Ag Weather Forum

Cold air that has been locked up in the Canadian Prairies and Northern Plains will spread south and east through the United States during the next few days, feeling like a slap to the face for a lot of us who have enjoyed some rather warm weather during the fall. But how long can this cold last?

The cold is coming via a large change in the jet stream. For most of the fall, a ridge has been persistent over the U.S., guiding the jet stream through Canada and keeping any thoughts of cold air locked up toward the North Pole. However, a shift this week has made that ridge dissipate and a new one develop over Alaska. The ridge, while warm, is able to gather colder air from up north and send it south through Western Canada. Several days of below-zero temperatures befell the Canadian Prairies and into the Northern Plains this week. Temperatures have not been as dramatically low elsewhere and in many cases across the south and east have remained well-above normal for late November.

But with the jet stream changing once again, small disturbances that have crossed into much of the U.S. and Canada are about to form a large and expansive upper-level trough across the Central and Eastern U.S. A system that is currently in the Southern Plains will quickly move through the Ohio Valley Wednesday night and into the Northeast on Thursday. There may be air cold enough to produce some areas of snow here. But the development of this trough behind the system will send the arctic air south and east through the U.S. during the next few days.

Temperatures will remain the furthest below normal in the Northern Plains, where low temperatures will cross below zero Fahrenheit through Dec. 2. But well-below normal temperatures in the single digits above zero could get as far south as Nebraska, Iowa and into Wisconsin. Lows in the 10s are likely in Kansas through the Ohio Valley. Depending on wind and cloud conditions, temperatures may even drop farther than the current forecast.

The question then becomes: How long does this last? Models insist the jet stream will not change a whole lot, at least through the first half of December, but that does not mean subtle changes in it could lead to large changes in temperatures across the middle of the country. That is because the ridge that is currently around Alaska will shift more to western North America this weekend and stagnate there for the following two weeks.

The western ridge and eastern trough will sort of battle it out across the middle of the continent and that could mean warmer weather returning to the Plains in the middle of next week, around Dec. 4. However, the trough will battle back and clippers coming down from Canada may reinforce some of the colder air as well.

The forecast could see temperatures shift back and forth as these clippers move through, depending on their orientation. If they dive farther southeast across the Central Plains to the Ohio Valley, expect lower temperatures to be reinforced. But if they move from the Northern Plains to the Great Lakes, the Plains and South will tend to be milder.

But that leaves the eastern portions of the country, and the Great Lakes and Northeast in particular, in the colder air for an extended period. The warm Great Lakes are primed to produce lake-effect snow when the cold, arctic air flows over it. Indeed, several days of lake-effect snow are forecast east and southeast of each of the Great Lakes, producing heavy snow in the most prone locations, especially in the Upper Peninsula of Michigan, from Cleveland to Buffalo, and on the Tug Hill in New York.

This could extend the cold air longer in these areas too, when the cold air eventually gets kicked out. Models suggest that right around the middle of December, the jet stream should change a bit, forcing the colder air out and bringing some milder air back. But the rest of the month may be chaotic and the cold could return around or after Christmas.

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John Baranick can be reached at john.baranick@dtn.com

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