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Northern California Surpassed the Wettest Year on Record

14 Apr 2017, 2:11 pm

Never in nearly a century of Department of Water Resources (DWR) record keeping has so much precipitation fallen in the Northern Sierra of California in a water year. DWR reported today that 89.7 inches of precipitation – rain and snowmelt – has been recorded by the eight weather stations it has monitored continuously since 1920 from Shasta Lake to the American River basin. Today’s total surpassed the previous record of 88.5 inches recorded in the entirety of Water Year 1983. The region’s annual average is 50 inches.

“It’s a 34-year-old record that’s been broken,” said Dr. F. Martin (Marty) Ralph, Director of the Center for Western Water and Weather Extremes (CW3E) at Scripps Institution of Oceanography at the University of California San Diego. Ralph says there could be even more rain to come, meaning the record will be broken by an even larger margin.

Rainfall is measured during “water years,” which run from Oct. 1 of one year to Sept. 30 of the next year, so there are still five months remaining in the water year 2017, though normally summers are dry. The six-station index in the Tulare Basin, often called ground zero of California’s five-year drought, which officially ended in most of California on April 7, has recorded 178 percent of the amount of precipitation that normally falls by this date during an average water year. Total precipitation so far is 45 inches, about 1.5 times the average annual precipitation of 29.3 inches in the basin.

The snow water equivalent of California’s snowpack is far above average throughout the Sierra Nevada — 176 percent of the April 13 average. DWR will conduct its final snow survey of the season on May 1 at Phillips Station in the Sierra 90 miles east of Sacramento.

Ralph and other scientists at CW3E study extreme weather events in the West, and their work has contributed to a better understanding of what leads to super wet years like this. One of the things they’ve discovered is that most of the rain in those 89.7 inches was from storms fed by “atmospheric rivers,” channels of water vapor driven by strong winds that can deliver intense amounts of precipitation in very short periods of time.

Normally a dozen or so atmospheric rivers hit Northern California each year.  They produce 40-50 percent of the annual precipitation in California (last year, 54 percent of rainfall was from atmospheric rivers), and they can be the determining factor in the state’s water supply. “If we don’t get enough atmospheric rivers, we end up heading into drought. If we get too many, we can have flooding,” Ralph said.

This year, CW3E created the first ever map of all the atmospheric rivers that have hit the West Coast this winter. About thirty atmospheric rivers hit California this year, and three of them were categorized as “extreme,” based on a strength scale developed by Ralph and colleagues. Northern California normally averages just one extreme atmospheric river every three years. But this winter there were two only a month apart, the second of which contributed to a spillway incident at Oroville Dam north of Sacramento and the subsequent evacuation of nearly 200,000 people in the communities below it.

With the never-before-observed back to back months of 20 inches per month in January and February 2017 (each with an extreme atmospheric river), it is fitting that this year has become the record for an entire water year, said California  State Climatologist Michael Anderson with the California Department of Water Resources.

In addition to the map, CW3E provides research, tracking, monitoring, and experimental forecasting of atmospheric rivers, so forecasters can look at storms coming and identify whether they are a run-of-the-mill storm or something that is likely to bring a deluge. As the Oroville crisis revealed, extreme weather events can cause significant damage to infrastructure and put communities in danger. CW3E’s mission is to provide research, technology, and outreach to help make society more resilient to extreme weather events, which are expected to become more extreme with climate change.

“It helps boil down what storms to pay attention to,” Ralph said. “A lot of storms don’t have much impact compared to an atmospheric river.”

“Another vital difference from 1983’s wet year: much more of the precipitation that year came down as snow. Although we broke the record for annual precipitation, the amount of snowpack in the Sierra (about 163 percent of normal through April 1, 2017) is much less than there was in 1983 (more than 227 percent of normal),” said Mike Dettinger of the U.S. Geological Survey and CW3E.

“The West has warmed up substantially since 1983,” Dettinger said. “We just haven’t had as cold winters, and one consequence is less of the precipitation fell as snow this year than it did in 1983.”

From Meteorologist Mace Michaels

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