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.
On Friday, Shasta Lake, the state’s largest reservoir, was just 24% full. Oroville Lake, the second-largest, was 29% full. The third-largest, Trinity Lake, in Trinity County, was 29% full.
Several rare, heavy storms in October got this winter season off to a very promising start, water experts say. But because of the extremely dry conditions, due in part to hotter-than-normal weather exacerbated by climate change, much of the rain soaked into the ground and didn’t run into reservoirs.
“We had fantastic amounts of rain hitting the landscape but a relatively modest response,” said Mike Anderson, the state climatologist and a supervising engineer with the California Department of Water Resources.
“The rivers came up a little bit, and some water got into Oroville and Folsom. Drought buster? Not so much. But it got the soils wet.”
Anderson said that in many areas of the state, at least 10 inches of rainfall are needed to saturate the soils before there is significant runoff that can quickly raise the levels of reservoirs. In Sierra Nevada watersheds that have lots of granite, 3 or 4 inches of rain can begin substantial runoff to Hetch Hetchy and other reservoirs.
The trouble is if a big storm arrives, but then it is dry for weeks and weeks, the ground begins to dry out again, he noted.
“Typically in November, we want to see storm regularity pick up, and maybe get one every couple of weeks,” he said. “Then in the middle of the winter season get a really stormy two-week period. We’re not seeing that yet.”
No significant rain is forecast for the next 10 days through Thanksgiving weekend in the Bay Area or Los Angeles.
But long-range computer models show that there may be another storm system setting up around Nov. 30 and Dec. 1.
“It looks like a pair of pretty big storms,” Anderson said. “Something to keep an eye on.”
It’s hard to know if that will happen. Long-range weather forecasting is not particularly accurate. The most recent winter outlook from NOAA, the National Oceanic and Atmospheric Administration, predicts it will be wet in the Pacific Northwest and dry in the Southwest through mid-February
For California, NOAA said continued drought conditions through February are likely. But the agency notes its confidence is only “low to moderate.” That’s because California is in the middle, Anderson noted.
California needs about a half-dozen more major atmospheric river storms between now and April to fill reservoirs and break the drought, he added.
“We’re off to a good start,” Anderson said. “But we need a lot more.”
