Interesting evolution of forecasted North American cold air outbreak over the next two weeks- is it or is it not a “#polarvortex” event? (1/9) 🧵 - ThreadSky

drahbutler.bsky.social • 72 days ago

Interesting evolution of forecasted North American cold air outbreak over the next two weeks- is it or is it not a “#polarvortex” event? (1/9) 🧵

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wxjay.bsky.social•72 days ago

Great thread! I agree the cause-effect is tough to disentangle from the polar vortex perspective.

One other element to this event evolution: According to the GEFS the last couple of days, the wave reflection index jumps above 1 on Jan 2 and stays there for a few days before dropping off.

wxjay.bsky.social•72 days ago

This is why we need more SNAPSI experiments! My PhD student Ollie has done some for a couple of past CAOs and has some interesting results. Stay tuned for the upcoming presentations and paper. 🙂

drahbutler.bsky.social•72 days ago

Can’t wait to see it!

charliepotatos.bsky.social•72 days ago

nice read
kudos 👏

drahbutler.bsky.social•72 days ago

Certainly by any common, zonal-mean metric of “coupling” between the stratosphere and troposphere, the stratosphere and troposphere have not been communicating much, and this is expected to continue over the next 2 weeks… (2/9)
https://bsky.app/profile/simonleewx.com/post/3lejg47657k27

drahbutler.bsky.social•72 days ago

The stratospheric #polarvortex has been roaringly fast in recent days—which, if the troposphere were listening, would lead to a +ve NAO. Instead the troposphere is predicted to heavily veer towards -ve NAO. This corresponds to a big old block over Greenland. (3/9)

drahbutler.bsky.social•72 days ago

Still, already I’ve seen posts pop up about the “stretching” of the vortex *causing* this event. Here’s a video of the forecasted polar vortex (note: bottom layer is at the lower stratosphere, not the surface) (4/9)

drahbutler.bsky.social•72 days ago

It appears that the vortex as a whole starts to stretch towards North America. In particular at the bottom of the vortex, a lob of Arctic air extends towards the Hudson bay by January 10, which could correspond to a southward shift of the polar jet stream. (5/9)

drahbutler.bsky.social•72 days ago

However, to me it’s not clear whether this stretching is *driving* the cold air outbreak, or whether it’s instead a *response* of the vortex to the forecasted blocking region over Greenland (and the Bering Strait) that essentially acts as a big boulder in the atmospheric flow. (6/9)

drahbutler.bsky.social•72 days ago

This might just seem like semantics- but the mechanism and directionality is important. Because if the root cause of the cold air outbreak is the tropospheric blocking pattern, the #polarvortex is just along for the ride… (7/9)

krasting.bsky.social•72 days ago

Great threat, @drahbutler.bsky.social !

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