A historic El Niño is almost certainly coming, government forecasters said Thursday, raising hopes that California could see a wet winter.
El Niño conditions occur when water temperatures in the equatorial Pacific Ocean rise by at least half a degree Celsius above normal for three consecutive months. Strong El Niños often, but not always, bring heavier winter rains to California, particularly Southern California.
The National Weather Service said Thursday that “forecaster consensus unanimously favors a strong El Niño” with ocean waters at least 1.5 degrees Celsius above normal.
“We are predicting this El Niño could be among the strongest El Niños in the historical record dating back to 1950,” said Mike Halpert, deputy director of the National Oceanic and Atmospheric Administration’s Climate Prediction Center. Government forecasters now predict “above average rainfall ... from Central California to Southern California,” Halpert said.
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Equatorial water temperatures were about 1.2 degrees above normal in July, Halpert said. Compared with past El Niños at this time of year, the July conditions ranked among the strongest on record. Only July 1997 ranked as significantly stronger. That winter, Sacramento saw 32 inches of rain.
But – as a cautionary note – conditions are roughly the same as they were in July 1987, a year that later brought just 15 inches of rain to Sacramento. The city normally gets about 18 inches of rain.
El Niño typically alters weather patterns throughout the Western Hemisphere. In the United States, the Northwest usually gets drier and the Southwest wetter. The Sacramento region sits between these focal points, and El Niño effects here can go either way.
Forecasters emphasized that El Niño is not a sure bet on increased rainfall. “Just because something is favored doesn’t guarantee it will happen,” Halpert said.
Added Kevin Werner, director of NOAA’s Western Region Climate Services: “There is a lot of uncertainty. A single El Niño year is very unlikely to erase four years of drought.” But, he said, “especially in Southern California, it stacks the deck in favor of an above-average year.”
While state water officials would welcome more rain in Southern California, the rivers of Northern California and snowmelt from the Sierra produce the bulk of fresh water used throughout the state. Government forecasters didn’t make predictions about rain and snow in Northern California on Thursday. Werner said there is “no correlation” between El Niño conditions and precipitation in Northern California.
The latest El Niño forecast comes as millions of Californians cut water use in response to prolonged drought. Under state mandates, most water districts in the Sacramento region must reduce water use by 28 percent to 36 percent compared with 2013. In July, the region’s water districts collectively cut use by 37 percent, building on a 35 percent reduction in June, the Regional Water Authority said Thursday.
State officials moved quickly to dispute the notion that major rainfall is certain this winter. Within hours of Thursday’s forecast, the California Department of Water Resources issued a news release entitled, “Do Not Count on El Niño to End Drought.”
“Historical weather data shows us that at best, there is a 50/50 chance of having a wetter winter,” state climatologist Michael Anderson said in the release. “Unfortunately, due to shifting climate patterns, we cannot even be that sure.”
During weak or moderate El Niño events, in which equatorial Pacific water temperatures rise by a modest amount, it’s hard to find a consistent rain pattern in Sacramento, according to a Bee review of data back to 1950. The average precipitation in those years was 18 inches – about normal for the city – and ranged from 7 inches to 31 inches.
Stronger El Niño years, when ocean temperatures rise by a significant amount, are more encouraging. During those years, rainfall in Sacramento averaged 24 inches, roughly 130 percent of normal. Six of the nine strong El Niño years that have occurred since 1950 resulted in more than 25 inches of rain in Sacramento.
Complicating the winter forecast this year is the presence of a blob of warm water in the northeastern Pacific. This warm water and a corresponding resilient ridge of high pressure have annoyed forecasters during the drought, pushing rain and snow away from California. Climatologists aren’t sure if the blob will disappear in the face of a strong El Niño, or how the two weather patterns might interact.
Halpert said ocean temperature and tropical rainfall shifts suggest that the high pressure ridge could “fade away toward normal,” allowing weather systems to move into California. He warned that Southern California could see “flooding in the midst of a severe drought. ... That’s certainly on the table.”
Local water officials said they don’t think the El Niño predictions will undermine conservation efforts.
“Until it happens, I believe our customers will continue to do what they need to do,” said Steve Nugent, general manager of the Carmichael Water District. Carmichael customers reduced consumption 39 percent in July, above the 36 percent mandated for the district.
In the city of Sacramento, savings hit 35 percent last month, well above the 28 percent mandate, and officials said they believe residents will stay the course.
“We would caution anyone (against) thinking that because they’re forecasting an El Niño, that our troubles are over,” said Bill Busath, the city’s utilities director.