San Diego may be riding a precipitation see-saw that’s swinging downward. After a wet year that followed an extremely dry one, rainfall could be scarce again during the 2019-20 rainfall season. It has been so far.
Or, the atmospheric patterns that led to a wet and reservoir-renewing winter in California last year could return.
There’s a “wam blob” in the Northern Pacific to consider, plus the influence of sea-surface temperatures along the equator. Or perhaps some unseen or poorly understood force will take charge and drive us into a wet — or dry — winter.
Some years, your guess is as good as anyone’s. This might be one of those years.
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“This is not one of our most confident forecasts,” Mike Halpert, deputy director of the Climate Prediction Center said last month as the agency issued its annual winter outlook.
Now it’s time for your guess and the 18th annual Union-Tribune Precipitation Prediction Contest.
Tell us how much rain you think San Diego will get this season, from Oct. 1, 2019, to Sept. 30, 2020. If your prediction is right as rain, you could win two-day ski lift tickets for four at Snow Valley Mountain Resort in Running Springs, plus two-nights lodging at Lake Arrowhead Resort & Spa. The second-place finisher will be able to accurately track future precipitation with a $50 rain gauge from Grangetto’s Farm and Garden Supply.
Details on entering the contest are below. Before you submit your prediction, read what some of the experts are thinking about the coming winter. But keep in mind, no one really knows what the weather spirits have in store for us.
Dry, wet, dry, wet
Since the 2013-14 rainfall year, which was dry, San Diego has alternated between dry and wet years. That wasn’t supposed to be case in 2015-16, because a massive El Niño was brewing in the equatorial Pacific.
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El Niños occur when the waters along the equator warm to above normal for several months. The warmer those waters, the better the chances of a strong jet stream delivering copious rainfall to Southern California. Or so the climatologists thought.
Despite some of the warmest equatorial waters on record, San Diego and the rest of California that year were left high and dry, and a multi-year, statewide drought continued.
While El Niño was fizzling that year and California was drying up, a huge swath of warm water in the Northern Pacific, which came to be known as the “warm blob,” persisted.
The blob is back this fall, with sea-surface temperatures well above normal from the Gulf of Alaska to off the coast of Oregon and Washington. Does that mean dryness will return, too? Not necessarily.
“It’s so strong in this case that it’s hard to imagine it won’t have some kind of influence on the coming winter,” said Dan Cayan, a climate researcher at the Scripps Institution of Oceanography at UC San Diego.
Just how big that influence will be is cloudy, so to speak. There haven’t been a lot of previous warm blobs to study, so it’s hard to know how much of a role — if any — the blob played in the 2015-16 dryness, and how big of a player it will be this year.
“I think the blob was part and parcel of the very persistent high-pressure system that sat over the North Pacific (in 2015),” Cayan said. “But it’s hard to say how much, by itself, it influenced the subsequent atmospheric circulation.”
If a storm does manage to form over the North Pacific this winter, the warmer waters could add moisture to any tempest that lashes California, Cayan said. Also, a strong storm could mix the warm surface waters with cooler, subsurface waters and wipe out the blob. The waters have cooled slightly in recent weeks, but they remain much warmer than normal.
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Halpert, of the Climate Prediction Center, said the blob’s role in the underperforming El Niño of 2015-16 has not been established.
“The ocean itself typically does not force the atmosphere in that region of the Pacific,” Halpert said. What was missing with the dud El Niño was storm formation in the tropical Pacific, which would normally extend the storm-steering jet stream and point it at California, he said.
Cayan agrees — ocean conditions in the tropical Pacific likely play a bigger role in West Coast weather patterns than what’s happening in the North Pacific. And for that reason, he thinks San Diego is probably in for a sub-normal rainfall year.
“Because the tropics have tended toward cooler, I suspect that at best, we’ll get a normal year,” he said.
Cayan thinks San Diego will probably get around 9 inches of rain. Normal is 10.34 inches.
“I don’t feel extremely confident, and I won’t be surprised if I’m wrong,” Cayan said.
San Diego’s annual rainfall total is often dependent on a just a handful of big storms. When an atmospheric river, a massive stream of moisture sometimes more than a thousand miles long is pointed at a region, rainfall totals go way up.
“If these systems linger for three or four days,” Cayan said, “they can generate a lot of precipitation.”
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The Climate Prediction Center does not speak in absolutes; its forecasts stress increased chances of one outcome or another. The CPC’s forecast map issued last month shows increased chances of drier-than-normal conditions in Northern and Central California in the late fall and early winter, with no slant toward dry or wet in Southern California.
But for the January through March period, the CPC’s map shows an increased likelihood of below-normal rain in Southern California.
Halpert said conditions in the equatorial Pacific are neutral, meaning neither an El Niño nor a rain-robbing La Niña is likely to develop before spring. In neutral years, the center has less confidence in its forecast.
Ivory Small, the science and operations officer at the weather service’s office in Rancho Bernardo, thinks the general trough pattern along the West Coast, which held for most of the year and was so conducive to storminess, was hanging on until recent weeks.
Now high pressure has dominated in the North Pacific and much of the West. San Diego has had no rain since Oct. 1.
Small said his “first inclination” was to go with an inch above normal rainfall in San Diego. That would mean 11.34 inches.
Alex Tardy, the local office’s warning coordination meteorologist, said the neutral conditions in the Central Pacific, combined with the warm waters in the Northern Pacific, create the potential for dry conditions in the fall and spring, or a shorter-than-normal winter rainy season. His prediction: 7.08 inches
Your turn
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, 2019, and Sept. 30, 2020. We typically receive more than 500 entries. So to break potential ties (three years ago, we had a three-way tie for first), also tell us the calendar day that you think will be the wettest. Including that tie-breaker improves your chances of winning.
There are three ways to enter the contest: Enter online at https://tinyurl.com/yhdanzgk 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 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. 25.
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