Monday, November 19, 2012

Teleconnection Talk: Cooler, Stormier Pattern Evolving

I said I would begin the new segment 'Teleconnection Talk' a while ago, but it is only now that I feel the time is right to debut this segment.

It appears that the three big indices tracked for effects on North America could be shifting in the next two weeks, possibly pointing to a pattern more favorable for cold, stormy weather by the start of December.

On the top left is the PNA forecast. The PNA, or Pacific North American index, tracks height anomalies in the Northeast Pacific. If that region has a high pressure system over it, the PNA is positive, and cold weather is known to invade much of the Lower 48. The storm track shifts north and hits the Midwest, Great Lakes and Ohio Valley as well. In a negative PNA, a low pressure system is sitting over the same area, bringing warm weather to the East US, but cool weather to the West US. The ensemble members from the Climate Prediction Center (CPC) show that the PNA could be trying to push positive. While there is a major spread (difference) in the forecast members on December 1st, this is an encouraging sign that the ensemble members are at least considering a change in the PNA.

On the top right is the AO forecast. The AO (Arctic Oscillation) is used to track the strength of a low pressure system over the Arctic Circle. When this low pressure system is strong, cold air is locked up north and the AO is positive. In a negative AO, the low pressure system is weak, and cold air flows south into the US. The ensemble members are pretty insistent on a major drop of the Arctic oscillation well into negative territory; something that bodes very well for cold air potentials over the Lower 48.

Finally, on the bottom half of the image we find the NAO (North Atlantic Oscillation) forecast. In a positive NAO, there is a stormy pattern over Greenland, leading to a warm East US and stormy, cool West US. A negative NAO brings cold and stormy weather to the East US, while delivering warm, dry weather to the central/west US. The CPC's ensembles are pretty spread out as time goes on, but there appears to be a general trend for a gradual weakening of the positive NAO and possibly slipping into weak negative NAO territory. If the lowest ensemble forecast members verify, the NAO is very negative. If the most-positive ensemble members verify, the NAO will stand at a weak positive phase. However, because most of the ensemble members are negative, I prefer to side with the negative solution.

Exciting things are happening, folks, and it looks like the trend for the end of November and into early December is looking cooler and stormier. Just how cool and stormy is yet to be seen, but things are looking relatively good at the moment.


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