A massive tornado that tore through the Oklahoma City suburb of Moore on Monday and killed scores of people was unusual not for its size or ferocity, but for the path it took, experts say.
"Strong tornadoes occur pretty much every year," said Christopher Karstens, a research scientist with the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma. "But most of them occur out in open flat areas, or areas where there aren't many people. It's rare that we get one of these going through a major populated area."
Scientists estimate that the devastating twister was half a mile (0.8 kilometers) wide and generated winds of up to 200 miles per hour (320 kilometers per hour).
During its 40-minute tour of destruction, Monday's tornado carved a 20-mile path through Newcastle, Moore, and South Oklahoma City, reducing entire neighborhoods and at least one elementary school to rubble. The official death toll on Tuesday morning stood in the 20s.
According to the National Weather Service, Monday's tornado was an EF5 on the enhanced Fujita scale—the most powerful type of twister.
"This is absolute devastation like nothing I've ever seen before," Betsy Randolph, with Oklahoma State Police, told Fox 25.
"This may be worse than the May 3, 1999, tornado," said Randolph, referring to a similarly strong tornado that affected some of the same communities.
Oklahoma City lies within an area of the Great Plains known as Tornado Alley, a region that stretches from South Dakota to central Texas and is particularly vulnerable to tornadoes.
Tornado Alley occupies a unique geographic position where warm moist air from the Gulf of Mexico and cool dry air from the Continental Polar Mass in Canada meet, explained NSSL's Karstens.
"In the springtime, those air masses tend to work together to create environments that we saw [on Monday]," he added.
While the United States has perhaps the best historical records for tornadoes, twisters also occur elsewhere, including in Italy, India, and South America. (See photos of extreme weather.)
What Tornadoes Have in Common
While tornadoes can differ in their size, strength, and location, they all share certain characteristics. They are spawned from a type of rotating storm called a supercell thunderstorm.
And they are all driven by atmospheric instability and by a phenomenon known as wind shear. This happens when "wind near the ground blows in one direction, but aloft it blows in another direction. This creates shear in the airflow," Karstens explained. "If you produce an updraft within that flow, the updraft will acquire the properties of the air, and the atmosphere begins to spin and rotate."
While scientists understand some of the basic setup conditions necessary for tornado formation, there are still many fundamental questions about tornadoes that remain unanswered.
Tim Samaras, a tornado chaser known for for getting instruments inside tornadoes to measure pressure and wind speeds, says we have a lot to learn about how tornadoes form.
"We still don't know why some thunderstorms create tornadoes while others don't," he said. "We're trying to collect as many observations as possible, both from outside and from the inside [of tornadoes]." (Related: "A Tornado Chaser Talks About His Science and Craft.")
Scientists also have a limited understanding about how tornadoes maintain their intensities and what causes them to fizzle out, Karstens said.
At the moment, tornadoes are much more difficult to forecast than hurricanes. For example, the National Hurricane Center was able to predict the path of last year's Hurricane Sandy with startling accuracy a full five days before it made landfall.
In contrast, residents of Moore had only 16 minutes after the first warning before the tornado touched down.
Part of the difficulty, Karstens said, is that tornadoes are much smaller than hurricanes.
"It's really a matter of scale," he explained. "With the hurricane being so large, we're able to populate our models with lots of points to resolve it and we can come up with much more accurate multiday forecasts."
Karstens is involved in an NSSL project that aims to predict a tornado's path shortly after it forms, called Warn-on-Forecast.
He's optimistic that tornado forecasting will improve as computers and tornado modeling software become more powerful, and as more environmental data such as temperature and dew point measurements are gathered close to tornado-spawning storms by instruments and tornado chasers.
"We've got a long way to go," he said, "but I think we're making steady progress."
Jane J. Lee contributed reporting to this article.