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Researchers said today that Northern California has been inundated with 228 centimeters (89.7 inches) of precipitation already this winter, surpassing 1983’s 225 centimeters (88.5 inches) for the full water year (ending in September) to make this the wettest full water year in recorded history in this key region. The nearly 90 inches is the combination of rain and the liquid equivalent of snow that has fallen.

“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.

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.”