Another Blast of Arctic Air: This Time, with a Stretched but Strong Polar Vortex
Posted
Last Updated
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.
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.
