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The jet stream took a sharp turn, and the US got unprecedented tornado weather in December

Extremely powerful winds swept across a large part of the U.S. on Dec. 15, 2021, . Record temperatures helped generate , winds spread and in Kansas, and wind damage was reported from northern and into the Midwest. The National Weather Service described it as a "historical weather day" with a "."

We asked , whose office at Iowa State University was at the heart of the storms, to explain what caused the extreme weather and why it was so unusual.

What happened in the atmosphere to trigger such extreme weather over such a wide area?

We were seeing very strong winds because of a very powerful disturbance in the . That disturbance helped to create a very intense , which creates strong winds and storms. But the low pressure wasn't what made this event unusual.

It was unprecedented because an incredible amount of warm air got pulled up from the south ahead of the storm.

Here in Iowa, they've ever been in December, with on Dec. 15, and a very unusual amount of humidity came up with those temperatures. That's why we were seeing —and reports of .

Tornadoes are . Minnesota, which had never had a tornado in December, also had tornado warnings and .

With anything going downward, gravity accelerates it, just like if you drop a ball from the top of a skyscraper. The . As they flow down the east face of the Rockies, they accelerate.

When you're on the leeward side of a mountain range, like Denver and Boulder, winds in those areas can as they're descending.

What role does the jet stream play in a storm like this?

When we get a low-pressure system, it's because of wiggles that are happening in the . We call them troughs in meteorology.

If you look at a map showing the jet stream, the jet stream —it oscillates up and down, from north to south. Any time you're out ahead of one of these troughs, where the jet stream bends down toward the south and then back toward the north, the air must rise out ahead of it, and this results in a low-pressure system. The winds that blow around it can become very strong.

In this case, there was an especially sharp trough in the jet stream, almost in a "V" shape, that intensified the effect.

Is there a connection between this storm and the deadly tornadoes that hit Kentucky and other states on Dec. 10–11?

It's hard to say if there was one trigger somewhere on the planet that managed to create these two different ripples in the jet stream.

What's interesting is that there is . When we have La Niña conditions, we often find that the far northern part of the United States ends up colder than normal and the south ends up warmer, so you have this bigger contrast in temperatures than normal and it often leads to a stronger jet stream.