Tornadoes in Wisconsin, Montana spur early warning system

The scientists knew that cold temperatures in the clouds were important for a tornado to form, but the team found that storms were more likely to be too cold to form tornadoes than earlier modeling studies suggested, says Harold Brooks, a researcher at the National Severe Storms Laboratory in Norman, Okla.

"Cold air is still important," he says. "But it's real easy for storms to be too cold."

Those cloud temperatures could then be factored into decisions on whether to issue tornado watches as a severe thunderstorm approached.

After that trait appeared, "there were some storms that weren't warned on that might have been warned on before," he says.

VORTEX 2, which took place in May and June of 2009, and again in May and June of this year, has also turned up some interesting traits in storms that generated tornadoes. But it's too early to tell whether the common traits are important or merely coincidence, Wurman says.

With the project just ended, "my team is basically spending this week inventorying the many terabytes of data that just my group has. We're making sure we have back-ups, we have all the data, we haven't left something on a thumb drive somewhere," he says.

Questions of which storms are most likely to generate tornadoes have also worked their way into discussions of global warming's potential effect on the formation of severe thunderstorms over the US.

Three years ago, Dr. Del Genio and colleagues at the GISS modeled the potential effect of temperature and moisture increases that could result if carbon dioxide concentrations reach twice preindustrial levels.

The results suggest that while the number of severe storms is unlikely to change, the proportion of the most severe storms could increase.

Last year, a team led by Robert Trapp, a climate scientist at Purdue University, found via a modeling exercise that the number of severe storms is likely to increase in the US under so-called business-as-usual carbon-dioxide emissions scenarios. But the increase varies by region.

Neither study says anything about tornadoes, which appear on scales too small for models to simulate – and based on field studies so far, they grow out of fewer supercell thunderstorms than scientists once thought.

From a climatological perspective, "there is not yet observational evidence that tornadoes have increased in the United States over the 20tH century," Del Genio cautions.

Tornado reports themselves have increased dramatically over that period, he notes, but that spike can be attributed to improved detection and more people moving into areas where tornadoes once spun harmlessly, he says.

Moreover, he says, trends show a decrease over time in the number of the most severe tornadoes.

IN PICTURES: Tornadoes

Related:

• Tornadoes tear through Southern states, but new alerts saved lives
• The chase is on: scientists launch largest-ever study of tornadoes

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