Thread by @wxjay

#S2S North America undergoes a significant pattern change this week, with much warmer conditions for the central and eastern North America with cooler, stormier weather for the West.

But... could we see a return of the cold in January 2023??? A somewhat technical 🧵 1/

If you recall, I talked about how a phenomenon called lower stratospheric reflection was a contributor to our recent massive cold air outbreak (several other factors in play too). Well, the GEFS is once again hinting at more wave reflection possible. 2/

Below is the reflection index (from Kretschmer and Matthias 2020). Sustained values above 1-1.5 indicate favorable conditions for "wave reflection." This phenomenon reflect upward propagating waves from Siberia that "reflect" back down over Canada due to wind shear. 3/

This plot shows vertical movement of waves ~15 km above the surface. Red/orange - more up than normal; Blue - less up than normal or even down.

Note the red areas over Siberia - that is indicating waves going up. We don't see very much blue over Canada, but there is some. 4/

I have some more confidence that we are going to see enhanced wave motion upward over Siberia. Why? Check out the mid-tropospheric height map. See the area of blues over eastern Siberia? That is a key source region for vertically moving waves. 5/

Now, I discussed the reflection indicators, which are favorable for short-term, significant cold air outbreaks over North America. But, I am not wholly certain that is the only story.

Why? Because waves can reflect OR break in the stratosphere, producing different conditions. 6/

This next plot shows the average magnitude of vertical propagation of waves from the surface to ~30 km over the Arctic (60-90 N) as a function of time. Red - more than average; Blue - less than average.

See the big red blob in January from 10-50 mb? 7/

That indicates that waves are not only entering the stratosphere and (possibly) getting reflected at 100 mb, but there are waves that make it way up to 30 km. Up there, these waves may actually breaking, causing the #PolarVortex to weaken there. 8/

Here's the animation of the forecast for the #PolarVortex from the GEFS for the same run, up at 10 mb (~30 km). The #PolarVortex starts out rather strong and symmetrical but then we see a big Aleutian High come in, distorting the vortex. 9/

Should the big low over Eastern Siberia persist, sending more wave flux into the stratosphere, this will only cause the mid-stratospheric vortex to get weaker with time (these waves, when they break, bring heat into the Arctic stratosphere, weakening the vortex). 10/

So, to sum up, there looks to be an interesting short-term and longer-term implications to the forecast pattern over the next two weeks. One would be wave reflection, which could reestablish the central/eastern North American trough, returning the area to colder-than-normal. 11/

The other would be if waves are actually able to penetrate further into the stratosphere, weakening the 10 mb vortex, which would then increase risks of longer-term colder-than-normal episodes, especially for Europe into the second half of Jan and Feb. 12/

Remember - this is ONE model suite, and forecasts for stratospheric weather patterns have poor predictability beyond ~10 days. However, the trends and signals are there, and it bears watching for subseasonal winter patterns in the new year. 13/13