Are long-range forecasts washed up? Your turn to predict rainfall

Long-range weather forecasters have been striking out lately.

Two years ago, they whiffed on El Niño, which was supposed to be a “monster” that would drench California and end four years of drought. Instead, the El Niño of 2015-16 turned out to be a mouse, and the drought dragged on.

Last year, the forecasters swung and missed on La Niña, the opposite of El Niño that they thought would leave the state parched. Instead, much of California had its wettest year ever, and state water managers worried about reservoirs failing, not water rationing.

Will the forecasters end their slump with their predictions for the winter of 2017-18? Are long-range forecasts worth paying attention to?

We want to hear what you think. Will San Diego drop back into drought, or be deluged like last year?

It’s time for the Union-Tribune’s 16th annual Precipitation Prediction Contest. Tell us how much rain you think San Diego will record this season, and you could win two-day ski lift tickets for four at Snow Valley Mountain Resort in Running Springs, plus two-nights lodging at Arrowhead Pine Rose Cabins. The second-place finisher gets a $50 rain gauge from Grangetto’s Farm and Garden Supply.

Rules and details on entering the contest are below. But before you prepare your prediction, here’s what some experts are thinking about the coming winter season.

Will La Niña matter?

La Niña will probably be back, says the Climate Prediction Center. Normally, that would strike fear in the hearts of rain lovers, because La Niñas tilt the odds in favor of drier-than-normal conditions in Southern California.

La Niña is declared when surface waters in the equatorial Pacific stay abnormally cool over a multi-month period. Historically, those cool waters, coupled with changes in the atmosphere, have tended to shift the storm track north, leaving Southern California out of the action most of the winter.

But La Niñas come in different sizes and strengths, and the one brewing now appears to be weak, with ocean temperatures just slightly below normal. In eight of the last 10 weak La Niñas, Southern California has ended up with below-normal rainfall. In the other two, the city received a surplus of rain. Last year was one of those two years.

“It doesn’t look like a big event, by any means,” said Mike Halpert, deputy director of the Climate Prediction Center in Maryland. “It could be comparable to last year’s.”

Halpert’s prediction for San Diego rainfall? 8.75 inches. Normal for the city is 10.34.

“But we’ve learned there are no sure bets in this business,” he said.

That sentiment gets a hearty endorsement from Jan Null, a former National Weather Service forecaster who runs a private weather consulting business called Golden Gate Weather. He focuses on the West Coast, mostly California.

“I don’t find any credibility in long-range forecasts,” Null said. “Go back year after year, and the forecast (accuracy) is not there on a consistent basis.

“You don’t see any rich (long-range) meteorologists out there. It’s not something I would reach into my wallet and pay for.”

Dan Cayan, a research meteorologist at the Scripps Institution of Oceanography at UC San Diego, agrees - to a degree.

“The (climate) system is complicated,” Cayan said. “There’s some predictability, but it’s marginal.

“When you look historically at seasonal forecasts, they are prone to some fairly substantial failures. The long-term average, it’s probably better than chance, but it’s not great.”

But that doesn’t mean long-range forecasting is without merit, Cayan said. The forecasts provide a platform to understand what might be coming.

The last couple of years have led to a growing awareness among long-range forecasters that the El Niño/La Niña cycle is not the only game in town, Cayan said. Other large-scale oscillations around the globe can and have overpowered the El Niño and La Niña signals.

However, Cayan is not optimistic about the chances of a wet winter this year. “A mild La Niña is developing. If we use that as an indicator, you’d expect Southern California to be on the drier side,” he said. “Last year was really unusual. To expect a carbon copy, I think it’s probably not likely to occur.”

His prediction for San Diego? 8.5 inches.

Thinking bigger

Ivory Small, science and operations officer at the National Weather Service’s office in Rancho Bernardo, said the pattern so far this year has not been unusual. He sees a continuation of the dry conditions through the fall, but a shift after the first of the year

“This is kind of a tricky one,” Small said. “It looks to me like probably not much more than normal. I’m thinking probably 12 inches.”

Bill Patzert, a research oceanographer at NASA's Jet Propulsion Laboratory in Pasadena, gave the same prediction: 12 inches. He said his prediction hinges on one key factor: the development of atmospheric rivers, streams of airborne moisture thousands of miles long that can be pointed at one area like a fire house. Atmospheric rivers can dump many inches of rain on a region in a matter of days.

Atmospheric rivers were a big part of the picture last year. Several of them took aim at Northern California, creating the biggest snowpack in northern Sierra history. Some of those rivers sagged into Southern California. San Diego ended up with 12.73 inches of rain.

Patzert said that while the waters in the equatorial Pacific are cool - the signature of La Niña - the waters just to the north are warm. He thinks those waters could generate a replay of last year’s atmospheric rivers. If those rivers don’t form, San Diego could end up with just 6 inches of rain, he said.

Cayan of Scripps said it’s currently not possible to predict atmospheric rivers on a seasonal scale, or even a month in advance. At best, forecasters can now predict these events a week or so beforehand, he said, although Scripps scientists are working to expand the predictability window.

The Climate Prediction Center’s Halpert said the presence of the warm water north of the equator is no guarantee atmospheric rivers will form. In the El Niño year of 2015-16, there was plenty of warm water in the region, but the rivers didn’t develop. Why they didn’t, and why they did last year during a La Niña, remains somewhat of a mystery.

Null, the weather consultant and former weather service forecaster, said long-range forecasters need to recognize their limitations.

“Sometimes, the most important thing to know,” Null said, “is what you don’t know.”

Your turn

We’ve heard from some experts, two of whom think San Diego will be a little wetter than normal, and two of whom think the city will be a little drier than normal.

What do you think? Tell us how much rain you think will fall (down to the hundredth of an inch) at San Diego International Airport, site of the city’s official weather station, between Oct. 1, 2017 and Sept. 30, 2018.

Last year, we received more than 650 entries. So to break potential ties (last year, there was a three-way tie for first), also tell us the calendar day that you think will be the wettest.

There are three ways to enter the contest: Enter online after reading rules and other details. You can also send your entry via email to rob.krier@sduniontribune.com. Be sure to include your full name, a phone number so we can reach you if you win, and the tie-breaker.

Entries can also be snail-mailed to: Precipitation Prediction Contest, C/O Robert Krier, San Diego Union-Tribune, 600 B St., #1201, San Diego CA 92101.

All entries must be received by Nov. 13.

Good luck.

rob.krier@sduniontribune.com

Twitter: sdutKrier

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