Notice how well the first week where we are getting almost purely a jet extension (previous post), actually fits with the jet extension composite (top row). (It’s a +PNA pattern).
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It’s not really until we get close to and beyond Christmas when we start to approach the adjacent poleward shift quadrant that the warmth really overspreads North America.
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Poleward shift in the pacific jet coupled with an extension is usually how we get a big torch over N America. A jet extension
on its own actually favors colder temps in the eastern US and is the kind of pattern we’ve seen most of Dec so far.
I just want to make the distinction here clear because there’s a huge difference in sensible impacts between a jet extension
on its own and a poleward shift in the jet
on its own. I think the jet extension (while accurate in this case) takes a little too much of the blame when these kind of patterns show up & most folks (including S2S forecasters) don’t pay enough attention to the latitude of the jet, which actually matters more in regulating the EPO. It usually takes more than a pacific jet extension by itself to get these kind of warm anomalies.
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