The record El Niño of 2015-2016 did not end the California drought
The presence of a strong El Niño in 2016 heightened anticipation of a drought-busting rain and snow season, but the results were mixed, and ultimately the observed rain and snowfall was not sufficient to lift California out of the drought that has prevailed since 2011.
El Niño helped steer enough rain and snow to the the northern part of California to help cut into the multi-year drought that’s plagued the state, but southern California did not receive nearly as much precipitation and dry conditions already began creeping back in April.
During the summer of 2016, 21 percent of the state remained in the US Drought Monitor's exceptional drought category, an improvement over the 46 percent seen from April through October of 2015, and the 58 percent from August through October of 2014. But El Niño was officially declared over on June 9, and the state remains in a precarious position.
Statewide snowpack ends below average...
California receives most of its precipitation during the winter months, which means snowpack serves as an important natural storage reservoir. In normal years, the snowpack supplies about 30 percent of California’s water needs as it melts in the spring and early summer. The greater the snowpack water content, the greater the likelihood California’s reservoirs will receive ample runoff as the snowpack melts to meet the state’s water demand in the summer and fall.
In 2016, and during a strong El Niño that typically increases rain and snowfall, the April 1 snowpack measurement in the state's north barely equaled the average, and in the south, which has historically benefited the most from El Niño storms, levels failed to rise enough to fill dried-out rivers and aquifers.
The statewide April 1 snowpack measurement in 2016 showed state water resources at 87 percent of the long-term average. The northern Sierra Nevada snowpack was at 97 percent of the long-term average; the central region was at 88 percent; and the southern region was at 72 percent. The April 1 snow survey is critical because these measurements generally represent when snowpack is supposed to be at its peak, and while 2016's average measurements brought relief, years of below average and record low rain and snowfall means the drought remains. Just one year before, in 2015, the April 1 statewide snowpack was at 5 percent of the long-term average, the record lowest for this date by far.
While for many parts of the state there will be both significant gains in both reservoir storage and stream flow, the effects of previous dry years will remain for now. - Frank Gehrke, chief of the California Cooperative Snow Surveys Program
...and melted rapidly due to persistent winter warmth
Climate change has intensified water deficits in California by fueling record-breaking temperatures that evaporate critically important snowpack, convert snow to rain, and dry out soils. January through March temperatures in California were 2 to 6°F above the 20th century average, with parts of coastal southern California seeing even higher temperature anomalies.
After April 1, 2016, the snowpack melted fast due largely to warm mountain temperatures that, as in the preceding drought years, persisted through the winter. On May 1, statewide snowpack was at 59 percent of the long-term average, and by June 1, it was at 23 percent.
Restoring the state's water supplies to their pre-drought levels would require several years of intense rain and snowfall. A June 2016 analysis finds that even with the strong 2015-2016 El Niño, the Sierra Nevada snowpack will likely take until 2019 to return to pre-drought levels.