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Twin tornadoes: Rare, powerfulTell North Platte what you think
 
Courtesy Photo­Image
Twin twisters, from StormChasinVideo.com­the Associated Press­accuweather.
Photo by Accuweather
Doppler radar images showing wind direction and speed. The tornadoes show up a double bulls' eyes.
Photo by Accuweather
Photo by Paul Huffman
Twin twisters in Elkhart, Ind. on April 11, 1965.

Twin tornadoes -- two simultaneously in the same thunderstorm -- are extremely rare but they formed Monday afternoon because the right atmospheric conditions converged near Pilger.

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Pilger was decimated by one of the tornadoes, which tore through the town of 350 people, ripping buildings apart and scattering the debris over a wide area.

According to an analysis by Accuweather, the conditions were right.

The first thing needed for a strong tornado is a thunderstorm.

Thunderstorms need rising, very warm, very moist air to get going. As the moist air begins to rise, it cools and forms towering clouds. Shifting winds with height, or wind shear, cause the storm to rotate.

The tendency of the air to spin is known as helicity. The extraordinary helicity of the Pilger thunderstorm was discovered by a special weather balloon released in mid-afternoon by the Omaha National Weather Service Office.

As the rotation continues, a narrow zone of high winds begins near the rear of a forward-moving severe thunderstorm.

While several severe thunderstorms capable of producing tornadoes can coexist over a general area, the strongest, longest-lived tornadoes tend to form in severe thunderstorms that are isolated from each other.

The thunderstorm that formed during the late afternoon on Monday, June 16, near Pilger was isolated. It essentially had the entire eastern half of Nebraska to work with.

According to Severe Weather Expert Henry Margusity, temperatures surged into the upper 80s to lower 90s. Dew point temperatures (the temperature at which the air become saturated when cooled) climbed into the lower 70s. There was no interference or competition from other storms nearby. Only a weak disturbance was producing other severe thunderstorms over the northern and central Plains, and those significant storms were more than 100 miles away from the Pilger thunderstorm.

"The Pilger storm essentially had the entire unstable atmosphere to tap into and grow into a monster," Margusity said.

"There was a screaming swath of high velocity, hot, humid air in the lowest levels of the atmosphere aimed right at eastern Nebraska," according to Senior Meteorologist Bernie Rayno.

The storm was able to rise tens of thousands of feet into the atmosphere, where the cloud was warmer than its environment. "This storm was able to single-handedly tap into the low-level jet and gain extra fuel and extra spin as a result," Rayno said.

The exact way that two large tornadoes were able to coexist for a dozen or so minutes is unclear. Usually, friction and other effects will cause one tornado to weaken quickly.

According to Mike Smith, senior vice president of AccuWeather Enterprise Solutions, "The tornadoes that hit northeastern Nebraska had a very high value of helicity and instability to work with."

"The atmosphere can only support so much rotation from a single vortex, so when there is too much rotation, multiple vortexes form," Smith said. 

Wedge

The storms near Pilger became wedge tornadoes. A wedge tornado is wider than it is tall and it is often violent. Wedge tornados form when the base of the thunderstorm cloud is close to the ground.

The tornadoes began to rotate around each other as the storm progressed, Smith said.

At least one of the tornadoes appeared to have multiple vortexes rotating around it. These vortexes contain higher winds than the parent tornado itself and can cause tremendous damage.

Twin tornadoes of this strength are extremely rare, but have occurred before.

"About once a decade, a severe thunderstorm is able to produce multiple major tornadoes simultaneously," Smith said.

During the Palm Sunday Outbreak of 1965, twin wedge tornadoes were captured on film at Elkhart, Ind. A storm in central Oklahoma on May 3, 1999 also produced twin tornadoes, but only one was considered to be a wedge tornado.


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The North Platte Bulletin - Published 6/17/2014
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