Another blast of Arctic air: this time, with a stretched but strong polar vortex

We’re briefly popping in because another surge of very cold air looks to drop down from the Arctic over a large region of the central US this weekend and into early next week. We know that the question will be asked: is the cold related to the polar vortex this time? So here we are to provide some answers.

There are two points we want to emphasize:

1. The polar vortex strength, as measured by the speed of the winds around the 60N latitude circle and 10 hPa pressure level, remains stronger than average, and is currently forecast by most models to return to near-record strong wind speeds into early February.

Observed and forecasted (NOAA GEFSv12) wind speed in the polar vortex compared to the natural range of variability (faint blue shading). Since mid-November, the winds at 60 degrees North (the mean location of the polar vortex) have been stronger than normal. According to the GEFSv12 forecast issued on January 15 2025, those winds are forecast to remain stronger than normal for at least the next few weeks (bold red line). NOAA Climate.gov image, adapted from original by Laura Ciasto.

Normally, if the polar vortex is communicating with the surface, which it finally has been in the last couple of days, a strong polar vortex would be associated with persistent warmth over much of Europe, Asia, and the eastern US. (A strong polar vortex is usually associated with a northward shifted jet stream that keeps the coldest air corralled over the pole.) Europe and Asia are indeed anticipating warmer than average conditions next week, but not the US. So something else is going on over the US that is overwhelming the “strong polar vortex” signal.

2. We discussed how we didn’t think the shape or stretching of the polar vortex contributed to the last cold air outbreak, because in the lower stratosphere the vortex was shifted towards Asia and not stretched over North America. However, in this case, the vortex is actually forecast to stretch throughout its entire depth (10-30 miles above the surface) over Canada and the Hudson Bay. So unlike last week, this time the stretched out polar vortex may be associated with the forecasted southward shift of the jet stream, which allows the troposphere’s cold Arctic air to spill into the continental US.

The forecasted structure of the tropospheric jet stream (yellow) and several levels of the stratospheric polar vortex from the lower stratosphere to the upper stratosphere in the NOAA GFS model for 17 January 2025 (initialized on 16 January 2025). The contours show how the stretched polar vortex corresponds to the southward shift of the jet stream over North America. NOAA Climate.gov image, adapted from original by Laura Ciasto.

However, we want to reemphasize that “associated with” still does not mean one thing caused another, and in this case, it’s still difficult to understand what is causing what. Additionally, a strong ridge of high pressure has been building up simultaneously near Alaska, which can also help force the jet stream to dive down south over the continental US and bring cold Arctic air with it, independent of the polar vortex.

To sum up: Unlike last time (Jan 5-7), the stretching of the polar vortex is extending through the entire column and is “in-sync” with the extension of the jet. But we don’t know the directionality (what caused what), and other tropospheric factors like the strong Alaskan ridging are definitely big players. And while things are more in-line this time, cold air outbreaks don’t only happen because of the polar vortex.