Special Long Range Lookout: Pattern Turning Favorable For Cold, Snow

This is a Special Long Range Lookout, published November 16, 2012.

Colder weather looks to be coming back in the not-so-distant future. The North Atlantic Oscillation (NAO) forecast on the left includes the black line of observed conditions and the red ensemble forecast members for the NAO. In a positive NAO, warm conditions are typically seen across the eastern US, while a negative NAO causes the jet stream to buckle south, usually into the Northeast. This results in an outbreak of cold air over the region. Storms that skirt the Gulf Coast also are pulled north and can turn into the famous Nor'easters.

The NAO is looking pretty negative over the next couple of weeks. While more recent forecasts are trending more positive, the GFS Ensembles have been very consistent with a strong negative NAO developing. This comes as East Asia indicates a stormy period is in store for the Northeast in the near future, aiding the -NAO potential.

The second image here on the left is the forecast for the Arctic Oscillation, or AO. The AO also has two phases: positive and negative. The NAO and AO are closely related in that both indices' positive phases involve warmth over much of the nation. Their negative phases also induce cold across the nation, more-so with the AO than the NAO, which is more influential in the Northeast than the AO. The ensemble forecast members, while varying in the long range, appear to be showing a fair negative trend in the long range.

Considering the consistency of the GFS Ensembles, East Asian signals and a few other significant factors, the Arctic Oscillation is likely to be heading on a more negative track in coming days. I find the presence of a negative NAO inviting to the potential of a negative Arctic Oscillation as well.

As for the Pacific-North American index, or PNA index, the outlook is not as favorable. Just like the NAO and AO, the PNA has a positive and negative phase, although they are much different than the effects from the NAO/AO. In a positive PNA, you will see an active storm track in the Plains, Midwest, Great Lakes and Ohio Valley, with cold air in those same regions. The Northeast is also affected. However, in a negative PNA, warm air is more common, and the storm track tends to stay away from these areas.

The persistent trough currently in place over the northeast Pacific is helping to keep the PNA negative, and the forecast ensemble members seem to have no trouble agreeing with this pattern continuing. Despite this, I believe that this persistent trough pattern will break in mid December.

Andrew