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Wintry Winter 2024-25 Discussion

The Pacific jet extension is not the true issue here late in Dec.

The poleward shift in the pacific jet has more to do with the mild spell out east late in the month than the jet extension.

Outside of big Nino winters, it’s only when a jet extension is coupled with a poleward shift that we see a big warm up over N America.

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Here’s the pacific jet phase diagram from WPC that runs this once a day on the 0z run:

North Pacific Jet Phase Diagram

Notice, the pacific jet is already extended at model initialization. Also note how it’s mostly a jet extension without much of an equatorward or poleward shift (points are orbiting well within the jet extension quadrant).

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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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Webberweather53 said:
The Pacific jet extension is not the true issue here late in Dec.

The poleward shift in the pacific jet has more to do with the mild spell out east late in the month than the jet extension.

Outside of big Nino winters, it’s only when a jet extension is coupled with a poleward shift that we see a big warm up over N America.

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Funny thing on the chart wording too is that the Poleward Shift has a farther east jet extension than the “Jet Extension” phase (I suppose it’s because the farther north location is caught in the stronger westerlies, carrying it farther east)
 
griteater said:
Funny thing on the chart wording too is that the Poleward Shift has a farther east jet extension than the “Jet Extension” phase (I suppose it’s because the farther north location is caught in the stronger westerlies, carrying it farther east)
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Yes.

This is because a poleward shift in the jet stream actually causes a further acceleration/extension of the jet through angular momentum conservation principles. Moving the jet stream closer to earth’s rotational axis/the North Pole forces it to accelerate (in much of the same way a figure skater pulls in her arms and spins more quickly).

You usually need a poleward shift or hybrid poleward shift/extension of the pacific jet to kick off a +EPO and really warm N America. A pacific jet extension on its own usually only regulates the PNA (& forces it to become positive).
 
Webb thanks for the post above on Jet extension and Poleward shifts. That was very easy to grasp and makes a lot of common sense to amateur like myself.
We need to be sweating the Pac Jets aiming point more than whether its a firehose or not.
 
CaryWx said:
I wouldn't count on February. Conditions will not be favorable. It's going to have to be January more or less
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I keep hearing this. How in the world is anyone about to predict how an entire specific month will turn out? I mean, I'm just as pessimistic as anyone when it comes to cold and snowy patterns around here, but there's no way we know whether or not something unforeseen might pop up to interfere with the hot February outlooks.

The only thing I can think of that holds water is that you can make the argument that February has been trash for years now, and that is the new normal (which i guess you could apply to modern winters as a whole). Otherwise, I submit that we don't know how it will turn out.
 
Rain Cold said:
I keep hearing this. How in the world is anyone about to predict how an entire specific month will turn out? I mean, I'm just as pessimistic as anyone when it comes to cold and snowy patterns around here, but there's no way we know whether or not something unforeseen might pop up to interfere with the hot February outlooks.

The only thing I can think of that holds water is that you can make the argument that February has been trash for years now, and that is the new normal (which i guess you could apply to modern winters as a whole). Otherwise, I submit that we don't know how it will turn out.
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I agree. Although a pessimist might say “if it walks like a duck and quacks like a duck”.

Febs have been awful lately so I can’t disparage someone for trashing it.
 
You can have 27 torch days in Feb. But if you can just find that 1 day to win the " SE has to Time Everything out just Right Lottery", Its usually a storm for the History books. Feb and March is usually Go Big or Go Home type deals.
 
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