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Why tornadoes are still hard to predict – even as storm forecasts are improving


Meteorologists began to warn of severe weather with the risk of tornadoes few days before storms passed the southeast and the Central US end of March 2023. At some point more than 28 million people were under a tornado lookout. But determining exactly where a tornado will touch down – like the tornadoes that hit Rolling Fork, Mississippion March 24, and cities in Arkansas, Illinois And several other states on March 31 – still relies heavily on seeing the storms developing on the radar. Chris Nowotarskian atmospheric scientist, explains why and how forecasting technology is improving.

Why are tornadoes still so hard to predict?

Meteorologists have gotten much better at predicting the conditions that make tornadoes more likely. But predicting exactly which thunderstorms will spawn a tornado and when is more difficult, and that’s what a lot of severe weather research focuses on these days.

Often you have a series of thunderstorms in an environment that seems favorable for tornadoes, and one storm may produce a tornado but not another.

The differences between them may be due to small differences in meteorological variables that are not resolved by our current observation networks or computer models. Even changes in the land surface – fields, forested areas or urban environments – can affect the formation of a tornado. These small changes in the storm environment can have major consequences for the processes within storms that can make or break a tornado.

One way scientists collect data to understand tornadoes is by chasing storms.
Annette Price/CIWRO, CC DOOR

One of the strongest predictors of whether a thunderstorm will produce a tornado relates to vertical wind shearthat’s how the wind changes direction or speed with height in the atmosphere.

How wind shear interacts with rain-cooled air during storms, which we call “outflow,” and how much precipitation evaporates can affect tornado formation. If you’ve ever been in a thunderstorm, you know that just before it starts to rain, you often get a gust of cold air from the storm. The characteristics of that cold air outflow are important to whether a tornado can form, because tornadoes usually form in that cooler part of the storm.

How far in advance can you tell if a tornado is likely to be large and powerful?

The vast majority of violent tornadoes originate from supercells, thunderstorms with a deep rotating updraft called a “mesocyclone”. Vertical wind shear can allow the storm’s center levels to rotate, and upward suction from this mesocyclone can amplify rotation within the storm’s outflow into a tornado.

If you have a supercell on the radar and it has a strong rotation above the ground, that is often a harbinger of a tornado. Some research suggests that a wider mesocyclone is more likely to create a stronger onelonger-lasting tornado than other storms.

Forecasters also look at the storm’s environmental conditions: temperature, humidity and wind shear. Those provide more clues that a storm is likely to produce a significant tornado.

What radar showed as a tornado heading toward Rolling Fork, Mississippi, on March 24, 2023.

The percentage of tornadoes that trigger an alert has increased in recent decades due to Doppler radar, improved modeling and better understanding of the storm environment. About 87% of deadly tornadoes from 2003 to 2017 had an advance warning.

The turnaround time of alerts has also been improved. Generally it is about 10 to 15 minutes now. That’s plenty of time to get to your basement or, if you’re in a trailer park or outdoors, to find a safe facility. Not every storm has that much run-up time, so it’s important to get to shelter quickly.

What are researchers discovering about tornadoes today that could help protect lives in the future?

If you think back to the movie TwisterIn the early 1990s, we started doing more fieldwork on tornadoes. We turned off radar in trucks and drove vehicles with roof-mounted instruments into storms. That’s when we really started to appreciate what we call the storm scale processes — the conditions inside the storm itself, how variations in temperature and humidity in outflow can affect the potential for tornadoes.

However, scientists can’t launch a weather balloon or send instruments into every storm. So we also use computers to model storms to understand what’s happening inside. We often perform different models, also known as ensembles. For example, if nine out of ten models produce a tornado, we know that the storm is likely to produce tornadoes.

The National Severe Storms Laboratory recently experimented with tornado warnings based on these models, called Prediction warningto extend the lead time for tornado warnings.

A destroyed house with only one wall standing and furniture scattered in Rolling Fork, Mississippi, after the March 24, 2023 tornado.
An early warning can be the difference between life and death for people in homes without basements or cellars.
Chandan Khanna/AFP via Getty Images

There are many more areas of research. For example, to better understand how storms form, I do a lot of idealized computer models. To do that, I use a model with a simplified storm environment and make small changes to the environment to see how that changes the physics within the storm itself.

There are also new tools in storm chasing. There has been an explosion in drone use – scientists are putting sensors in unmanned aerial vehicles and they fly close and sometimes in the storm.

The focus of tornado research has also shifted from the Great Plains – the traditional “tornado alley” – to the southeast.

What’s Different About Tornadoes in the Southeast?

In the Southeast, there are different influences on storms compared to the Great Plains. The southeast has more trees and more varied terrain, as well as more moisture in the atmosphere because it is close to the Gulf of Mexico. They usually are kill more also in the southeast, as more tornadoes form at night.

US map showing highest number of tornadoes in Mississippi, Alabama and western Tennessee.
A map of severe tornado days from 1986 to 2015 shows a large number in the Southeast.
NOAA Storm Forecast Center

We tend to see more tornadoes in the Southeast that are in rows of thunderstorms called “quasi-linear convective systems.” The processes that lead to tornadoes in these storms may differ, and scientists are learning more about them.

Some studies have also suggested the onset of a climatic shift in tornadoes heading southeast. However, it can be difficult to disentangle an increase in storms from better technology noticing more tornadoes. So more research is needed.

This article was updated March 31, 2023, with tornadoes in Arkansas and the central US.

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