Another explosion of Arctic air: this time with a stretched but strong polar viral

On By Littum

We are breaking in briefly because another wave of very cold air seems to fall from the Arctic over a large region in the central United States over the weekend and into the beginning of next week. We know the question will be asked: Is the cold related to the polar virtue this time? So here we have to give some answers.

There are two points we will emphasize:

1. The polar viral strength, measured at the speed of winds around 60n latitude and 10 hpa pressure levels, remains stronger than average and is currently expected by most models to return to strong wind speeds nearby in early February.

Observed and expected (NOAA GEFSV12) Wind speed in the polar vertebra compared to the natural area of ​​variation (weak blue shade). Since mid -November, the wind at 60 degrees north (the average location of the polar vertebra) has been stronger than usual. According to the GEFSV12 forecast issued on January 15, 2025, these winds are expected to remain stronger than usual for at least the next few weeks (bold red line). Noaa Climate.gov picture, adapted from Original by Laura Ciasto.

Usually, if the polar vortex communicates with the surface as it has finally been in the last few days, a strong polar whirl would be associated with sustained heat Over large parts of Europe, Asia and the eastern United States. (A strong polar vortex is usually associated with a north -shift jet stream that holds the coldest air correlated over Poland.) Europe and Asia actually expect warmer than average conditions next week but not the United States. So something else is going on over the United States that overwhelm the signal “strong polar vertebra”.

2nd we discussed how we did not think that the shape or stretch of the polar virtue contributed to the last cold air outbreak because the vertebra of the lower stratosphere was changed towards Asia and not stretched over North America. In this case, however, Vortex is actually expected to extend throughout its depth (10-30 miles above the surface) over Canada and Hudson Bay. So, unlike last week, this time the stretched polar vertebra can be In connection with the expected south change of jet stream, which allows the troposphere’s cold Arctic air to be wasted into the continental United States.

The expected structure of the tropospheric jet stream (yellow) and several levels of the stratospheric polar vertebra from the lower stratosphere to the upper stratosphere of the NOAA GFS model to 17 January 2025 (initialized on January 16, 2025). The contours show how the stretched polar virgin is similar to the southern shift of jet stream over North America. Noaa Climate.gov picture, adapted from Original by Laura Ciasto.

However, we want to emphasize that “associated with” still does not mean that one thing caused another, and in this case it is still difficult to understand what is causing what. In addition, a strong back of high pressure has built up at the same time near Alaska, which can also help forcing the jet stream to dive south across the continental United States and bring cold Arctic air with it, independent of the polar vertebra.

To summarize: Unlike last time (5-7 January), the stretch of the polar vertebra stretches throughout the column and is “in synchronization” with the extension of the jet. But we don’t know the direction (what caused what), and other tropospheric factors like the strong Alaskan -ridging are definitely big players. And although things are more in line this time, not only cold air outbreaks happen due to the polar vertebra.