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‘Horizontal hurricanes’ – atmospheric rivers pose extreme storm risk but also drought-busting opportunity for California; plans for new 2-3 week forecast and ranking system similar to hurricanes

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By Kurtis Alexander September 27, 2017 read full SF Chronicle article here

…California is facing its own threat of bigger and more destructive storms. Mounting research, much of it done in the wake of the near-record rains that pulled California out of a five-year drought this past winter, shows that seasonal soakers may not come as often as they used to, but could pack more punch when they do arrive.

….The massive weather systems that rise out of the Pacific Ocean, now popularly called atmospheric rivers, can drop as much water as Hurricane Irma dumped on Florida this month — billions of gallons that submerged cities and towns.

…U.S. Geological Survey ran a simulation of what a sequence of severe atmospheric rivers might look like in California…. 23 days of rain and wind that caused floods and landslides to the tune of $300 billion in property damage….big drenchers earlier this year…caused mass flooding in San Jose and other cities and triggered a near-catastrophe at Lake Oroville when a pair of dam spillways failed.

…A group of Southern California scientists, some of whom are partnering with Sonoma County, plan to begin publishing regular 14-day to 21-day outlooks on atmospheric rivers as soon as this winter. Current forecasts typically don’t anticipate the events more than a week out.

…Marty Ralph, director of the Center for Western Weather and Water Extremes at UC San Diego’s Scripps Institution of Oceanography, is also working to develop a rating system for the storms.  The blasts will be evaluated for intensity on a scale of 1 to 5, similar to hurricanes. A test run of the ratings last winter pegged the strong atmospheric river in February, which contributed to the damage at Oroville Dam, as a category 5 eventResearch published in the journal Geophysical Research Letters in August, which Ralph participated in, indicates that atmospheric rivers are carrying increasing amounts of water.

…Sometimes called horizontal hurricanes, atmospheric rivers are exactly what they sound like: airborne channels of water that develop over the Pacific Ocean and are pushed along by strong winds toward the West Coast during the winter….providing as much as 50 percent of the state’s annual rainfall in a matter of days — dumpings that are critical to water supplies but, at times, bring on disaster…

Another study [published in September] in the journal Scientific Reports, suggests that the swelling intensity of atmospheric rivers — while carrying the potential for more damage — will also help California maintain typical rainfall levels amid a drying climate….

During the height of the drought, California went as long as a year without seeing a single atmospheric river. But the state was hit by more than 30 of the systems last winter, according to Scripps researchers. That explains why the season was one of the wettest on record….

Alexander Gershunov, Tamara Shulgina, F. Martin Ralph, David A. Lavers, Jonathan J. Rutz. Assessing the climate-scale variability of atmospheric rivers affecting western North America. Geophysical Research Letters. Volume 44, Issue 15 16 August 2017. Pages 7900–7908 DOI: 10.1002/2017GL074175

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