sealPurdue News

March 21, 1997

Radar provides 'signature' of unexpected twisters

WEST LAFAYETTE, Ind. -- By studying the radar pattern of two past tornadoes, a Purdue University expert on severe storms has identified another specific pattern that can alert meteorologists that a tornado may occur.

Ernest M. Agee's study of two Indiana tornadoes -- one of which occurred without warning -- has helped the National Weather Service develop more effective ways of monitoring certain types of thunderstorm activity to better recognize conditions that might lead to a tornado.

Agee, professor of atmospheric sciences who has studied tornadoes for 30 years, says Doppler radar systems, first developed for weather forecasting in 1989, now are widely used to provide scientists with new information on how tornadoes form.

"Though full details on how tornadoes form are not known, information from new-generation Doppler systems is exposing the unique and interesting ways that tornadoes and severe weather can develop," Agee says.

He and his colleagues used records from a Doppler system to track weather conditions leading to two 1994 tornadoes. Thunderstorm activity in both areas was generated from isolated storms, which seldom produce tornadoes. But Doppler records showed that additional storms to the southwest had generated wind gusts strong enough to cause the two storms to merge.

Initially, the storms appeared on the radar as two separate incidents, in patterns recognized by meteorologists as a hook-echo and a bow-echo pattern. Once they merged, the storms created an "S-shaped" pattern on the radar.

The S-pattern is now widely recognized as a signal of the threat of severe storm activity and the possible need to issue tornado warnings, Agee says.

"Though meteorologists have long recognized that isolated storm cells moving ahead of other storm lines can produce tornadoes, they didn't fully understand the mechanisms behind these interactions," he says. "By identifying these interactions on a radar, we can better understand how such storms occur, and use the signature "S" shape to recognize areas where a tornado may occur."

The tornado "season" typically runs from March to July in most areas, with April and May usually producing the most tornadoes.

"Spring is a prime time for tornadoes because the contrast in weather systems can be so great," Agee says. "When cold, dry air aloft and warm, moist air at the surface are accompanied by strong winds, lines of thunderstorms can form that are capable of spawning tornadoes."

The warmer temperatures during the past month throughout much of the Midwest could slightly tip the scales in favor of tornadoes, he adds.

"For a tornado to occur, data suggest that ground temperatures at the four-inch level must be at least 45 degrees Fahrenheit and the surface dew point must be 55 F or higher," Agee says. "This year, if ground temperatures are warmer than usual, the chances for tornado activity early in the season may increase."

The highest number of tornadoes recorded for Indiana is 48 in 1973, and the annual average is 21.

Agee advises people to take precautions during severe thunderstorms, especially during tornado season.

"Most people don't think of thunderstorms as dangerous, but the thunderstorm is the parent to tornadoes," he says. "A single thunderstorm can give birth to a family of tornadoes over its lifetime. As many as seven different tornadoes have been documented from a single thunderstorm."

Agee adds that tornadoes can occur at any time during the year, and people should take precautions to protect themselves during violent storms.

"What is most important is to have a plan of action," Agee says. "Tornadoes can strike as a thunderstorm passes to the north of your location, and they sometimes occur without precipitation, and maybe even during sunshine."

Basements or interior rooms on a ground floor are generally the safest places during a tornado or severe thunderstorm. Agee points out that most danger in such a situation is from flying debris, and people should take refuge away from windows or glass doors, preferably covering themselves with blankets and taking shelter under a desk or furniture.

Source: Ernest Agee, (765) 494-3282; e-mail,
Writer: Susan Gaidos, (765) 494-2081; e-mail,
Purdue News Service: (765) 494-2096; e-mail,

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