Signals for a potential storm system to impact the Northeast to close out the month of November have been in place the past several days and as we move closer to the end of the month these signals remain in place with confidence growing. The signals have been the following;
1) A sharp rise in the Pacific-North American (PNA) teleconnection pattern (PNA becoming more positive)
2) A rapid drop in both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) (AO and NAO becoming less positive)
Without going in great scientific depth regarding these teleconnection's and oscillation's when the PNA becomes increasingly positive, this typically corresponds with a ridge across the western United States with a trough either across the eastern United States. When the AO/NAO become increasingly negative, this typically correlates to a trough across the eastern United States. However, it needs to be noted and clearly stated that there is more to whether or not these are positive or negative, the strength, placement of the height anomalies associated with each, and structure of the anomalies play a significant role in connection to the pattern across the United States.
Moving into the final week of November and into the first of December, forecast models indicate a sharp rise in the PNA as shown below:
At the same time, forecast models indicate a sharp drop in the AO and NAO as shown below:
How can we visualize what this means on the weather pattern? I am going to use the GFS forecast model and the 500mb height anomalies to provide a visualization. Here we see a ridge (orange/red shadings) across the western United States with a trough (darker blue shadings) across the eastern United States:
So the question becomes, for New England (since my blog typically focuses on New England) why aren't we looking at the chance for snow? Well snow can't be ruled out, especially in the higher elevations of northern New England, but unfortunately snow lovers, this just is not a look for snow.
Perhaps the biggest reason overall is there is just lack of sufficient cold out ahead of the developing weather system. Looking at around November 27 (a few days before we can expect a storm to materialize) and looking at 850mb temperature anomalies across Canada there just isn't much in the way of cold. Now...if this were say January or February, 850mb temperature anomalies might not be sufficient to look at here. Given those months are getting into the climatological coldest part of the season, even "anomalous warmth" at this level could still be composed of an air mass more than sufficient for snow, however, we are just entering December so above-average temperatures just won't cut it:
Another big factor is where does cyclogenesis take place? Note: It is never a good idea to judge this based on one forecast model, however, for illustrative purposes I will use the same graphic that was posted above the 850 temperature anomaly graphic. While using that for illustrative purposes, analysis of other long-range forecast models plus model ensembles was conducted to generate a forecast:
Based on the analysis, the best likelihood for cyclogenesis would be right around the base of center of the black line indicating trough. This would promote low pressure development well West of New England which favors warmth ahead of the system. This is also shown by the positive height anomalies (red shadings) into New England.
At this juncture, the period between November 30 - December 2 is likely to see a significant weather system develop. This would include heavy rainfall, gusty winds, and perhaps potential for thunderstorms. Again, some snow can't be ruled out across the higher elevations of northern New England.
So what happens behind this...well I want to again post the PNA and AO/NAO charts from above to highlight the look just to the right of what I have circled:
Notice how there is "chaos" going on. Does the AO/NO head well negative, does it trend more positive? Does the PNA quickly become less positive? This chaos indicates uncertainty with how the weather pattern will evolve post-storm. While we may face a decent blast of cold air behind this storm, unfortunately, the pattern does not look very supportive to maintain this cold and we may quickly go back into a pattern which favors average-to-above-average temperatures. Those hoping for snow moving into December...good luck.
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