Wednesday, January 4, 2012

December 2011 vs. December 2010

Mother Nature is indeed fickle.  
Avenues Foothills, 29 Nov 2010
Avenues Foothills, 28 Nov 2011
Little Cottonwood Canyon, 24 Dec 2010
Little Cottonwood Canyon, 18 Dec 2011
No doubt about it, December was a miserable month for Utah skiers and it is quite remarkable how different it was from last December.  Here's an interesting comparison:

December 2011
1 Dec Alta-Collins Snow Depth: 25 inches
31 Dec Alta-Collins Snow Depth: 35 inches

December 2010
1 Dec Alta-Collins Snow Depth: 63 inches
31 Dec Alta-Collins Snow Depth: 116 inches

Based on data at, only 15.5 inches of snow fell from 1–22 December this year.  For some reason they haven't included additional storms since then, but they were small, so it is safe to say that the monthly snowfall in December 2011 was less than 25".  How much fell in December 2010?  131.5"  Ah, the good old days.

We've already taken a look at the large-scale circulation from this December, but it's worth doing a comparison with December 2010.  To do this, we'll take a look at the 500-mb geopotential height, which is commonly used by meteorologists to examine the upper level flow.  Essentially a 500-mb geopotential height analysis is a topographic map of the 500-mb surface.  Areas of low heights are associated with upper level troughs and areas of high heights are associated with upper-level ridges, with the flow generally following the height contours.

This December (2011), the mean 500-mb flow featured pronounced split flow over the western United States, with the northern branch of the jet traversing southwest Canada and the southern branch of the jet traversing northern Mexico.  It is not uncommon for split flow to occur over the western United States, but the split this December was quite pronounced.
Another perspective is provided by a 500-mb height anomaly plot in which we plot the difference between this December's mean 500-mb height and the long-term climatology.  This helps us to identify the anomalous ridge that persisted off the Pacific Northwest coast (warm colors) and trough over Baja California.  Also evident is an anomalous trough/ridge dipole over the North Atlantic with the trough over the high latitudes which is characteristic of the positive phase of the North Atlantic Oscillation.
 In contrast, the pattern was markedly different in December 2010.  In particular, there was a mean trough of the US west coast, with Utah in broad west-southwesterly large-scale flow.  Further, the North Atlantic Oscillation was in it's negative phase, with anomalous ridging over the high latitudes of the North Atlantic and troughing over the mid latitudes.
Much was made a few months ago about this being a double dip La Nina.  Some suspected that we could have another huge snow year, but as we have discussed, La Nina is only one contributor to the large-scale circulation.  Aspects of the large-scale circulation that cannot be predicted on monthly or seasonal time scales can also play a role.  Perhaps the La Nina signal will more strongly emerge in the coming months, but for the Wasatch, that signal is fairly weak.  We are largely at the mercy of atmospheric processes that, based on today's understanding, are not predictable on monthly to seasonal time scales.  In other words, I don't know what is going to happen for the rest of the winter in the Wasatch Range. 


  1. This certainly seems like an atypical La Nina year so far, with even most of the Pacific Northwest (especially Oregon) being dry. The difference in the position of the high-latitude cold pools between this year and last year is striking in the plots you posted. A "polar low" position around northeastern Canada coupled with another near the Bering Sea, which is the current situation, seems to almost guarantee dry weather here. There has also been a lot of trough activity in the tropical eastern Pacific near 15N, maybe just a result of the split pattern but perhaps reinforcing it also.

  2. To me the most striking part about this winter so far has been the widespread nature of the anomaly. It's not just in the West. Typically with a big ridge in the West you'd get a deep cold trough in the East, but not this year. Buffalo had only 3.8" of snow Oct-Dec, 3rd lowest ever. Chicago had only 1.7" of snow, 11th lowest ever. Instead of the highly amplified pattern of last winter, we're stuck with a low amplitude pattern this year. Largest anomalies are the low heights over the pole. The arctic isn't sharing its cold air this year.

  3. That's a good observation, which I suspect is partly related to the strong positive phase of the NAO, which is typically associated with above average temperatures in the eastern US.