After three years of drought, reservoirs across California finally filled up this past winter with plentiful rainfall -- driven by El Niño conditions, where Pacific Ocean waters warm up, often bringing wet winters for the state.
But now, as the next winter rainfall season approaches, water experts are closely watching the emergence of the opposite trend -- La Niña -- a shift that increases the chances of a dry year ahead for much of California.
Since May, Pacific Ocean temperatures along the equator have been steadily cooling, a classic symptom of a La Niña, according to scientists at the National Oceanic and Atmospheric Administration.
"For Central and Southern California, it looks like a drier-than-normal winter, and it may be for Northern California also, but there's still a lot of uncertainty," said Jon Gottschalck, head of forecast operations at NOAA's Climate Prediction Center in Washington, D.C.
Since 1950, there have been 19 years with La Niña conditions. In those years, rainfall in San Francisco has averaged 90 percent of normal, and in Los Angeles 80 percent of normal. During the most recent La Niña event, in 2007-08, rainfall was 79 percent of normal in San Francisco.
But it's not a certainty that Northern California lawns will be brown come next spring. That's because in some La Niña years, rainfall has been more plentiful than normal. In one such year, 1999, San Francisco rainfall hit 115 percent of normal.
Overall, however, drier winters are more likely during La Niñas, said Jan Null, a former lead forecaster with the National Weather Service who now runs Golden Gate Weather Services in Saratoga.
"It could be a dry winter," Null said. "But it's too early to panic."
The reason? We have some water in the bank now.
Rainfall in San Jose in the year that ended June 30 was 114 percent of normal and 106 percent of normal in San Francisco. Today, underground aquifers have refilled and reservoirs across the state are in good shape.
"Water managers would much rather hear El Niño than La Niña," Null said. "But they are much more comfortable now that they have some reserves."
During the 18 El Niño winters since 1950, Null notes, the average rainfall in San Francisco was 117 percent of normal, and 132 percent of normal in Los Angeles.
To understand major climate trends, scientists collect temperature readings every week from dozens of buoys in the Pacific Ocean. They combine them with satellite images and temperature readings from ships, and then plug the data into computer models from scientific agencies around the world.
Although scientists don't know the exact causes, El Niño conditions begin when trade winds that normally blow westward across the Pacific weaken.
That allows warm Pacific Ocean water near the equator to spread east, toward South America. The opposite effect, a strengthening of the trade winds and cooling of the water in the Pacific, is a La Niña. Neither trend is related to the ongoing warming of the planet from climate change, NOAA's Gottschalck said.
The term El Niño -- or "little boy" in Spanish -- was originally used by fishermen along the coasts of Ecuador and Peru to refer to "the Christ child" because the phenomenon appeared every three to eight years around Christmas. La Niña, which means "little girl," grew from that term.
Currently, water along the equator in the Pacific is between 1 and 2 degrees Celsius cooler than normal. In extreme La Niña events, it can get up to 4 degrees Celsius, or 7 degrees Fahrenheit, cooler, affecting everything from the frequency of storms to catches of commercial fishermen.
Scientists say they will know more in the months ahead as California's traditional rainy months of December, January and February near. In the meantime, water districts are holding their breath.