A second wave of deadly tornadoes and thunderstorms that ripped through Oklahoma Friday night is not related to the colossal twister that tore through the same region less than two weeks ago, scientists say.
Unlike earthquakes, which are often followed by aftershocks, the storms that birth tornadoes are independent of one another.
"By pure chance, we had two separate weather systems, both in very close locations and that ended up looking quite similar," said Christopher Karstens, a research scientist with National Oceanic and Atmospheric Administration's (NOAA) National Severe Storms Laboratory (NSSL) in Norman, Oklahoma.
In addition to spawning multiple tornadoes, Friday night's severe weather triggered a flash flood and hail the size of grapefruits. At least 5 people, including a woman and her baby, are reported killed. (Related: "A Tornado Chaser Talks About His Science and Craft.")
This week's tornadoes follow closely upon another devastating twister that touched down near the city of Moore, Oklahoma on May 20, which reduced entire neighborhoods to rubble and killed 24 people.
What makes Oklahoma so tornado-prone? The state lies within an area of the Great Plains known as Tornado Alley, a region that stretches from South Dakota to central Texas. (Related: "Is There a Tornado/Climate Change Connection?")
Tornado Alley occupies a unique geographic position where warm humid air from the Gulf of Mexico, hot dry air from Arizona and New Mexico, and cool dry air from Canada meet, explained NSSL's Karstens.
"In the springtime, those air masses tend to work together to create environments that we saw [on May 20 and on Friday]," he added.
"Sometimes they collide in Oklahoma, sometimes in Texas, and sometimes in Kansas."
While the United States has perhaps the best historical records for tornadoes, twisters also occur elsewhere, including in Italy, India, and South America. (Related: "Lessons From Joplin's Recovery")
Another area with similar conditions to Tornado Alley is Bangladesh, said Chris Weiss, an atmospheric scientist at Texas Tech University.
"They have a lot of violent tornadoes - some would argue even stronger [storms] and tornadoes - over there," Weiss said. "But a lot go unreported because they don't have nearly the observation network over there that we have over here."
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]."
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, even though residents of Moore had advanced warning that a potentially dangerous storm was moving in, they had only 16 minutes after the first warning on May 20 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."
Secondly, while current computer models can predict when a supercell storm is likely to form, not all supercell storms give rise to tornadoes.
"That's one of the questions we're struggling with as scientists: which storms will be the ones to go on producing tornadoes and which ones won't?" Karstens said.
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 researchers.
"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.