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From the Los Angeles Times

There are better odds that the area around Lake Oroville, California’s second-largest reservoir, will have above-normal precipitation — now more than a 40% chance, up from a more than 33% chance in last month’s forecast. San Francisco now has more than a 50% shot of a wetter-than-average winter, up from a more than 40% probability.

Los Angeles continues to have more than a 60% probability of a wet winter during the months of January, February and March.

Here are some questions and answers about the coming winter.

Why is there more confidence that Northern California will have a wetter-than-normal winter?

Not only are we getting closer to winter, but El Niño is maintaining its strength and even getting stronger, said Matthew Rosencrans, head of operations for the National Weather Service’s Climate Prediction Center.

“From the latest observation, it’s still on an upward trend,” he said, “not even topping out right now.”

Not really. There is a 95% chance that El Niño will persist through the spring, the Climate Prediction Center says.

El Niño is a warming of ocean waters west of Peru that can cause dramatic changes to the atmosphere, altering weather patterns worldwide. In the past, it has meant that the path of winter storms that normally keeps the jungles of southern Mexico and Central America wet moves north, over California.

That pattern has traditionally meant drought in Central America and southern Mexico, and a wet winter for northern Mexico and the southern United States. (It has also meant the best surfing season in a generation, from the coast of British Columbia to Costa Rica.)

Why are scientists so confident that El Niño won’t suddenly disappear earlier than expected?

The pool of warm water in the Pacific Ocean west of Peru is huge and very deep. “There’s been a tremendous distribution of heat, and that is definitely not going away” anytime soon, said Bill Patzert, climatologist with NASA’s Jet Propulsion Laboratory in La Cañada Flintridge. “I’m quite optimistic that the entire state is going to get hosed.”

How big is this warm water in the Pacific Ocean west of Peru?

“It’s about 8 million square miles of overheated ocean,” Patzert said. “The United States is only 3 million square miles. So this is about two and a half times the size of the continental United States.

“This thing is pumping moisture out of the overheated ocean into the atmosphere above,” which of course means it’s having a huge impact and rearranging all the pieces on the motherboard across the planet.”

When are the El Niño rains expected to come?

In 1983, El Niño rains came in earnest in January, Patzert said. In 1998, the biggest storms statewide didn’t kick in until February, he said.

Daniel Swain, climate scientist for Stanford University, said he suspects that at some point during December, the weather pattern will change, “and certainly by January, February and March we’ll see above-average precipitation — potentially well above-average.”

By then, Patzert said, Californians should expect “mudslides, heavy rainfall, one storm after another like a conveyor belt.”

Actually, no, Patzert says. While El Niño storms come straight from the west from an area of the Pacific Ocean north of Hawaii, November’s snowstorms originated from the Gulf of Alaska.

That’s what brought very chilly weather and lots of snow to the resorts at Lake Tahoe. One single storm brought 20 inches of powder to Mammoth Mountain, cheering skiers and snowboarders.

Is that a sign that the persistent weather pattern that plunged us into a deep, devastating drought is weakening?

Yes, Patzert said. The drought in recent years has been worsened by a mass of high pressure deflecting typical winter storms that swoop into California from the Gulf of Alaska.

That mass of high pressure generated warmer-than-normal ocean water in the northern Pacific, which became dubbed “the blob.”

November’s snowstorms suggested that the drought-causing mass of high pressure “doesn’t seem to be playing the role it has been in the last few years,” Patzert said.

“This is the earliest the ski resorts have been opened in many years. … They rarely open before Thanksgiving.”

Especially important is that it has been cold.

“For California ski resorts, snow is good but what really counts is temperature. So the key for Big Bear and Mammoth is that the temperatures are cold enough to make snow. And it has been cold,” Patzert said.

Whether the cold snap lasts through spring is an open question. The Climate Prediction Center has forecast above-normal temperatures this winter for California.

Overall, what’s the outlook for the ski season?

It will probably remain the best ski season in years, because ski resorts are so high in elevation. “The really high elevations in the Sierra Nevada will do well,” Swain said.

But what about the mountains that are important to the state’s water supply?

No one has a good answer for this. The mid-elevation mountains of Northern California are very important to the state’s water supply, and it’s important that precipitation comes as snow, not rain.

Too much rain all at once will force excess water to be flushed out to sea to prevent dams from being overwhelmed. But if snow falls, it can melt slowly in the spring and summer, gently replenishing reservoirs.

Scientists generally agree that more precipitation is likely in these mid-elevation mountains. But whether it’ll fall as snow or rain isn’t known.

What have El Niño winters brought to California in the past?

A lot of rain, snow and devastation. Los Angeles in the 1997-98 season saw double the amount of rainfall, and the mountains of Northern California saw double the snowpack. The storms themselves were not particularly intense, but the problem was that there were so many of them and they came one after another at a relentless pace. Downtown Los Angeles in February 1998 saw nearly a year’s worth of rain in that single month.

That winter, 17 people died in California, and more than half a billion of dollars’ worth of damage occurred. Flood-control channels overflowed, mudslides destroyed hillside homes and roads and railroad tracks were washed away.

What isn’t expected in an El Niño?

Patzert says Pineapple Express storms — the kind that come from south of Hawaii and bring excessive rain in a short amount of time — aren’t typically seen during an El Niño.

“They’re the storms where you get 10 inches in 24 hours. The El Niño storms aren’t like that,” he said.

An example of a Pineapple Express storm was the storm in 2010 that dumped rain at an alarming rate over the mountains that burned in the massive Station fire, unleashing a torrent of mud that inundated more than 40 houses in La Cañada Flintridge, Patzert said.

How hot is this El Niño? 

Ocean waters west of Peru are now hotter than recorded in at least 25 years, surpassing the temperatures during the record 1997 El Niño. It is the highest such weekly temperature recorded in 25 years of modern record-keeping in this key region of the Pacific Ocean west of Peru.

Temperatures in this key area of the Pacific Ocean rose to 5.4 degrees above average for the week of Nov. 11. That exceeds the highest comparable reading for the most powerful El Niño on record, when temperatures rose 5 degrees above the average the week of Thanksgiving in 1997.

In fact, last week was the hottest this area of the Pacific Ocean has been since 1990, when records began being kept meticulously. It was 85.46 degrees as of Nov. 11, surpassing the 85.1-degree record hit a week before. Prior to that, the record high was 84.92 degrees set the week of Thanksgiving in 1997.