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Valerie Trouet, a tree-ring specialist at the University of Arizona and the lead scientist on the study said:

“Snowpack conditions in the Sierra Nevada mountains present an ominous sign of the severity of this drought.”

“We were expecting that 2015 would be extreme, but nothing like this.”

The winter snowpack in 2015 was the worst in the past 500 years, and the water content in the snow that remained was ALSO at historic lows. In this fourth year of California’s drought, the water content of the snow was only 5% of its historical average over the same 500-year period, the scientists reported.

In California, the melting snowpack generally supplies at least ONE-THIRD of the state’s water each year, through the network of reservoirs, and the snowpack also replenishes the groundwater in California’s deep aquifers. As reported on SFGate.com:

Trouet and her team of scientists examined the evidence of past snows and rainfall year after year as they are reflected in the varying widths of California tree rings over the centuries, using thousands of details kept in an international tree-ring data bank maintained in Boulder, Colo., by the National Oceanic and Atmospheric Administration.

Trouet’s survey involved counting and measuring the annual rings in more than 1,500 blue oak trees acquired at varying levels in the Sierra and coastal ranges, some from core samples taken from living trees, and others that were measured across stumps left from trees killed in storms. During wet years, those rings, which indicate a tree’s annual growth rate, are typically wider. In dry years, they are narrower.

Separately from the study:

The promise of a wet fall and winter appears to be increasing as forecasters see El Niño conditions strengthening across the Pacific with tropical ocean temperatures rising toward record highs, said Daniel Swain, a Stanford graduate student who maintains the highly regarded California Weather Blog.

Swain, a member of Stanford’s Climate and Earth System Dynamics Science Group whose scientists have tied the drought to the increased pace of global warming, noted that while the future impact on California of the strengthening El Niño conditions in the tropical Pacific are uncertain, the evidence that the Pacific Ocean’s water temperatures are rising is clear.

“El Niño conditions tend to shift the odds away from a dry winter,” Swain said. “And water temperatures should be higher than normal.”

Swain points out that, while precipitation in the highest peaks of the Sierra could fall as snow, much of the precipitation at lower elevations (where most of the snowpack USUALLY is) could fall as rain. No matter what, even the wettest winter on record cannot end California’s historic drought. A single wet winter could help to restore California’s reservoirs but could not refill the aquifers below the Sacramento and San Joaquin valleys, upon with so much of California’s agriculture, and the U.S. food supply, depend.