El Niño is flexing its muscles. What does this mean for California?

El Niño is expected to gain strength and flaunt its muscle this winter, and forecasters are closely watching ocean temperatures to determine just how strong the El Niño weather pattern that developed over the summer will get in the coming months.

The El Niño forecast is always of particular interest in California, where it has near-celebrity status. The state has seen some of its wettest winters in El Niño years, such as the winters of 1982-83 and 1997-98, and many in California associate the weather phenomenon with flooding, mudslides and massive snowpacks. But researchers say the state can also see above- or below-normal precipitation in an El Niño year, as it did in 2015-16.

“The El Niño-Southern Oscillation climate pattern is, by itself, not a good predictor of wet or dry years for California,” Michael Anderson, state climatologist with the California Department of Water Resources, wrote in an email.

“We have had El Niño events that brought us above normal precipitation, and we’ve also had El Niño years when we’ve seen notably drier conditions,” added Sean Miller, a meteorologist with the National Weather Service.

El Niño and its cousin La Niña develop in the equatorial waters of the Pacific Ocean far from California. In an El Niño year, the trade winds ease and warm ocean water collects at the equator off the west coast of South America. La Niña is the exact opposite of El Niño, and occurs when those same winds gain strength, pushing the warm water off South America to the west toward Asia.

The fluctuating sea surface temperature in an El Niño or La Niña year can cause the jet stream — high-altitude winds that generally move west to east over the continents — to shift. Movement in the jet stream can impact weather across the globe, especially in winter months, leading some areas to be drier and some areas to be wetter than is normal. 

The National Oceanic and Atmospheric Administration’s Climate Prediction Center said in its latest forecast that there’s a 75% to 85% chance the current weather pattern will be strong from November through January. Perhaps an even more interesting data point: There’s a 30% chance this winter’s El Niño will be historically strong — matching the power of 2015-16, 1997-98 and 1982-83, the three strongest El Niños in recent years, said Michael McPhaden, a NOAA senior scientist.

“These are the three big events of the last 40 years, and they all rattled the planet,” McPhaden said. 

Some have been using the term “super El Niño” to describe the historic weather pattern that may develop, but McPhaden said it’s not an official term and he prefers to describe these events as “extreme.” 

The stronger El Niño is, the more likely it will impact global temperatures and rain and snow patterns, but it’s difficult to forecast with precision where and when these impacts will occur and at what strength. The jet stream is usually steered southward in an El Niño year. McPhaden said this leads the northern part of the United States to see above-normal precipitation and temperatures in winter, while the southern half is cooler and wetter than normal. Florida generally sees a wet winter. “This is because the storms that are usually pushed by the jet stream across the center of the United States are pushed south,” he said. “That is the expectation. It’s not guaranteed.”

California stretches across both the northern and southern parts of the country, and its winter precipitation trends are in part dependent on where the jet stream sets up. There is a slight tendency in El Niño years for Southern California to receive above-normal rainfall in winter, while there’s no defined trend in Northern California based on historical data.

“The northern part of California falls in this in-between area,” McPhaden said. “It depends on where that jet stream falls. Northern California can be dry, but if the jet stream doesn’t shift south, Northern California can be wet.” 

The weather prediction center’s seasonal outlook shows equal chances of above-normal or below-normal precipitation for October through December for California. In January through March, the same forecast holds for Northern and Central California, but tilts toward increased likelihood of above-normal precipitation for Southern California.

While El Niño and La Niña can impact locations around the world, McPhaden said there’s a “zoo of climate oscillations” that can also impact weather. “Sometimes they work in concert and sometimes they work against each other,” he said.

“There’s a lot more going on in the climate system besides El Niño, and you also have day-to-day weather,” McPhaden said. “El Niño can be overridden by things that happen locally that are just random chaotic weather variations in the atmosphere.”