Quirky weather puzzles experts

As the summer of 2010 was winding down and transitioning into fall, all the usual indicators seemed to be pointing in the same direction: a potent La Niña winter was on the way.

Weather prognosticators quickly spread the news. Ski resort operators throughout the region brimmed with optimism at the prospect of plenty of mountain snow. And the spring and summer runoff from a large snowpack would provide ample capacity for generating hydroelectric power and keep irrigation reservoirs filled. After experiencing dry and mild conditions during the previous winter, it seemed La Niña was just what the doctor ordered.

As expected, the ol' girl burst on the scene in December with some very active weather. Sisters received double the normal amount of precipitation as frequent accumulations of snow fell throughout the month. Temperatures were near normal.

Winter was unfolding just as advertised. But as the Sisters Country welcomed in the new year, things changed. We entered a prolonged period of warm and dry weather. Weather records for January showed that Sisters only got about half of its normal precipitation, and most of that fell mid-month as warm rain that melted away most of the snowpack that was gained a month before. Except for the first four days of 2011, January's average temperature was almost seven degrees Fahrenheit above normal.

What happened?

Before we can begin to answer that question, an explanation of the El Niño/La Niña phenomenon is in order.

The El Niño/La Niña Southern Oscillation, or ENSO, is a semi-periodic shift of the climate across the tropical Pacific Ocean, and it consists of a water temperature component and an atmospheric component that are tightly coupled. When sea surface temperatures are warmer than normal across the eastern Pacific, an El Niño is said to exist. It's a La Niña when this region experiences sufficiently colder than normal temperatures.

The second aspect of ENSO is the Southern Oscillation. This is the periodic fluctuation of the surface winds across the Pacific. It is measured by the difference in atmospheric pressure between the island of Tahiti and Darwin, Australia.

When barometers read higher in Tahiti than in Darwin, the easterly trade winds strengthen and tend to push warm surface water toward the western Pacific area, thus promoting upwelling (cooling) to occur in the eastern Pacific (La Niña.) However, when the barometric pressure is higher in Darwin, the trade winds are weakened, or even become westerly, allowing warm water to be transported into the eastern Pacific (El Niño.)

But not all El Niños and La Niñas are created equal.

An ENSO index was developed to determine the onset and strength of these changing weather patterns across the Pacific Ocean. When the value of this index is between -0.5 and +0.5, a neutral state is said to exist. In other words, conditions over the ocean are normal. Values between +0.5 and +0.75 indicate a weak El Niño; at +0.75 to +1.0 the signal is of moderate strength, and anything greater than +1.0 is a strong event. It's the same for a La Niña except the numbers are negative.

According to information gleaned from the Western Regional Climate Center in Reno, Nevada, there is about a 65 to 75 percent chance of cooler and wetter winters at many locations in the Pacific Northwest when a La Niña is in effect. So, when in September and October of last year the ENSO index was in the neighborhood of -1.8 (a very strong La Niña signal), it seemed foolish not to call for a cool and wet winter for 2010-11.

December was well behaved, as a series of storms marched across the Pacific and into the Northwest. But by January a large upper-level ridge developed over the West Coast that diverted approaching storms far to the north into British Columbia and southeastern Alaska. As these storms crested the top of the ridge, they plunged southward through Canada and into the midsection of the country, dragging down bitterly cold arctic air behind them.

So, while the West Coast basked in warm and dry weather, the eastern half of the U.S. took the full brunt of Old Man Winter.

It was primarily the strength and duration of this atmospheric feature that perplexed forecasters. Occasionally ridges do form at that location during a La Niña winter, but this one was stuck in place for a good six weeks. That's a long time for any weather configuration to remain essentially unchanged.

On January 24, Clifford Mass, a highly regarded professor of meteorology at the University of Washington, wrote the following on his blog: "The atmosphere seems to be locked in a very anomalous pattern right now and my profession really can't explain it..." He added, "Whatever is going on seems to be overwhelming the normal La Niña pattern around here and over the rest of the western U.S."

This event will surely be the focus of study for some time to come.

At this writing, the ridge aloft that held sway for so long has finally broken down and the High Desert is getting some much-needed snow. But is it too late? Can the Cascades make up its snowpack deficit and equal or exceed normal levels by winter's end?

We'll know soon enough.

Ron Thorkildson is a retired meteorologist. He lives in Sisters.