A rare double tornado mowed down the small town of Pilger, Nebraska (map), late Monday afternoon. The storm killed a five-year-old girl, and a second person died in what was likely a weather-related traffic accident, according to CNN. The twisters injured 19 people who were sent to hospitals for treatment. (See: "Startling Pictures: Twin Tornadoes Slam Nebraska.")
Nebraska's governor, Dave Heineman, issued a state of emergency, allowing use of the National Guard if needed. The National Oceanic and Atmospheric Administration's (NOAA's) Storm Prediction Center said there was a slight chance Tuesday of more severe storms across 14 states from Montana to New York.
Double, or twin, tornadoes are unusual, says Greg Carbin, a meteorologist at NOAA's Storm Prediction Center in Norman, Oklahoma. Tornadoes like yesterday's twins occur perhaps every 10 to 15 years or so, he added.
All tornadoes, whether single or double, are "associated with supercell thunderstorms, which are well-organized, persistent storms," Carbin explained. Supercells have a large, vertical column of rotating air that can spawn tornadoes in about 30 percent of cases.
But "we still don't know why some thunderstorms create tornadoes while others don't," said tornado-chaser Tim Samaras in early 2013. Samaras, a scientist and National Geographic grantee, was killed by a twister on May 31, 2013, in El Reno, Oklahoma. (Read "The Last Chase" in National Geographic magazine.)
Scientists believe that high relative humidity and strong changes in winds in the lowest parts of the atmosphere are essential to how tornadoes form. Harold Brooks, a research meteorologist with NOAA's National Severe Storms Laboratory in Norman, explained in an April interview that there's a third key ingredient: A downdraft needs to occur in just the right part of the storm. (See: "Tornadoes: The Science Behind the Destruction.")
All in the Family
A twin tornado is produced by a single supercell, says Carbin, as opposed to tornado outbreaks, where multiple tornadoes are associated with separate supercells.
"The twin tornadoes yesterday," he said, "were associated with one parent supercell."
Yet there are several ways to get twins, Carbin says. Sometimes, a new tornado spins up before an old one dies out. During the time they overlap, there are two tornadoes on the ground from the same supercell.
A "satellite" tornado can also form on the periphery of a primary one, after which "it orbits the first tornado," said Carbin. The tornado outbreak in Oklahoma City on May 3, 1999, is an example of this. "Before the F5 tornado moved in, there was a very well-defined satellite tornado," he said. (Scientists rate the strength of tornadoes on a 0-to-5 scale, known as the Enhanced Fujita scale, based on wind speeds and the amount of damage they inflict. F5 tornadoes are the strongest.)
The third way to get twin tornadoes occurs during a particularly violent storm. "There's so much turbulence and rotation going on within a tornado itself that it spins off smaller-scale vortices that are very intense," said Carbin. But, he added, that's probably not what happened yesterday in Pilger.
It's unclear how strong yesterday's twin tornadoes were, Carbin says, though conditions indicate they were fairly mighty. "The atmosphere was incredibly supportive of very strong tornadoes," he said, "and by all accounts, that was a violent tornado."
Officials are still conducting surveys of the damaged areas and hope to have a categorization later today.
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