Tornado Destruction
© Wikimedia Commons
Destruction from a tornado in Joplin, Mo., May 23, 2011.
The unusually large number of severe tornadoes this year may be a sign of large scale changes in the jet stream.

John Harrington Jr., a professor of geography at Kansas State University, notes that severe tornadoes are not unheard of historically. But when the events happen frequently such as the the destruction of Joplin, Mo., the outbreak of multiple tornadoes in Alabama, and yesterday's northeast outbreak in Massachusetts, it becomes a warning that there are changes afoot in the jet stream.

"The fact that this is happening all in one year and in a relatively short time frame is unusual," Harrington said in a Kansas State press release.

The jet stream in the upper atmosphere flows from west to east and tends to meander over the southern states during the winter and the northern states in the summer. Tornadoes tend to strike most during the spring and fall shifts of the jet stream.

Thunder Storm
© Wikimedia Commons
NOAA satellite image of a thunder storm minutes before a large tornado formed over Joplin, Missouri, May 22, 2011.
"We have these good historical precedents for specific synoptic [large-scale] events, but they're starting to come more frequently together. That's what is very interesting, is that this weather system seems to be getting more variable," Harrington said.

For a tornado to occur in the Great Plains, humid air from the Gulf of Mexico must combine with the right jet stream patterns, said Harrington. Uplift from the jet stream helps create towering thunderstorm clouds.

Then low altitude winds from the south, mid-level winds from the southwest, and high altitude northwest winds start the weather system rotating.

A rotating thunderstorm cloud can mean tornadoes, but they are hard to predict.

Scientist use observations of the jet stream to predict the likelihood of tornadoes, though they can't predict specifically when or where they will hit.

"Unfortunately in terms of death and destruction, we've had too many of those events this year," Harrington said.