“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.”

A timely example of how the nuances in predictive models have a way of getting lost.

Rong-Gong Lin II writing for the LA Times.

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 heralder of drought.

But the would-be 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 dumping eye-popping snowfall in the mountains.

The Sierra Nevada has a snowpack of 240% of average for the date, and 126% of where it should be by the start of April. San Francisco was 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 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. But for Southern California, the agency reported there was a 33% to 50% chance of below-normal precipitation.

Taking the midpoint of that forecast — say, 40% — that meant 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.

“These probabilities are going to be relatively modest ... because that is the state of the science,” DeWitt said.

Those subtleties, however, tend to get less attention. Easier to understand was the bottom line, as a center’s statement noted: “The greatest chances for drier-than-average conditions are forecast in portions of California,” as well as other southern parts of the nation. 

...

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, which came in at record strength.

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.”