The southern jet stream, or subtropical jet stream, has been quite active during the last few weeks. Storm systems have routinely moved out of the Rockies, through the Southern Plains, and exited the East Coast at various points, but have not been followed by significantly cold air we typically see over the Plains and Midwest. Instead, temperatures have remained sometimes well-above normal.
Reports from areas of the Western Corn Belt that fieldwork and even tiling can still be accomplished this late in the year are quite the surprise, especially if you compare it to the end of 2019. There is relatively little to no frozen ground yet and it doesn't look like that will change much during at least the next week.
And this is somewhat unusual when we look at the polar jet stream, or polar vortex. During the last several weeks the jet has been quite strong, locking the cold arctic air near the North Pole. But the index that measures its strength has trended negative (weaker) in the minus 1 to minus 2 range during the last few days and maintains this strength through Christmas. This usually suggests pieces of the polar vortex are shifting southward through the continental Northern Hemisphere.
However, models continue to suggest, at least for the next week, temperatures over North America will largely remain above normal. So where is the cold air going? Well it seems Siberia and Kazakhstan are more in line for the chilly conditions than our side of the globe.
The southern jet stream still remains active, though. Storm systems will continue to move through the country, bringing showers and even moderate snow at times. In fact, one late this week and weekend is set to do just that. Its track is still uncertain, but an area from Kansas to Michigan looks to have the best chance for snowfall accumulations.
But the temperatures that follow behind it are not those we are used to seeing at this time of year. Areas of the south such as the Southern Plains, Delta, and Southeast will see temperatures go below normal. But oddly enough, temperatures will remain above normal across the Northern Plains and much of the Midwest.
Snowpack has been lacking this year. When snow has occurred, it has melted off fairly quickly. The lack of a snowpack has contributed to the above-normal temperatures as well. Snow will reflect a better portion of sunlight back out into space. And the energy that is absorbed goes more into melting the snow than warming up the air, both contributing to cooler air temperatures. The snowpack is far enough into Canada to limit the cooling that takes place over the northern tier of the country, allowing it to modify and warm on its way south.
This is at least for the next week. Models suggest next week the weaker polar vortex could start to shift below normal temperatures further south through Canada, while keeping the U.S. warm. The limited snowpack may contribute to this as well. But as we all know, models can be wrong. The suggestion of a weaker polar vortex should give us a heads up that the colder arctic air may be on its way. Will it make it here before the end of the year? That's the main question.
John Baranick can be reached at email@example.com
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