What if we could predict tornadoes a month out? Scientists make strides.

The team analyzed tornado data gathered between 1979 and 2010 and identified 10 atmospheric features as potential candidates for their forecasting approach.

After some sifting, the researchers found two that appeared mostly closely tied to tornado formation. One is the intensity of rainfall in storms with strong updrafts, and the other is the amount of wind shear available to impart spin to the storm.

The team used those observed features to develop a monthly index of tornado activity. To test the index's predictive power, they replaced the index's observed values with values as forecast by the National Weather Service's latest climate-forecast model. Then the researchers compared the index-based forecast of tornado activity with the monthly averages observed over the 1979-2010 period.

For every month but September and October, the team's tornado forecast tool showed statistically significant skill at reproducing the number of tornadoes in a given month and the regions of the US where they occurred.

The team's model focuses on conditions affecting supercell thunderstorms – the vast, stand-alone thunderstorms that pop up in the Plains and Midwest in late spring and early summer. The model doesn't do as good of a job of reproducing tornado numbers in areas where the twisters are associated with storm fronts, such as the twisters that hit the South this week.

Still, other tornado researchers say the Tippett team's results are encouraging.

The approach, detailed in a recent issue of the journal Geophysical Review Letters, represents "a solid contribution" to tornado climatology, writes Grady Dixon, a tornado researcher at Mississippi State University, in an e-mail exchange. "The potential usefulness is obvious."  

Indeed, over the past several months, Tippett's group – along with Brooks, researchers from Purdue University, and representatives from the National Oceanic and Atmospheric Administration's Climate Prediction Center – have been holding conversations aimed at moving toward longer-term forecasts of conditions that favor tornado formation.

One key element that is yet to be resolved: What aspect of tornado activity should be the forecast's focus?

From a public-safety standpoint, monthly tornado counts don't tell the whole story, Dr. Dixon explains. Tornadoes in late winter or early spring often affect more areas than do late-spring or summer twisters. This is because the late-winter, early-spring tornadoes tend to be embedded in fast-moving storm systems.

Thus, should a forecast focus on numbers, or expected length of tracks? Brooks notes that these issues remain to be resolved.

As for the the new model's acknowledged weak spots, "even incorrect predictions by this model can be learning opportunities for researchers as we move forward," Dixon says.

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