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      How rare are twin tornadoes?

      photo via AP/Eric Anderson

      NEW BLOOMFIELD, MO ?? Experiencing one large tornado can be scary enough, but two, side-by-side, is horrifying. Seeing two violent tornadoes side-by-side prompted many to ask themselves this question: How rare are twin tornadoes like the ones that touched down in Pilger, NE on Monday night?

      The answer to that question is not very rare at all. There are many documented cases of multiple tornadoes touching down from the same parent supercell thunderstorm. What makes these tornadoes rare is the fact that there was two large, violent tornadoes on the ground at the same time within roughly a mile of each other. Usually when there are two tornadoes on the ground simultaneously, there is one stronger tornado and one tornado that is much weaker and usually the weaker one rotates around the parent tornado. Both of these twin tornadoes were very large and destructive.

      On average, twin tornadoes of this size and magnitude only occur roughly every 10-15 years, says Greg Carbin, a meteorologist at NOAA??s Storm Prediction Center in Norman, OK. The last documented large twin tornadoes occurred on May 3, 1999 in Oklahoma during a severe weather outbreak, prior to that, twin tornadoes were recorded during the Palm Sunday tornado outbreak in Indiana in 1965. Historically, in Nebraska, June is the month with the highest number of tornadoes.

      Average number of tornadoes for Nebraska in the month of June from 1984-2013 via NOAA.

      Photo of the two tornadoes near Pilger, NE -- photo via AP/Eric Anderson

      How do these occur?

      One way that twin tornadoes can occur is through a processes called occlusion. This happens when one tornado starts to dissipate as cool, moist air wraps around the tornado while another tornado begins to form in a more favorable part of the thunderstorm. Another term for this is cycling and that process is fairly common in very strong thunderstorms. The overlap time for both tornadoes to be on the ground concurrently is a very short time.

      Another way two tornadoes can form is from a multiple-vortex tornado. This occurs when much smaller, rapidly spinning whirls rotate inside a larger tornado ?? also termed ??suction vortices??. These ??suction vortices?? inside of a larger tornado are usually responsible for some of the most intense damage in a tornadoes path. This is because of smaller tornadoes inside the larger tornado have stronger winds and are spinning much faster than the larger tornado. The tornado in Pilger was not a multi-vortex tornado.

      The last way that twin tornadoes can occur is by satellite tornadoes. This occurs when a second tornado develops independently of the primary tornado. The satellite tornado orbits the primary tornado but is located inside the same mesocyclone ?? this means there are two separate areas of rotation occurring at the same time in the atmosphere.

      At this time, it is still unclear exactly what caused these two tornadoes, the National Weather Service will analyze radar data and make that determination. This thunderstorm and its tornadoes will be a studied by meteorologists and scientists for many years to come. It will take some time analyze this thunderstorm to understand exactly how and why two large, violent tornadoes occurred so close to each other.

      The National Weather Service confirmed that at one point there were three tornadoes on the ground.

      Radar view showing what the winds looked like inside of the thunderstorm. The three white ovals show the location of the three tornadoes that were on the ground at once.

      Aftermath of the Tornado

      The National Weather Service in Omaha, NE did a damage survey of Pilger. Their preliminary rating was EF-4 with winds somewhere between 166 and 200 mph. The National Weather Service has yet to fully complete their damage surveys and come out with a final report from these tornadoes. Once they consult structural engineers and look at more data, they will make a final determination of what the maximum winds were with these tornadoes.

      Pilger, NE is a town of nearly 350 people and local law enforcement report that nearly 75% of the town was destroy. Two died, including a five year old girl and at least 19 people were injured.

      Pilger has a pretty incredible town motto: ??We??re too tough to die.??