A La Niña forecast, so why all the rain?
For decades, two climate patterns in the Pacific Ocean have loomed large in predicting weather in California and other parts of the globe. El Niño — a warming of sea-surface temperatures in the tropical Pacific — seemed synonymous with wet winters for Southern California, while La Niña was a harbinger of drought.
But the model didn’t hold up this winter. Despite La Niña’s presence, a robust series of 10 storms brought impressive precipitation across California, spurring floods and landslides, increasing reservoir levels and bringing eye-popping snowfall in the mountains.
The Sierra Nevada has a snowpack of 240% of the average for the date and 126% of where it should be by the start of April. San Francisco has been drenched with more than 18 inches of rain since Christmas, posting its wettest 22-day period since 1862. Downtown Los Angeles has logged more than 13 inches of rain since October — more than 90% of its annual average of 14.25 inches.
Though winter isn’t over,
and a renewed dry spell can’t be ruled out, the significant storms have defied expectations of a dry winter.
The forecast in October by the Climate Prediction Center, a division of the National Weather Service, indicated that the odds were stacked against the Golden State: A rare third year of La Niña was expected. And California had already recorded its three driest years in the historical record.
The center’s seasonal forecast for December, January and February said there were equal chances of a dry or wet season in Northern California. Areas farther south in California were a bit more likely to get below-normal precipitation than in Central California.
But there is still a significant chance of an average year of precipitation. Say the forecast was for a 40% chance of below-normal precipitation for Southern California this winter: That still means there was a 35% probability of near-normal precipitation and a 25% chance of above-normal precipitation, said David DeWitt, director of the Climate Prediction Center.
Those subtleties, however, tend to get less attention. Easier to understand was the bottom line, as the center’s statement noted: “The greatest chances for drierthan-average conditions are forecast in portions of California,” as well as other southern parts of the nation.
Typically, La Niña produces dry winters in Southern California, and that pattern fit during the last two years.
But this winter, it changed. Since the start of December, downtown L.A. has received more than 11 inches of rain — more than double the average 4.91 inches for that time and above the entire December-January-February average of 9.41 inches.
Still, this shift is not an anomaly. In fact, La Niña was present during a spectacularly wet season: the winter of 2016-17.
Storms were so intense across California that they ended a punishing drought that ran from 2012 to 2017.
One meteorologist who has warned against putting too many eggs in the La Niña basket is Jan Null, a former lead forecaster for the National Weather Service. In late 2020, as La Niña was developing, he tweeted of the phenomenon: “What does it mean for California and U.S. rainfall? Almost anything!”
Four months ago, he again tweeted: “Does La Niña automatically mean another dry winter for California? Not necessarily.”
Null, an adjunct professor at San Jose State’s Department of Meteorology and Climate Sciences, has put together data comparing La Niña and El Niño years and what they’ve meant for California’s precipitation.
If you look at all the La Niña events over roughly the last 75 years, Southern California does tend to get below-average rainfall, while for Northern California, it’s more a roll of the dice. During La Niña seasons since the 1950s, coastal Southern California got just 80% of its average annual rainfall, while the San Francisco Bay Area got 93%, according to Null’s website.
On the f lip side, during all El Niños since the 1950s, coastal Southern California got 126% of its average precipitation, while the Bay Area got 109%, Null calculated.
But there are also plenty of memorable exceptions in which wet years coincided with La Niña events. One was in the winter of 2010-11, which brought 142% of downtown L.A.’s average annual rainfall.
A big reason El Niño and La Niña are fixed in the minds of many Californians as the crystal balls of winter weather is how the science developed in the 1980s and ’90s after particularly punishing winter storms in 1982-83.
There was no way to predict El Niño back then. Scientists “really didn’t even understand at that time that it was even occurring,” DeWitt said, but it later was associated with significant mayhem worldwide.
In California, the 1982-83 season brought the secondwettest annual precipitation to the northern Sierra and, according to data Null has compiled, $2.3 billion in damage in today’s dollars — one of the costliest f lood seasons in the state in the last half a century.
It was that jolt that pushed scientists to figure out ways to predict the next El Niño. The failure to forecast the 1982-83 event led to the development of a range of tools that successfully predicted another El Niño in 1997-98.
There was “massive flooding over the West Coast, especially California. And it was well predicted,” DeWitt said. The damage in California was severe — with at least 17 deaths — and brought Los Angeles its wettest February on record.
“And then the next year, 1998-99, was a strong La Niña, and you saw exactly the opposite ... these very dry conditions,” DeWitt said.
“And that imprinted on a lot of people — including the scientific community — a couple of messages: one, that that was what you were always going to see with El Niño and La Niña, especially significant-strength ones; and that basically, this was a solved problem.
“And not one of those was ever true,” DeWitt added.
He remembers his predecessor at the Climate Prediction Center testifying to Congress about the upcoming 1997-98 El Niño and its predicted effects, a forecast that ended up being on the money. “And it created this confidence that you could always rely on just knowing ... the El Niño/La Niña phase, and that would be able to give you a very accurate prediction for precipitation, especially for California. And that is just not scientifically true.”
So when a third supersized El Niño event emerged in 2015, there was hope it would raise the prospect of big, drought-quenching storms for California. Instead, downtown L.A. got less than 50% of its average rainfall; San Francisco broke even, with its average annual tally; and the northern Sierra got only 9% above its average yearly precipitation.
“The bottom line is that if you count every El Niño as wet and every La Niña as dry, sooner or later you’re gonna get embarrassed,” said retired climatologist Bill Patzert.
Patzert developed a reputation of being “as right as rain” on El Niño- and La Niña-influenced weather patterns, but he was among those who bet on a wet winter in California in 2015-16. He notes, however, that Texas was hit hard by floods that spring, and southern Texas does tend to get above-average precipitation during an El Niño event.
In fact, recent correlations between El Niño and La Niña and precipitation in California haven’t “really proven out as well,” said Marty Ralph, director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography at UC San Diego.
The correlation is imperfect, which could mean “there’s some processes that are interrupting and interfering with that correlation,” Ralph said.
So while La Niña and El Niño do factor into Southern California weather, another phenomenon known as the Madden-Julian Oscillation can affect whether storms hit. And instead of being forecast months in advance, they can be predicted only weeks ahead of time.
The Madden-Julian Oscillation — or MJO — is separate from La Niña and El Niño. It starts as a massive expanse of extra large precipitation, often in the form of tropical thunderstorms, that tend to move from the Indian Ocean eastward into the Pacific Ocean, Ralph said. That can release an enormous amount of latent heat into the atmosphere, and through that action, it can influence storm tracks and whether they hit California.
But even the MJO correlation isn’t perfect. Ralph remembers that over a fouryear period, there were two years where the MJO correlated with atmospheric rivers reaching California. The other two years, the relationship didn’t appear.
“The bottom line is: There’s a lot more to learn about what’s controlling the precipitation anomalies for the season, for the wet seasons on the West Coast,” Ralph said.
As for the rest of the winter, California is in a dry spell for the next couple of weeks. But DeWitt is looking closely at the MJO for clues to what February will bring to California’s water supply. Already, some specific phases of the observed MJO have been tied to the recent series of atmospheric river-fueled storms.
And now, conditions suggest the start of a new MJO. If it stays in its first few phases, then dies, that would bode poorly for more rain for California. But if it continues, the state could face similar heavy-rain conditions, around the middle or third week of February, DeWitt said.
“It doesn’t mean we’re going to have as intense atmospheric rivers ... but it would have an enhanced probability of precipitation over California,” he said.