The general circulation of the northern hemisphere is in the very beginning stages of undergoing a change from what has been the normal for roughly the first half of the 2011-2012 winter season. This change will primarily be felt after January 15th in Southeast Michigan, as temperature and precipitation patterns return to a more characteristic winter state.
The origin of this pattern change is a strong jet in the western Pacific that will send a series of strong waves toward North America (Figure 1). One such wave has the potential to result in a storm system over the east-central United States in the January 11-12 period. Whether or not our area is impacted by this potential system is independent of the pattern change itself. The reason is because another strong low over the east-central Pacific will aggressively build ridging into Alaska around the same time period, thereby disrupting/displacing the Alaskan polar vortex, which been a dominant feature of the general circulation so far this winter. That is not to say that the mid-week (Jan 11-12) system is not important in terms of the pattern evolution, regardless of whether it affects our area. After it lifts out of the United States, it will likely contribute in some fashion to a newly forming polar vortex over Hudson Bay/Eastern Canada during the latter half of January (Figure 2).
Confidence in this change toward a more winter-like pattern as a result of the strong west Pacific jet is high. In addition, recent long range model runs have occasionally hinted at additional possibilities unfolding once the change in the pattern is underway, although confidence in these possibilities is considerably lower. For example, as the midweek energy lifts into northern Canada late in the week, downstream ridge amplification may reinforce the effects of a wave break in the high latitudes of the north Atlantic, resulting in a decline of the well-known NAO index from its persistently positive state to one that is neutral or negative. A higher-confidence possibility, however, is that an anticyclonic wave break in the east-central Pacific will create potential for upstream blocking that may add some stability to the new pattern.
In summary, interests in southeast Michigan can expect the pattern beyond January 15th to be winter-like, a stark contrast from most of the season so far. That is to say that temperatures will be most likely near-to-below normal (instead of near to above normal as they have been recently) along with the regular chances for snow that residents of southeast Michigan are accustomed to. Whether or not the new pattern develops real staying power will at least partially depend on how the aforementioned factors above (e.g. blocking potential) evolve.
Nonetheless, confidence that the second half of January will average much cooler in our area and with more snowfall than the first half is high.
Posted January 6th, 2012