'Anything goes' in winter forecast

With the arrival of autumn, climate experts are weighing in on what kind of a winter we might expect here in the Pacific Northwest. After serving as a caldron where one of the strongest El Niños in 65 years was brewed last season, conditions in the tropical Pacific Ocean have returned to more normal levels. What atmospheric scientists need to figure out now is where to go from here.

Several climatic entities are currently under scrutiny. The Madden-Julian Oscillation is the major fluctuation in tropical weather on weekly to monthly timescales. It is characterized as an eastward propagation of clouds and rainfall near the equator that typically recurs every 30 to 60 days. The Pacific Decadal Oscillation Index is the leading principal component of North Pacific monthly sea surface temperature variability poleward of 20 degrees north latitude.

Then there's the Arctic Oscillation, a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55 degrees north latitude. When these winds are strong the coldest air in the hemisphere tends to stay bottled up in the polar regions; when they weaken frigid air is allowed to spill southward into more temperate climes. Some researchers even introduce sunspot cycles into the equation.

But the single most important climatic indicator is still the El Niño Southern Oscillation (ENSO). Surface water temperatures and atmospheric conditions in the tropical Pacific Ocean are the key. When strong tradewinds push the warmest waters into the western Pacific Ocean, the ENSO is said to be in its "cool" phase and is called a La Niña.

Conversely, when trade winds are weak, or become westerly, the highest sea-surface temperatures tend to migrate into the eastern Pacific, putting the ENSO in its "warm" phase, or El Niño.

At our geographical location on the planet, winters tend to be warmer and drier than normal during El Niños; cooler and wetter when La Niñas rule.

Under the influence of last season's super El Niño, weather across the country went pretty much as expected. It was milder than normal in the Midwest and Northeast and wetter in the southern states from Texas eastward. But something went awry in the western U.S. Instead of warm and dry weather here in the Pacific Northwest, it was cool and wet, except for February.

Portland set an all-time record for rainfall in December.

The anomalous weather in California was even more dramatic. The monster El Niño was supposed to inundate Southern California with torrential rains, causing mudslides and flooding. What actually occurred, however, were precipitation totals that barely registered half of what is normal.

There is currently no general agreement among climate researchers as to what may have led to the unexpected conditions in the west.

So, what do current indicators tell weather analysts about the upcoming 2016-17 winter season?

Late last spring federal forecasters believed a mild was developing. But that has changed, according to Kathie Dello, deputy director of the Oregon Climate Service at Oregon State University.

Dello said the current outlook is for an ENSO neutral state, or La Nada.

The modified forecast was issued in part because of a persistent blob of unusually warm water in the north Pacific that the Climate Prediction Center thinks will result in above-normal temperatures from October to December across the Northwest. But Dello warns that ENSO-neutral doesn't necessarily equate to an average winter. La Nadas are also associated with less-predictable winters.

"Well, really, anything goes," Dello said. "We've had warm ENSO neutral years. We've had cool ones. Wet ones, dry ones."

Cliff Mass, University of Washington Professor of Atmospheric Science, knows that neutral winters carry some risk of unusual or severe weather.

"The biggest windstorms, strongest atmospheric rivers and largest snowstorms tend to like neutral years," Mass wrote in an email. "These are infrequent events, but when they happen, major problems occur."

Mass thinks that if the current outlook pans out, water supplies "should be average for next summer."

The current ENSO-neutral signal lowers confidence in coming up with an accurate winter forecast, but Dello is betting on a warmer-than-average winter in the Northwest.

Dello says the blob had started to break apart earlier this year but has since reformed. "The blob feature is new, in terms of the way we've been thinking about these things," Dello says. "And it has certainly played a role in our weather."