Twin tornadoes, two simultaneously existing from the same thunderstorm, are extremely rare but still occurred Monday afternoon under the right atmospheric conditions in Pilger, Nebraska.
The first thing needed for a tornado to form is a thunderstorm. A thunderstorms needs rising, very warm, very moist air to get going. Warm, moist air begins to rise, and the ability to which this occurs is known as instability.
As the air rises, it cools and forms towering clouds. Shifting winds with height, or wind shear, cause the storm to rotate. The ability of the atmosphere to cause rotation is known as helicity.
As this rotation continues, a narrow zone of high winds begins to extend 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 and longest-lived tornadoes tend to form in severe thunderstorms that are isolated from each other.
The thunderstorm that formed during the late afternoon hours on Monday, June 16, 2014, near Pilger, Nebraska, 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 F, and 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.
A weak disturbance was producing other severe thunderstorms over the northern and central Plains, but the significant members were more than 100 miles away from the the Pilger thunderstorm.
"The Pilger storm essentially had the entire unstable atmosphere to tap into and grow into a monster," Margusity said.
Adding to the development of the rare twin twisters was high wind energy.
According to Senior Meteorologist Bernie Rayno, "There was a screaming swath of high velocity, hot, humid air in the lowest levels of the atmosphere aimed right at eastern Nebraska Monday afternoon."
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 at the same time is unclear. Usually friction and other effects will cause one tornado to weaken quickly.
"Both tornadoes formed on the same hook part of the storm," Marguisty said.
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 extraordinary helicity was discovered by a special weather balloon released by the Omaha National Weather Service Office during mid-afternoon.
"The atmosphere can only support so much rotation from a single vortex, so when there is too much rotation, multiple vortexes form," Smith said.
The storms near Pilger became wedge tornadoes. A wedge tornado is wider than it is tall and is often violent. This type of tornado forms 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 they have occurred before.
"About once a decade, a severe thunderstorm is able to produce multiple major tornadoes simultaneously," Smith said.
It was during the Palm Sunday Outbreak of 1965, that twin wedge tornadoes were captured on film at Elkhart, Indiana.
A storm in central Oklahoma on May 3, 1999, also produced twin tornadoes, but only one was considered to be a wedge tornado.