La Niña is back in control of California's weather, and like last winter, her temper seems to be flaring.

First it was strafing November winds. Then persistent December frost. Now half the state is dry – not a drop of rain in Sacramento this month, and Lake Tahoe ski resorts are yearning for snow. And at the other end, Los Angeles is breaking rainfall records.

"It's been a strange La Niña," said Ken Clark, a senior meteorologist at AccuWeather based in Southern California. "It's playing havoc with our precipitation amounts so far. We've got a lot of signs that were right on track, but the weather pattern's been kind if screwy."

What's screwy is a high-pressure system parked offshore of Northern California, diverting storms around the region and into Southern California instead. That's not normal La Niña stuff.

La Niña is a periodic cooling of the equatorial Pacific Ocean that causes the jet stream to arc far to the north. It is a fickle phenomenon in California because the position of the jet stream determines whether the state is wet or dry in La Niña conditions.

In Central California latitudes, including Sacramento and Lake Tahoe, the outcome is often a tossup.

Last winter's La Niña was wet and snowy, partly because other phenomena pressed the jet stream well south. Among them: a negative Arctic oscillation, which pushed bitter northern cold into the United States, and a periodic cooling of the North Pacific, known as the negative phase of the Pacific decadal oscillation, which amplifies a La Niña effect.

The Arctic influence is not in play so far this year, but the Pacific oscillation remains negative.

More importantly, La Niña itself is tamer than last year. The National Weather Service calls this winter's version moderate.

Even so, in a prediction for the December through February period, released Nov. 30, the Weather Service predicted wetter-than-normal conditions for California roughly north of Sacramento.

Bill Patzert, a climatologist at NASA's Jet Propulsion Laboratory in Pasadena, agrees with that prediction.

"I would say, look for the impacts of last winter, but muted," he said. "I don't see mammoth breaking news again."

That ought to please ski towns in the Sierra Nevada, where the winter economy depends on plentiful snow but too much can be crippling, as was the case in some areas last winter.

"Last year was exceptional," said Gary Bell, longtime owner of Sierra Ski and Cycle Works in South Lake Tahoe. "It made a lot of us forget that this is a little bit closer to an average start to winter."

Bell is still renting bikes at his shop and waiting for ski season to really kick off. Most Tahoe ski resorts are open but with limited terrain available, and most of that thanks to snow-making equipment.

"I want to do snow dances," Bell said.

Last year's big La Niña broke a three-year drought in California. Recent history has brought the state other big winters that falsely promised to end a drought that proved longer lasting; but forecasters don't seem worried about that yet. The state's reservoirs are still splashing with last winter's bounty, and winter hasn't even officially begun yet. That happens Dec. 22 with the winter solstice.

Anybody brave enough to make a 10-day forecast, however, sees no break in the current cold and dry conditions.

"We're thinking it will be later in December and more into January and February when things really pick up," Clark said.

Once the pattern does change, officials will be bracing for the kind of extreme events that have made La Niña famous.

California's Central Valley has seen many of its most extreme flood emergencies during the Pacific's cool phase, not its warmer El Niño sibling. Weather experts say atmospheric rivers, of which the feared "Pineapple Express" is but one variety, are more likely in La Niña years.

An atmospheric river is California's version of a hurricane, the difference being that the water and wind are strung across the Pacific Ocean from the tropics like a fire hose, rather than concentrated in a swirling mass.

Scientists say such extreme weather events are becoming more common due to climate change. It is a creeping effect that is difficult to measure, said Thomas Peterson, chief scientist at the National Climatic Data Center in North Carolina, an arm of the National Oceanic and Atmospheric Administration.

Borrowing a metaphor from another researcher, Peterson compared it to a home-run hitter who starts taking steroids. As the drug's effect begins to take hold, you might see a slight increase in homers, but no clear pattern gives away the cause.

Another season or two go by, however, and suddenly there's a string of home-run records to look at and you can see that a change has clearly taken place.

Greenhouse gases are like the steroids of the atmosphere, Peterson said, and we're only now beginning to notice the patterns.

NOAA reported last week that the nation set a record in 2011 with a dozen extreme weather events, measured as those that caused at least $1 billion in damages.

"Heavy precipitation is definitely increasing. Heat waves are definitely increasing. Both are attributable to climate change," said Peterson. "We are changing the weather. Absolutely."

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Call The Bee's Matt Weiser, (916) 321-1264. The Bee's Bill Lindelof contributed to this report.

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