Sub-seasonal predictability of winter tornado regimes in Europe captured through disruptions to the stratospheric polar vortex

Europe is often thought of as a place where the sky is generally calm, a stark contrast to the violent tornado outbreaks that dominate headlines from the American Midwest. Yet, the continent is not immune to severe storms. While meteorologists can reliably predict dangerous weather a few days in advance, looking further ahead—into the sub-seasonal window of three to four weeks—has remained a frustrating blind spot. This gap leaves emergency planners and communities without crucial early warnings for events that could cause significant damage. The challenge lies in the chaotic nature of the lower atmosphere, where weather patterns tend to dissolve and become unpredictable after about a week. To see further, scientists must look higher, beyond the weather we feel, into the stratosphere, a layer of the atmosphere that changes much more slowly and can influence the air below it.

A recent study by Matthieu Lacroix and colleagues at the University of Lyon and the University of Évora has cracked a piece of this long-standing puzzle. The researchers discovered that the risk of tornadoes in Europe during the winter months is not random, but is instead tied to massive disruptions in the stratospheric polar vortex, a ring of strong winds circling the North Pole. However, the story does not end there. The study reveals that a disruption in the upper atmosphere does not automatically create a tornado threat on the ground. Instead, the outcome depends entirely on a specific, pre-existing pattern of air pressure over the Atlantic Ocean. By combining these two factors, the team has developed a new way to forecast high-risk windows for severe storms up to a month in advance, offering a tool that could fundamentally change how Europe prepares for winter weather.

For decades, scientists have known that the stratosphere can act as a reservoir of predictability. The polar vortex is a giant, swirling column of cold air that sits over the North Pole during winter. Occasionally, this vortex is disrupted by a sudden stratospheric warming event, where the temperature in the upper atmosphere spikes by dozens of degrees in just a few days, causing the winds to slow down or even reverse direction. These events are well-documented, and it has been known for some time that they send a signal downward, eventually shifting the jet stream—the fast-moving river of air that steers weather systems across the continent—toward the equator. This shift often brings cold spells and snow to Europe. But the question remained: does this same shift also create the specific, dangerous conditions needed for tornadoes?

To answer this, the researchers first had to solve a measurement problem. In the United States, scientists use a formula to predict tornado risk that multiplies two ingredients: heat energy in the atmosphere and wind speed changes with height. If either ingredient is missing, the formula says no tornado will form. This works well in America, where storms often need both intense heat and strong winds. However, European winter tornadoes are different. They frequently occur in cold air where there is very little heat energy, but the winds are exceptionally strong and change direction sharply near the ground. The American formula, which relies on multiplying these factors, would incorrectly predict zero risk for these European storms because the heat ingredient is missing. The team therefore created a new index, the European Tornado Environment Index. Instead of multiplying the ingredients, they added them together. This simple change allowed the index to recognize that strong winds alone, even in cold air, can create a dangerous environment for tornadoes. They trained this new tool using decades of storm reports and atmospheric data, proving it could accurately identify the specific conditions that lead to severe weather across Europe.

With a reliable way to measure the risk, the team looked at 50 major stratospheric warming events that occurred between 1950 and 2025. They tracked what happened to the tornado risk index in the weeks following these events. The results were surprising. When they looked at all the events together, the signal was messy and unclear. Sometimes the risk went up, sometimes it went down, and often it seemed to disappear entirely. This lack of a clear pattern suggested that the stratospheric signal was being blocked or altered by something else in the lower atmosphere. The researchers realized that the atmosphere does not react to a single push in a uniform way; the reaction depends on the state of the atmosphere at the moment the signal arrives.

The key to unlocking the pattern was the East Atlantic pattern, a large-scale arrangement of high and low pressure over the ocean west of Europe. The researchers split their 50 events into two groups based on whether this pattern was in a "positive" or "negative" phase at the time the stratospheric signal reached the ground. The difference was dramatic. When the stratospheric disruption happened while the East Atlantic pattern was in a positive phase, the risk of severe tornado environments doubled or even tripled in specific regions. The displaced jet stream, combined with the warm, moist air flowing north from the Mediterranean and the subtropical Atlantic, created a perfect recipe for storms. This high-risk zone stretched from Portugal and Spain across southern France and into Italy and the Balkans.

In stark contrast, when the same stratospheric disruption occurred during a negative phase of the East Atlantic pattern, the risk of tornadoes vanished across almost the entire continent. In this scenario, the atmosphere was dominated by cold, dry air and high-pressure blocks that prevented the warm, unstable air from moving north. Even though the jet stream still shifted south, the lack of moisture and heat meant that the conditions for severe storms could not form. The stratospheric signal was effectively neutralized by the cold, stable air below. This finding explains why previous attempts to predict these events failed; without knowing the state of the East Atlantic pattern, the stratospheric signal is just noise.

The study concludes that forecasting severe winter weather in Europe requires a two-step approach. First, meteorologists must monitor the stratosphere for signs of a polar vortex disruption. Second, they must check the phase of the East Atlantic pattern. If a disruption occurs while the East Atlantic pattern is in a positive phase, it serves as a powerful early warning that a window of high risk for severe storms will open three to four weeks later, particularly over southern and western Europe. This is not a guarantee that a tornado will strike on a specific day, but it identifies a period where the atmospheric ingredients are aligned to make such events much more likely. By integrating this knowledge, forecasters can move beyond the current one-week limit and provide communities with a much-needed heads-up, allowing for better preparation and resource allocation before the storms arrive.