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Environment

Watch intense rivers of atmospheric moisture spray the Pacific Northwest like a firehose

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In recent weeks, atmospheric rivers have also helped ease the drought in Northern California.

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The amount of precipitable water in the atmosphere over the northern Pacific is seen in this animation created using data from microwave observations by polar orbiting satellites. The animation covers the period between Nov. 5th and 7th, 2016. (Source: Cooperative Institute for Meteorological Satellite Studies.) During this past weekend and into Monday of this week, parts of British Columbia were hosed with copious, flood-inducing precipitation, thanks to at least two so-called 'atmospheric rivers' originating far to the south and west. You can see them in the animation above, which shows the amount of precipitable water in the atmosphere. Watch for long, thin plumes depicted in green, yellow and orange, stretching from near Hawaii and hitting the Pacific Northwest. Atmospheric rivers contain an almost unimaginable amount of moisture. A strong one can move water vapor "roughly equivalent to 7.5–15 times the average flow of liquid water at the mouth of the Mississippi River, according to NOAA's Earth System Research Laboratory. To offer another comparison, one study found that atmospheric rivers can even carry more moisture than the mighty Amazon River, with flows on the order of 200,000 tons or more per second.

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An animation of GOES-West weather satellite images shows copious atmospheric water vapor flowing toward the Pacific Northwest between Nov. 4th and 7th. (Source: NOAA Earth System Research Laboratory) With this in mind, it shouldn't be surprising that between 30 and 50 percent of the annual precipitation falling on the U.S. West Coast states occurs in just a few atmospheric river events. In recent weeks, such events have helped ease the drought in Northern California. "October brought a couple of ARs to NorCal, and contributed significantly to early season precipitation there," says F. Martin Ralph, who directs the Center for Western Weather and Water Extremes at the University of California San Diego. That's fantastic news for the state. But atmospheric rivers can also have a downside: flooding. While relatively modest in strength, the two that have been aimed at British Columbia in recent days have done just that — including some flooding in a park in the city of Courtenay that actually brought salmon along with it: https://twitter.com/KrissyVann/status/794953182984048640 This flooding looks relatively modest compared to far more devastating events in the past associated with atmospheric rivers. Consider these examples from the Earth System Research Laboratory:

What might the next few weeks bring for California and other parts of the West Coast? "It's not really possible to get a good look ahead that far," says Ralph. "Though seasonally, one would expect more action normally during the next couple of months." California could sure use that action — but without the flooding.

  • Research has shown there were 42 ARs that impacted CA during the winters from 1997 to 2006, and the resulting seven floods that occurred on the Russian River watershed northwest of San Francisco during this period were all associated with AR conditions.

  • A major flood in California, known as the "New years Day Flood" in 1997 caused over $1 Billion in damages and had a well-defined AR.

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