Ordinarily, Dr. Tippett spends his time developing or improving ways to make extended-range forecasts of tropical cyclones, or swings in natural climate cycles such as El Niño or the Arctic Oscillation. But that changed last April, when the US experienced its worst tornado outbreak on record. The three-day outbreak from April 25 to 28 spawned 359 tornadoes in 21 states, including four tornadoes that reached EF5, the most destructive category. The outbreak and the thunderstorms that spawned them inflicted at least $11 billion in damage and killed 322 people.
At the time, Tippett says, he noted that forecasters at the Storm Prediction Center in Norman, Okla., "had identified large regions where they thought there was going to be trouble, maybe four or five days in advance."
That implied the presence of large-scale, predictable features in the atmosphere that favor the formation of severe storms.
Researchers have applied the same general concept to produce seasonal hurricane forecasts. Tippett says it dawned on him that key atmospheric features also may encourage tornado-spawning storms to form.
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.