Winter finally arrives in Sisters

Last October, regional long-range prognosticators made public their views concerning what the Pacific Northwest might expect this winter in the way of weather: A second straight La Niña phase, which would mean cooler and wetter-than-normal conditions; and active weather wouldn't kick in until sometime in December.

There was expected to be a better-than-even chance of seeing at least one significant arctic air outbreak; a greater than normal snowpack in the Central Oregon Cascades.

Yet, in the middle of January, folks in Sisters Country were wondering when the action would begin. With the advent of snow in the Cascades and a dusting in Sisters, winter may have finally arrived. It took its time. Despite a moderately strong La Niña signal, the active weather normally associated with her presence has been late getting started.

Something similar occurred last winter. After a series of low-pressure systems put down a pretty good snowpack in the Cascades (and even a few inches in the high desert) during December 2010, conditions suddenly went dead in January through the first half of February. There is some evidence to suggest that an eastward propagating tropical feature, called the Madden-Julian Oscillation (MJO), temporarily overpowered La Niña and helped to build and maintain a blocking ridge over the U.S. West Coast that deflected incoming Pacific storms. Eventually the ridge collapsed and La Niña came back with a vengeance, with cooler and wetter weather that persisted well into June.

The winter of 2011-12 has started out dry, with warmer-than-normal daytime temperatures, but colder nighttime temperatures, indicative of mostly clear skies. December is normally the wettest month of the year in Sisters, but until the last four days of the month no measurable precipitation had been recorded. A couple of warm weather systems did make it over the Cascades near the end of the month, resulting in a total of 2.01 inches of precipitation for December, just 0.25 of an inch short of what's normal.

But the first nine days of 2012 were warm and bone-dry. Another persistent ridge aloft is to blame. But this time MJO doesn't appear to be the culprit.

What's different this season is that the unusually mild early winter weather isn't confined to the Pacific Northwest. It extends from the West Coast, across the upper Midwest, though the Great Lakes area and into New England. Temperatures in eastern Montana and parts of North Dakota and Minnesota averaged more than 10 degrees above normal in December, and six to eight degrees above average in many Great Lakes and New England states. As of early January only 19 percent of the U.S. was covered in snow, less than half the average snow-cover over the past five years, according to the National Weather Service.

There is evidence to suggest that the abnormal weather is also occurring in many other regions across the northern hemisphere.

In the United Kingdom, the average temperature for December 2010 was the coldest on record. Last December, however, an unimpeded mild westerly flow of air kept temperatures on the moderate side. And on January 6, 2012, Beijing, China, received its latest snowfall in 60 years.

Is there, then, an explanation for this wide-spread mild weather? Perhaps there is: enter the Arctic Oscillation (AO).

The AO refers to the fluctuation of opposing atmospheric pressures between the mid-latitudes and the polar region and is expressed as a numerical index that ranges from about +5 to -5.

When pressures are lower at the pole and higher at mid-latitudes (say, about 45 degrees), the phase is said to be positive. In the positive phase, stronger-than-normal jet stream winds tend to keep the coldest air bottled up near the pole, as well as carry ocean storms along a track farther north than normal. This usually brings wetter weather to Alaska and drier conditions to the western U.S.

When the AO phase is negative, higher pressure builds in the polar region while relatively lower pressure exists at the mid-latitudes. This causes jet stream winds to weaken, allowing occasional excursions of arctic air to sag southward.

Can we use the AO index to help explain the anomalous weather we're experiencing now?

During November the index was mostly negative, with a minimum value of -3.8. However, the phase switched in December, reaching a maximum value of +4.0

This does seem to square with some of the aforementioned observations. But because the AO varies irregularly over short periods of time, it is of little use for long-range forecasting. Complicated mathematical models are currently being applied to improve AO forecasting, with only marginal success thus far.

While we still don't know for sure what the rest of the winter will bring, we can now pass judgment on a couple of items in the original forecast. The active weather kicking in in December is definitely a miss.

The rest of the forecast may yet verify, but only if La Niña is allowed to do her thing, and soon.

At this writing, medium-range forecasting models are hinting at a fundamental change in the broad-scale weather pattern over our region. And snow has returned to the high desert.