La Niña: Is California heading into another drought?

Federal scientists say that La Niña — the phenomenon where Pacific Ocean waters off South America are cooler than normal — is underway this winter.

A commonly held assumption among many Californians is that La Niña means a dry winter is coming, and in years when the opposite occurs, El Niño, a wet winter is considered more likely.

So brown lawns and water rationing are just around the corner, right? Not necessarily. Looking at historical records, there isn’t a clear pattern. In the Bay Area, La Niña years have been drier than normal only about half the time.

Since 1954, there have been 22 years when La Niña conditions were present.

In 10 of them, the Bay Area had a dry winter, receiving less than 80% of its average rainfall. In eight of those years, however, rainfall was normal — between 80% and 120% of average. And in four, it was a very wet winter, with rainfall above 120% of average.

“People generally think El Niño is going to mean a wet winter and La Niña is going to mean a dry winter. The reality is that’s not always true,” said Jan Null, a meteorologist with Golden Gate Weather Services in Half Moon Bay who compiled the statistics.

But in Southern California, La Niña has meant dry winters much more often, with 15 out of 22 La Niña winters delivering less than 80% of normal rainfall.

To be sure, there have been famously dry winters when La Niña conditions were present. Like in 2011-2012, when California’s most recent, and most devastating drought began. But five years later, in the winter of 2016-2017, when that drought ended as relentless atmospheric river storms wrecked the spillway at Oroville Dam and caused $100 million in damage during floods through downtown San Jose? That also was a La Niña winter.

Some La Niña events are considered stronger or weaker, depending upon how much colder Pacific waters are than normal at the equator off Chile. But regardless, the same general pattern has played out on average. Northern California has received 91% of its historic average rainfall during those 22 La Niña years since 1954, while Southern California has received 76%.

The issue is gaining a higher profile of late. California is on edge. The state has had a rough summer.

Rainfall in San Francisco and other Northern California cities was just half of normal last winter. The Sierra Nevada snowpack on April 1 was only 54% of its historic average. Temperatures in August and September were the hottest August and September ever recorded in California history since modern records began in 1895. Combined with dr y lightning storms, dead trees from the last drought, and decades of overgrown forests due to a century of fire suppression, 4.1 million acres have burned in wildfires statewide this year, more than double the previous record.

Another dry year would increase fire risk in 2021, and could lead to tighter water supplies.

Typically in Northern California, rains begin in mid-November, and continue through April.

“This looks like the first year of a drought,” said Jay Lund, director of the UC Davis Center for Watershed Sciences. “But we don’t know if we are going to see a second year, or a third year or a fourth year. You basically have to have at least two dry years in a row before there’s a drought in California. By March or April we’ll have a pretty good sense.”

The state’s reservoirs are in OK shape.

On Monday, the massive Shasta Lake near Redding, which is California’s largest reservoir and a key water source for millions of people, was 47% full, or 79% of its historic average for that date. Similarly, Lake Oroville, the state’s second-largest reservoir, in Butte County, was 44% full, or 73% of normal. San Luis Reservoir near Los Banos was 47% full or 93% of its historic average.

Much of California, including San Jose, Fresno and Los Angeles, receives only about 15 inches of rain a year on average. That’s the same as Casablanca, Morocco.

“It’s good to be a little nervous,” Lund said. “It’s always good in California to be a little nervous about water.”

Last week, NOAA, the parent agency of the National Weather Service, issued its annual winter weather outlook. The agency notes that a mild but streng thening La Niña is underway and that there is an 85% chance it will continue into the spring. NOA A researchers say that means odds of a hotter, drier winter are elevated in the southern half of the United States, including much of California, with northern states more likely to be wetter and cooler.

But they note the forecasts deal only in general probabilities.

“Other outcomes are always possible, just less likely,” said Mike Halpert, deputy director of NOAA’s Climate Prediction Center in Maryland.

T he agenc y ’ s lon grange forecasts have had a spotty success record for California. Last year, for example, the winter outlook showed greater chances of dr y winter weather for Southern California and normal rainfall for far Northern California. The opposite happened.

“The NOAA long-term winter forecasts are nice efforts, but they have very little skill about the predictions being actually true,” Lund said. “It’s a little bit like forecasting the stock market.”

Mea nwhile, 67% of California is now classified as being in at least ‘moderate drought’ by the U. S. Drought Monitor, a weekly report. That’s the highest mid- October level since 2016.

“Given the impacts of climate change on California’s extreme weather variability,” said Michael Anderson, state climatologist with the Department of Water Resources in Sacramento, “Californians should always be prepared for dry conditions and should continue to make water conservation part of their every day lives.”