Water & Drought

La Niña arrives in California. What that means for the drought.

La Niña has arrived, bringing California the possibility of a relatively dry winter.

With the state entering its fifth year of drought, the National Weather Service made it official Thursday, following weeks of speculation, declaring that the country will see a winter of La Niña. That’s a weather phenomenon associated with cooler-than-average temperatures in the surface of the eastern tropical Pacific Ocean near the equator.

Jim Mathews, a weather service meteorologist in Sacramento, said the phenomenon is expected to bring somewhere between a normal or somewhat drier winter.

“The odds favor normal to below-normal (precipitation) for us, for Northern California,” he said.

But he said a lot of uncertainty persists. Northern California just finished one of its wettest Octobers on record, and more rain is expected next Tuesday and Wednesday. But the effects of La Niña won’t really begin to be felt until December.

“We’re just getting into the winter season,” he said.

The La Niña phenomenon often follows an El Niño season. Last winter was one of the strongest El Niño seasons on record, although it only brought normal levels of precipitation in Northern California and below-normal rains in Southern California.

This year’s La Niña effect is weak, leading other California forecasters to express doubt that it will bring more dry weather to California.

State climatologist Michael Anderson said La Niña tends to create a looping jet stream centered around high pressure in the Pacific Ocean. If the high pressure anchors around the international dateline, La Niña tends to bring wetter weather to North America. If it centers in the eastern Pacific, it tends to bring drier weather. If it moves around, the results are periods of wet and dry weather.

“Right now, it’s looking like choice three,” he said, referring to a looping jet stream with a shifting high pressure center – and mixed periods of wet and dry weather.

Bill Patzert, an oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena, said the effects of La Niña in California are likely to be “pretty marginal” though it seems to be contributing to drought conditions in the American Southeast.

“It’s feeble,” Patzert said of this year’s La Niña. “In some ways that is good news. A weak to nonexistent La Niña opens the door to north Pacific storms and atmospheric rivers like we had last month.”

The last La Niña winter was in 2011-12, which was a dry winter.

Dale Kasler: 916-321-1066, @dakasler