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Science Spotlight: How did Monday's deadly 'twin tornadoes' form?

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Scott Sutherland
Meteorologist, theweathernetwork.com

Tuesday, June 17, 2014, 2:08 PM - On Monday afternoon, a series of powerful storms swept through the U.S. Midwest, breaking a nearly two-week-long lull for tornadoes. One particular storm that swept through Stanton County, Nebraska, was so intense that it actually spawned one of the rarest severe weather events we know about - twin tornadoes.

Although tornadoes generally 'cook up' from the same recipe - a mixing of warm humid air, warm dry air, and cool dry air, producing storms with strong wind sheer and powerful updrafts and downdrafts - they are random, chaotic and given the right conditions, they can take some very bizarre forms. They can get into a cycle where they form, tear along a path, then dissipate, only to reform from the same vortex a short distance away to continue on, and they can repeat this several times. They can spin off satellite tornadoes - smaller funnels that usually spin in the opposite direction of the 'parent' tornado (a 'reverse' tornado), and sometimes orbit around the parent. They can take the form of multi-vortex tornadoes, where two twisters form inside a larger swirling vortex, to rotate around each other like a pair of dancers. They can spawn also in 'families', either separately or at the same time, and the members of these 'families' tend to compete with each other for the energy to grow and maintain their strength. This 'sibling rivalry' could mean that one tornado dominates, stifling the formation of any siblings, or siblings that spawn can rob their elder of energy as they form and grow.

The powerful supercell thunderstorm that formed in northeastern Nebraska on Monday was a special case that isn't often seen, though. It's size and raw power were enough that it produced a pair of tornadoes that could best be described as twins. 

Although footage of the storm showed that one twister definitely spawned before the other, the second one formed too far away to make the two a multi-vortex tornado, and the second wasn't a satellite, as it was spinning in the same direction as the first. It certainly qualified as part of a tornado family, but the storm was so large and powerful that no sibling rivalry was necessary. With each maintaining their distance from the other (up to around 1.5 km apart at times) and tapping into an ample supply of everything they needed to grow and maintain their strength - directly from this massive storm - both were able to grow to roughly the same size and strength. 

As the storm tracked to the east, these twins tore through Stanton County, spreading devastation through the town of Pilger, about 100 kilometres southwest of Sioux City, Iowa. According to reports more than half the town's buildings were destroyed, resulting in two deaths and at least another 19 people taken to hospital, some with critical injuries.

Although definitely rare, twin or double tornadoes have been witnessed before. Some of the most famous documented cases came out the Midwest, on April 11-12, 1965 - the Palm Sunday tornado outbreak - although some of these may have been multi-vortex tornadoes (as they were very close together). More recently, double tornadoes were reported during the Oklahoma/Kansas tornado outbreak on May 3, 1999, and others have been witnessed or reported during outbreaks in 2012 and 2013. 

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