Wednesday, January 11, 2012

Large-Scale Pattern Changes

As one might gather taking a look at some of the comments to the blog over the past couple of days, change is coming to the large-scale pattern over the northern hemisphere over the next several days.  At issue is whether or not it will bring an end to the prolonged dry spell that has plagued the Wasatch Mountains.

The December mean 500-mb circulation featured anomalous troughing over the high latitude North Atlantic and anomalous ridging over the mid latitude North Atlantic, consistent with a strong positive phase of the North Atlantic Oscillation.  Further west, an anomalous ridge off the Pacific Northwest coast and trough over the southwest led to strong flow splitting over the western US and our dry December.  Anomalous troughing also extended across Alaska and the Behring Sea in the high latitudes. 
Over the last seven days the pattern has featured a generally similar pattern of anomalies, with a few subtle differences.  For example, the high latitude troughing over the North Atlantic is a bit weaker and rather than split flow, we've simply had an anomalous ridge over the western United States.  Troughing is a bit stronger and more isolated over Alaska.  Nevertheless, the pattern is broadly consistent with that which dominated in January.  

The long-range forecast models are, however, beginning to converge on a solution that suggests a major pattern change.  In particular, the GEFS ensemble shows the development of a high amplitude ridge centered near the dateline in the high latitudes and a deep trough over the Pacific Northwest. 

Source: Penn State E-wall
This is good news for the Pacific Northwest, but the Wasatch remain on the edge of the action in the 1200 UTC GFS forecast (through 180 hours).  However, there are other models and ensemble members that are more optimistic for Wasatch snow.  The CPC 6-10 day precipitation outlook doesn't shift the odds toward a wetter or drier pattern over northern Utah because of this.  

We'll have to see how the details emerge in the coming days.  Do others have a clearer cyrstal ball than I do?  

1 comment:

  1. I am finding that there is a correlation between being at home with the flu and looking at 10+ day forecasts. The models have been pretty consistent with the blocking pattern and (at least the GFS) with the main jet core spending a lot of time in the 25-35N latitude band underneath it, over the central Pacific. In this area the SST's are warm, with the 70F isotherm around 30N and some temps close to 80F at 25N. I think the probability is very high for significant "atmospheric river" type of activity affecting the west coast in 7-14 day time frame, most likely OR or northern CA. I think we will get in on some decent moisture, but if the main jet core remains north of us it may be a lot warmer than we would like.