Given the inconsistency in expectations among meteorologists over the last few weeks on December’s expected pattern evolution, I thought I would try to clear up where we are and where we could be going. Given the model flip-flopping this forecast could prove highly volatile, however I am giving it a go.
Over the past few weeks the pattern has been dominated by a strong ridge configuration over the eastern CONUS. This ridge has allowed unseasonable warmth to take hold. What is interesting is this has happened during a negative AO period.
The past and future AO forecast can be seen at the above link. The GFS and EURO ensembles both support this continued dip in the AO, with the GFS ensembles taking it to very anomalous negative numbers next week. Statistically this would strongly favor a pattern change and indeed we have seen hints of this change on the GFS and EURO ensemble runs over the past week as well, however there has been little consistency or overlap in the two models until this afternoon's runs.
Given this is the first run of such, we should continue to hold some skepticism on a complete pattern reversal, but I do believe a significant pattern change is likely and we’ll see it take shape as we progress through December. In the Image section I have taken the last 2 week’s 500mb pattern over the north hemisphere and dissected what has happened and what could happen.
Ultimately I expect a strong system to form late next week over the plains and Midwest due to the deepening negative AO and warm anomalies building over the Arctic. This storm will be forced due to a frontal system surging into the lower 48. This system will likely spark a 500mb low over the 50/50 lat-long location. This is key, as it will allow the blocking near the poles to organize and build towards western Greenland, forcing the PV to weaken and setting up a possible favorable cold and snow pattern near the holidays.