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. 

Source: NOAA/ESRL/PSD

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.  

Source: NOAA/ESRL/PSD

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.  

Source: NOAA/NWS/CPC

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