringing downpours and flooding.

When they hit California, they pack an amazing punch. Just one can carry 15 times as much water as the Mississippi River.

“It’s essentially a fire hose of water brought up from the tropics that comes up and crashes into the West Coast,” said Michael Dettinger, an atmospheric scientist with the Scripps Institution of Oceanography in La Jolla.

And one is hitting now, moving up from Hawaii.

By Monday, forecasters expect 2 to 4 inches of rain to have fallen over San Francisco and the East Bay, 2 inches over the South Bay, 4 to 6 inches over the Santa Cruz Mountains and Marin County, and a soaking 7 to 9 inches over parts of Sonoma and Mendocino counties. The Sierra could get 3 feet of snow.

That will almost certainly rank as the biggest storm to hit the Bay Area in 14 months, since San Francisco received 1.39 inches of rain on Nov. 30, 2012. It may even be the biggest in four years, when 2.48 inches drenched San Francisco on Oct. 13, 2009.

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