 By LEE BOWMAN Scripps Howard News Service June 12, 2005
Researchers analyzed reports of nearly 15,000 tornados that occurred in the continental United States between 1986 and 1999. The date Doppler was installed at each of 121 weather service forecast offices was used to divide the sample for comparison. Weather Surveillance Radar-1988, also called NEXRAD (Next Generation Radar), was installed during the late 1980s and early 1990s as part of the weather service's $4.5 billion modernization program. The radar measures the shift in the signal from radio waves bounced back to it from precipitation in clouds to measure how fast winds in a storm are moving, and in what direction. In the same way that 19th-century Austrian scientist Christian Doppler noted a shift in sound-wave frequency as an object like a locomotive moves closer, and then away, the frequency of radio waves coming back to radar antenna increases as objects like rain bands move toward it, and decreases as objects move away. This ability to "see" the wind inside a storm allows meteorologists to detect tornado formation and quickly issue warnings, often before the funnel clouds reach the ground. The review was carried out by Kevin Simmons of Austin College in Sherman, Texas, and Daniel Sutter at the University of Oklahoma in Norman. It found that, over all, the percentage of tornados for which warnings were issued almost doubled - from 35 percent before the radar was installed to 60 percent after the system was in place.
Performance was particularly improved for warnings of storms rated at F3 (winds of 158 mph) or higher. After the new radars were installed, warnings were sounded for 85 percent of those most dangerous storms, with an average lead time of about 15 minutes. In addition, the average advance warning for all tornados increased by more than four minutes, from 5.3 to 9.5 minutes before a twister reached a warned area. "Our study provides strong evidence that this investment has had a significant effect on reducing injuries and fatalities from these storms," Simmons said. The results were published in the June edition of Weather and Forecasting, a publication of the American Meteorological Society. Every year, an average of 1,000 tornados kill about 80 Americans, injure 1,500 others and cause more than $400 million in damage. Without Doppler, these numbers would have been much higher. Considered nature's most violent storms, the strongest tornados can pack winds in excess of 250 mph, and spread damage along a path up to a mile wide and 50 miles long. Although they're often considered a phenomena of the country's midsection, twisters can strike virtually anywhere in the country, in any month, although activity tends to peak between March and July in most years. National Weather Service offices issue about 3,000 tornado warnings a year. The average warning lead time for all twisters has continued to increase since the study period ended. In 2004 it was 15 minutes for all tornados. Simmons and Sutter also found that expected fatalities and expected injuries were 45 percent and 40 percent lower, respectively, for twisters that occurred after NEXRAD came online at a local weather office. The research found that expected casualties were significantly lower for tornados that happened during the day and evening, as opposed to late at night, when people seldom have radio or television on to hear broadcast warnings. The researchers said this gives further indirect evidence of the life-saving effects of improved tornado warnings.
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