BOSTON — AS spring unfolds across North America, tornadoes once again are in the news. It's a reminder that the United States is the severe-storm capital of the world.
Describing this status recently in the Bulletin of the American Meteorological Society, Elbert W. Friday Jr., National Weather Service director, observed that ``the United States experiences more severe local storms and flooding than any other country in the world.'' He added that a typical year brings ``some 10,000 violent thunderstorms, 5,000 floods, and 1,000 tornadoes.''
Yet the country is not helpless before this onslaught. Thanks to advances in meteorological knowledge and in the forecast and warning system, the tornado death rate, for example, has been cut in half in recent decades. It's down from nearly 2,000 per decade 60 years ago to less than 1,000 per decade today.
Now the weather service is poised for what Mr. Friday calls ``a meteorological revolution.'' Sharp-eyed new radars, more vigilant weather satellites, and computerized-information - handling will bring what he calls ``dramatic improvements in ... forecasts and ... detection of and warnings for severe weather.'' This is particularly true for tornadoes.
These funnel-shaped circulations develop in association with severe thunderstorms. As the National Center for Atmospheric Research (NCAR) in Boulder, Colo., explains, the funnels often form at the thundercloud base. But tornado spotters also have to watch what's happening on the ground. The first clue may be swirling dust and debris.
Of the 710 to 1,100 tornadoes reported annually in the US, about 79 percent are what the American Meteorological Society calls ``weak.'' About 20 percent are ``strong.'' About 1 percent are ``violent.''
Weak funnels last under 10 minutes and have wind speeds on the order of 110 miles per hour (m.p.h.). They leave ground tracks less than a mile long and 100 yards wide. Although called ``weak,'' they are potentially dangerous, while their short lifetimes make timely warnings difficult.
Strong tornadoes last from 10 minutes to more than two hours. Maximum winds, as estimated from damage surveys, range up to 280 m.p.h. or higher. A single thunderstorm cell may produce these powerful tornadoes in cycles. Each such sequence may last for tens of minutes. It can leave damage trails over 100 miles long by 1,000 yards wide.
Tornadoes have touched down throughout North America in every month of the year. But NCAR notes that they occur predominantly over the Great Plains and Midwest and are common in Eastern states and the Gulf of Mexico coast. Their region of most frequent occurrence begins near the Gulf Coast in March and shifts toward Kansas, Iowa, and Nebraska by May and June.
A weather satellite launched April 13 will help forecasters monitor this tornado ``season''. The $220 million GOES-8 (Geostationary Operational Environmental Satellite), the first of five improved weather ``eyes,'' can pinpoint storms to within 1.2 miles, compared with 6.2 to 12.4 miles for the old system.
A new class of radars is also part of the meteorological ``revolution.'' Unlike their predecessors, they sense motion of clouds, rain, and wind-borne debris. There will be 150 such radar sites. The National Weather Service will have 121. The Federal Aviation Agency and the Department of Defense will operate the other installations and share data with the Weather Service.
Preliminary experience at six sites shows ``dramatically'' improved ability to detect and track severe storms and tornadoes. Paul D. Polger and several colleagues at Weather Service headquarters in Silver Spring, Md., recently reported in the American Meteorological Society Bulletin that lead times for tornado detection increased by four minutes. Also, the fraction of unwarned tornadoes dropped from 33 percent to 13 percent of severe-strom events when compared with 1989 through 1991.
The new radar system ``is proving to be the most significant improvement to [thunderstorm and tornado] prediction and tracking since the introduction of weather satellites,'' they reported. To which the American Meteorological Society adds, ``The success of the forecast and warning system ... also hinges on the ability of the public to respond quickly and properly to the information that is presented.''