When Tornadoes Collide

What happens when one funnel cloud bumps into another?

Tornado damage

Southeasterners reported at least 230 tornadoes over the weekend, with 90 sightings in North Carolina alone. With all those funnel clouds bouncing around, could two twisters collide? If so, what would happen?

Very little. There is no record of two tornadoes joining forces. On rare occasions, a single thunderstorm spawns a new tornado just as an old one is dying off, and then the two offspring of the same thunderstorm system run into each other. The result isn’t nearly as cataclysmic as it sounds, though. Most meteorologists think that when a large tornado absorbs a smaller one, it makes no measurable difference to the strength of the surviving funnel cloud. The smaller twister just disappears.

So why can’t a pair of healthy tornadoes from different storms pull a Voltron? It’s not unheard of for two distinct thunderstorm systems to slam together. As they approach each other, however, the updraft of air that sustains the smaller thunderstorm gets sucked into the larger storm. When that happens, the tornado associated with the smaller storm disappears; it’s starved to death before it has a chance to join forces with the other twister.

The most talked-about twin tornadoes were born in Kansas on March 13, 1990. After an F5-strength twister—the most intense kind, of which there have been 52 in U.S. historypassed through Hesston and began to weaken, the same thunderstorm that produced it generated a new tornado. Some observers described the twisters as combining into a single, more powerful funnel cloud, but most meteorologists don’t think that’s quite what happened. The two tornadoes did come into close proximity, they say, and their paths definitely overlapped. But that doesn’t mean the two vortices joined up to create a single massive tornado. Instead, the dying tornado happened to peter out as it moved into the path of the growing tornado. The former probably had little or no effect on the latter. (You can find video footage of the Hesston double tornado on YouTube.)

Sometimes a single tornado looks like a whole group of tornadoes colliding with one another. That’s because a single, very strong twister can develop between two and six individual funnel clouds spinning around its center. From the ground, this resembles several independent tornadoes merging and separating. But meteorologists consider these systems to be one entity.

Because tornadoes are transient and small, averaging just over 500 feet across, meteorologists don’t have much of an opportunity to study rare and fleeting events like twister collisions. Hurricanes, which average more than 300 miles across and can last for days, are much better research subjects. When two hurricanes get within 900 miles of each other, they often begin to spin around a common center of rotation in what’s called a Fujiwhara interaction. It looks like they’re circling a drain. If the attraction between the cyclones is strong enough and they’re rotating in the same direction, the larger storm eventually absorbs the smaller. As with tornadoes, meteorologists don’t typically observe a strengthening effect, because the hurricane’s fuel supply—the warm water beneath it—doesn’t get bigger.

Bonus Explainer: Can a tornado run into a hurricane? It depends on what you mean. Hurricanes generate thunderstorms, which generate tornadoes. So the weather phenomena are closely related. It’s common to see tornadoes forming on the northeast corners of hurricanes that make landfall near the Gulf of Mexico. But hurricane storms tend to produce weaker, shorter-lived twisters than the ones in tornado alley.

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Explainer thanks Harold Brooks of NOAA and John T. Snow of the University of Oklahoma.