Weather experts studying the May 22 Joplin tornado report its 205 mph winds and mile-wide path were caused by the unlikely merger of small and large storm cells in the region.
Bill Davis, chief meteorologist with the National Weather Service station in nearby Springfield, Mo., said the consolidation "added steroids to this storm," turning it into a monster.
"We could have had an EF-2 or an EF-3 tornado without the merger," he said. "But something happened and it had to fit perfectly to produce this EF-5. The odds of that happening are astronomical."
The storm destroyed one-third of Joplin, killing 159 people, the most in a U.S. tornado in six decades. Damage has been estimated at $3 billion.
Davis and other meteorologists at the Weather Service and the national Storm Prediction Center in Norman, Okla., said developing weather patterns near Joplin before the EF-5 tornado formed indicated a much smaller twister.
Then suddenly, they said, a large storm cell blew in from Southeast Kansas and merged with a smaller storm cell from Northeast Oklahoma to enlarge the force and size of the tornado headed toward Joplin, located in the southwest corner of Missouri that borders on Oklahoma, Kansas and Arkansas.
"This was a tornado that was pieced together from several circulation mergers to make the perfect storm," said Davis.
Tornado warnings about the ferocity of the storm were sent out immediately, giving Joplin residents about a half-hour to get ready for its impact at 5:33 p.m. on May 22.
Two days before that, computer modeling by the Storm Prediction Center suggested there was a 30 percent chance or slight risk of severe thunderstorms in the Joplin area that day.
Still, there were some early signs, the meteorologists said.
They said the upper-level jet stream in the region was farther south than it should have been for late May, and the boundary between rain and dry conditions had shifted unexpectedly to the east.
Yet, said Greg Carbin, the storm center's chief meteorologist, "it was a typical day in May for widespread severe weather. It was what you would expect. Nothing really stood out."
He said that made it different than the series of tornadoes that devastated Alabama and other southern states one month earlier.
"We knew a tornado outbreak would happen (in the south) on April 27 with large, long-tracked and violent tornadoes," said Carbin. "The forecast for Joplin was much more ambiguous. It was not considered an outbreak day."
Bill Gallus, a professor of meteorology at Iowa State University, said the Joplin tornado was not like any other EF-5 twister because it contained an outer boundry of rain that befuddled forecasters.
"What you had was a bunch of storms come together," he said. "It did not look like a classic storm that would produce an EF-5. With most EF-5s, you see them coming."
A weather observer in nearby Commerce, Okla., spotted the developing monster tornado headed toward Joplin.
"There was not an actual funnel, but you could see the clouds rotating extremely fast up above," said Ernie Shelby, Commerce's emergency management director. "It wasn't but a few minutes after that we heard that Joplin had been hit."
Details for this story were provided by the Joplin, Mo., Globe.