Normally we see stratospheric warming events in late December or early January, so this one is a little late to the party. Any image in this post can be enlarged by clicking on it.
For those that are just learning, every winter we see a stratospheric vortex form over the pole. Don't confuse the stratospheric polar vortex with the tropospheric polar vortex, even though somewhat related, they work at different levels in the atmosphere, but changes in the stratospheric vortex are usually reflected to some extent in the the tropospheric vortex. For more views of the stratosphere, I have multiple pages with tons of information:
What happens is that waves, called Rossby waves, move poleward through the troposphere taking heat along with them. When this heat flux and wave are strong enough, these waves travel upward to the stratosphere and are dissipated there, decelerating the winds and warming the Arctic. If the heating is rapid and strong enough, it can cause the high level winds to slow or even reverse direction, which in turn causes disruption in stratospheric vortex, which in turn propagates down and interferes with the tropospheric jet stream which induces large amplitude blocking. In other words, a stratospheric warming event is beneficial for us as is helps to push the cold air off the pole and move it south though various blocking regimes. The blocking can also cause a buckling of the jet stream which can cause a very potent blast of arctic air as well as big storms on the periphery of the cold.
The next 7 images are slices of the stratospheric atmosphere, beginning with 1hPa and going down to 70hPa. What we see in these images are a top-down warming that's underway. Notice how much stronger the temps are at 1hPa and how it diminishes as you go down in levels. Also take note of how steep the ramp up is.
And here are a few Tweets regarding the strat warming that is underway.
"Something always didn't sit right with me about the upcoming 10 days. I clearly see the -NAO (North Atlantic Oscillation), the -AO (Arctic Oscillation), the split flow etc. but yet my mind was sort of cautious when it came to the epic cold/snow calls. There are things in the back of my mind that are keeping me from hyping this period more than usual. Obviously the progressive, active Pacific Jet aided by a very strong El Niño would be one concern.