So La Nina, where is all the snow you promised?

All right, we were all told this was supposed to be an epic season for snow, you know, like it was last year. But here we are nearing the end of December and the local resorts are still pressing their luck opening new terrain. Rocks scatter the runs. Stumps await skiers in the trees. Basically, areas that were open by the beginning of December last year are presently still far from skiable.

According to the Natural Resources Conservation Service, our snowpack is at 65 percent of normal for this time of year. So what gives? Have we been duped?

Well, according to the National Weather Service in Missoula, there is no reason to put away the powder skis and boards for good. Here's their recently released update on where the heck La Nina might be hiding:
NOAA's National Weather Service Climate Prediction Center has been forecasting a La Ni--a phase of the El Ni--o Southern Oscillation (ENSO) index for the northern hemisphere winter of 2011-2012. Indeed, La Ni--a conditions (anomalously cool surface water temperatures) have been present across much of the eastern Pacific Ocean near the equator since about Aug. 1, 2011. This indicates that La Ni--a is occurring.
However, the weather since late November over the Conterminous United States (CONUS) has not been what experts expect with La Ni--a. Instead, La Ni--a pattern typically brings frequent storms moving across the Northern Rocky Mountains, leading to a healthy mountain snowpack. Beginning in late November 2011, the storm track has steered around the Northern Rockies, favoring the desert southwest states and the southcentral CONUS, including drought stricken Texas.
So, when will La Ni--a weather patterns arrive? It is important to remember that the forecast for La Ni--a represents a seasonal forecast, for conditions expected for the December to March winter season. At this point, in early December, it is too early to determine the ultimate accuracy of the La Ni--a seasonal forecast, according to the National Weather Service. At some point late in December or early in 2012, the weather will likely begin to show more characteristics of a La Ni--a pattern, including frequent storms, adding to the mountain snowpack of Montana and northern Idaho.
What would signal such a change in the weather? One possible phenomenon that could trigger the expected change may be another large-scale feature known as the "Arctic Oscillation", or AO. The AO has been anomalously positive since the second half of November 2011. This is in contrast to the onset of winter in late 2010 (also a La Ni--a condition winter), when the AO was very negative for December 2010 and much of January 2011. So, perhaps when a change in the Arctic Oscillation occurs in the coming weeks and months, the La Ni--a pattern will begin to influence the day-to-day weather in Western Montana and Northern Idaho.