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March-April 2011

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Weather Front

Top-Down View Improves Thunderstorm Forecasts

While long-range forecasting affords outdoor enthusiasts and municipal safety offices a heads up in their planning, it is the short-range forecast, three to four hours out, that really makes a difference in preparing for bad weather. But when it comes to thunderstorms, those forecasts are often unreliable.

“Predicting [thunderstorms] a few hours out is one of the great problems of meteorology,” said Chian-Yi Liu, a postdoctoral researcher at the University of Wisconsin–Madison. “Even major storms can be missed,” he said, citing a 2007 storm that dumped 10 inches of rain and caused major flooding in La Crosse, Wisconsin. “Predictions for the day said a moderate chance of thunderstorms.”

Liu and his colleagues at the Cooperative Institute of Meteorological Satellite Studies (CIMSS) at UW–Madison said they can improve short-range forecasting by adding data on conditions at 15,000–32,000 feet of altitude. Traditionally, the major source of forecast data comes from surface or balloon observations.

The researchers collected data on 400 different events from sensors on NASA's Aqua satellite that measure conditions at different altitudes. “Our analysis shows that if there is instability at around 30,000 feet, with other storm-favorable conditions, a convective storm will develop in the following three to five hours,” said Liu. “Using the top-down view of a satellite reverses our usual way of thinking about convective storms, and may suggest an explanation for storms that arise when they would not be predicted using conventional methods.”

“For a long time, we have looked at convection and instability from a near-surface perspective,” said colleague Steve Ackerman, a professor of meteorology and director of CIMSS. “What Chian-Yi has showed is that this is not always the case, you can drive instability from upper troposphere too.”

KIMBRA CUTLIP is a freelance writer and former assistant editor for Weatherwise.

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