Note California heat as fires expanding in LA, meanwhile cold jetstream trough settles in East of the Rockies.
Frigid air that normally stays trapped in the Arctic has escaped, plunging deep into the United States for an extended visit that is expected to provoke teeth-chattering but not be record-shattering.
It’s a cold air outbreak that some experts say is happening more frequently, and paradoxically, because of a warming world. Such cold air blasts have become known as the polar vortex. It’s a long-established weather term that’s become mainstream as its technical meaning changed a bit on the way.
What it really means to average Americans in areas where the cold air comes: brrrrr.
What’s happening is the jet stream — that usually west-to-east river of air way above ground that moves weather systems along — has made a roller-coaster like dip from the Pacific Northwest to the Southeast and is stuck on that wavy track. To the west of that plunge, in California, it’s hot and dry. But to the east and just above the dip, it’s a taste of the North Pole.
“We’re just getting a lot of cold Canadian and Arctic air that’s being just channeled from north to south,” said Dan DePodwin, AccuWeather director of forecast operations. “We really expect this to be more of a prolonged stretch of well below historical average temperatures. We’re talking 12 to 25 degrees Fahrenheit (7 to 14 degrees Celsius) across a large portion of the eastern half of the country.”
NOAA weighs in, yes, it’s the jet stream, but no, not a classic Polar Vortex set up as we have seen in years past.
I’m going to have to read this more than once to get clear.
A common way in which the stratospheric polar vortex contributes to cold air outbreaks over the eastern U.S. usually involves two key ingredients. First, the westerly (west-to-east) winds ~19 miles above us at 60 degrees North weaken substantially or even reverse direction (easterly winds = major sudden stratospheric warming).
Second, those changes in the polar vortex must communicate down through the troposphere to the surface (stratosphere-troposphere coupling). If that communication and coupling has happened, we would expect to see a wavier-than-normal tropospheric jet stream with plenty of troughs and dips [footnote #2]. If one of those dips stretches southward down over North America, the cold Arctic air comes with it.
Now, yes, that is what the jet stream is doing right now. But no, it’s probably not the polar vortex’s fault.
For one, the polar vortex winds are strong right now…much stronger than normal. In fact, at the end of December, the westerly winds were stronger than they have been since at least 1990. You can see that in the graph below.
Furthermore, the troposphere seems pretty uninterested in the strong polar vortex winds. If the strong polar vortex nudged the troposphere to follow its behavior, we’d expect to see the low thickness anomalies in the stratosphere drip down into the troposphere leading to a much less wavy jet stream, which would keep the cold air confined to the Arctic.
Is the case closed? As the graphics above show, the polar vortex winds are very strong with very little interaction between stratosphere and troposphere. So, the polar vortex isn’t the reason why I’m shoveling my driveway?
Well…even though we think it’s unlikely, we can’t rule it out entirely because the polar vortex and the tropospheric jet stream do agree on one thing: they both extend down over eastern North America.
As we mentioned in a previous post, there’s been some interesting research that suggests the shape of the jet stream that can bring cold air outbreaks is associated with an elongated polar vortex. But it’s not clear that this is actually happening. “Associated with” just means they seem to occur at the same time, not that they are connected. In fact, looking at the vertical structure of the polar vortex, that elongation over North America doesn’t extend to the lower stratosphere. Down there, it’s stretched more toward northern Europe.
The short answer after the long explanation is that the stratosphere is not impacting the surface the way we normally expect, through a weakened polar vortex. The stratosphere and the troposphere are moving around in similar ways, but these similarities don’t continue all the way from the surface to the stratosphere. It’s also important to remember cold, snowy weather doesn’t need a stratospheric culprit, sometimes it’s just…weather.
Is the polar vortex being set up for future troublemaking?
Current forecasts of the polar vortex don’t indicate any significant weakening of the winds in the next month or so. Interestingly though, the strong polar vortex winds make for an excellent conduit for tropospheric wave activity to propagate upward. This is one of the ingredients needed to disrupt the vortex, but additional steps would have to occur for these waves to break and slow the vortex. For now, the winds are so strong that any wave breaking would only weaken the vortex down to its normal speeds for this time of year. If we imagine the polar vortex as a spinning toy top, it might only take one good nudge or two to send it into an irreversible tail spin. If the troposphere keeps up its wavy weather antics, that might be just the nudge needed to get the polar vortex to respond.