Why thunderstorms and high winds are so rare in Minnesota in December. And why that could be changing.

17December 2021

Minnesotans are used to extreme weather in December, but it usually involves snow. But on Wednesday evening, a severe wind storm that meteorologists and climatologists call “unprecedented” swept through south and southeastern Minnesota, a confirmed tornado in Winona County – the first tornado to be recorded in December in Minnesota history.

“What’s unprecedented is having a damaging severe weather outbreak in Minnesota during December,” said Kenny Blumenfeld, a climatologist at Minnesota’s Department of Natural Resources.

“It takes a very strong thunderstorm to blow things over, and it takes an incredibly strong thunderstorm to have a continuous track of wind damage like we saw Wednesday night where there was just damage from area to area as you go across south, central and southeastern Minnesota,” said Blumenfeld.

The storm was caused by a massive low-pressure system that moved through much of the central U.S. Low-pressure systems alone are not unusual for winter months — in fact low-pressure systems are associated with clouds and precipitation, or more specific to winter, snow. What made Wednesday’s weather unique was the presence of warm, humid air caused by unseasonably high temperatures. Many cities in southern Minnesota set record high temperatures on Wednesday, and in some cases a record high for the month of December.

“There had been signals early on that this could be a very unusual type of event for this time of year with the warmer, more humid air that we would more typically see during the warmer months, reaching all the way up into Minnesota,” said Jeff Makowski, a meteorologist at the National Weather Service’s La Crosse, Wisconsin branch.

Pete Boulay, a climatologist at the DNR explained that the combination of high humidity and warm temperature creates “warm, muggy air”  which, when lifted higher into the atmosphere by a low pressure system, condenses into clouds. Boulay said this then allows the storm to form, and in this case “a destructive line of thunderstorms.”

“Having the dynamics to produce thunderstorms in December” is unusual, Boulay siad. “This is one of the most bizarre weather moments in our recorded history.”

Is the storm linked to climate change?

Given the role played by high temperatures in fueling the storm, one might ask whether Wednesday’s unprecedented damage is a symptom of climate change. A warming winter was undoubtedly an ingredient in causing the storm, Blumenfeld says, but the link between extreme storms and climate change is more nuanced.

“It’s hard to say that climate change had any influence on the main kind of weather system itself, but it had an influence on the different ingredients in the air that the weather system was working with,” Blumenfeld said. “And those all have kind of signs of being related to the changing climate because we’re seeing warmer and more humid conditions kind of creep into winter in ways that we hadn’t seen before.”

“You could at least make a connection and say the thunderstorms being that intense may be a sign that the season for severe weather is expanding in ways that we hadn’t seen before,” Blumenfeld said.

Meteorologists at the National Weather Service said a similar storm is not in the forecast for the immediate future. But Blumenfeld said that another severe thunderstorm may become an event that is not unprecedented in future winters, though it will still not be a common occurrence.

“It’s kind of a wake up call that this kind of thing can happen, right? Just the fact that it did happen [Wednesday], we now have evidence that it certainly can happen and probably will happen,” Blumenfeld said. “Maybe not this year or next year, but you know, maybe another time this decade and maybe at other times of our cold season, we’ll just overall start seeing more of these.”

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