Is California headed for a dry winter?

Despite forecast for La Niña, weather patterns show that’s less likely in Bay Area than in L.A.

You may have seen it on social media or heard it while talking to a friend: This is a La Niña year, so California won’t get any rain this winter and the severe drought is only going to get worse. Right?

Maybe not. Although that’s a common belief, it’s not supported by past history. The reality is that a lot depends on where you live.

“The message most people get about La Niña seems to be biased by Southern California,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay.

“There is a really good connection between La Niña and drier-than-normal weather in Southern California. But in Northern California, it’s a coin flip.”

La Niña conditions occur when Pacific Ocean waters off South America are cooler than normal. They are the opposite of El Niño, the atmospheric trend when waters there are warmer than average.

Null, a former lead forecaster with the National Weather Service, has spent years tracking the amount of rainfall California receives every winter and looking for trends.

Since 1950, there have been 23 winters with La Niña conditions, his records show. Although some were dry, like last year or 1976-77, some also were very wet, such as the winter of 2016-17, when relentless atmospheric river storms caused the near-failure of Oroville Dam.

Rainfall that winter ended the state’s previous drought and prompted widespread flooding in downtown San Jose.

The average rainfall over those 23 years was 93% of normal.

Similarly, the region farther north, where California’s largest reservoirs are located, including Shasta, Oroville, Trinity and Folsom, has received 97% of normal rainfall, on average, in La Niña years. And the rugged coast near the California-oregon border has received an average of 103% of normal rainfall during La Niña years.

So why does the stubborn belief persist that La Niña guarantees dry weather?

The farther south one goes, the drier it has been in La Niña years.

Null’s data shows that La Niña years have brought only 79% of normal rainfall, on average, to the Los Angeles-san Diego area.

“Fortunately the state’s biggest reservoirs are not in Southern California,” he said.

The engineers who built California’s largest reservoirs in the 1940s, ’50s and ’60s didn’t have 70 years of precise climate data, precise satellite images and computer weather models.

They had slide rules and hand-drawn blueprints. But they did know from previous generations which watersheds of the state tended to deliver the most rain or melting snow, and that’s where they built many of the big dams to catch the water for cities and farms.

Many of those reservoirs have fallen to very low levels as Northern California has received less rain in the past two years than any two-year period since 1976-77.