When clouds billow and skies darken over the gentle undulations of central Texas in late May, the only thing residents usually have to contend with is heavy rain. In a typical year, the spring tornado season has passed.
But for survivors picking up the pieces from Tuesday's deadly tornado outbreak, 1997 will not go down as a typical year.
The outbreak, which began near Waco and continued through early evening into the Austin area, claimed at least 27 lives. Jarrell, a town of 1,000 about 40 miles north of Austin, was hit hardest, with the debris so scattered that officials were having trouble yesterday compiling a death toll. In addition to the dead, as many as two dozen town residents were missing.
The National Weather Service was able to give 20 minutes' warning of the impending twisters. But as the death toll mounts from the 1997 tornado season, researchers are poring over new research in hope of providing even longer warning times.
But understanding these storms is work that takes years - and the latest data from forecasting offices and from two years of fieldwork are raising more questions than they are answering.
Roger Wakimoto, professor of meteorology at the University of California at Los Angeles, says the data suggest more research is needed on what is happening during a storm within a mile above the ground. Only 30 percent to 50 percent of all rotating thunderstorms produce tornadoes, he notes, a number that researchers keep reducing as they collect data from the National Weather Service's growing network of advanced Doppler radar. The radar reveals the inner motions of more storms.
The Texas disaster also reminds Americans of the nation's dubious distinction as tornado capital of the world. America's unique geography provides the right mix of twister-making ingredients: a source of warm, moist air moving up from the south in spring, and mountain ranges to the west that lift and cool the eastward-flowing air as it hurdles the Rockies. When the two air masses meet in midcontinent, they spawn conditions that can turn thunderstorms into violent breeding grounds for tornadoes.
Twisters can occur in any season, but they are most common in from early spring to late summer. By May and June, the "season" has moved north from the Gulf states to reach its peak in Kansas, Iowa, and Nebraska.
Outside the normal pattern
Tuesday's storms were "outside the normal pattern of tornado formation," notes David Imy, operations branch chief at the National Weather Service's Storm Prediction Center in Norman, Okla. Typically, tornado-bearing storms move generally northeast. "These thunderstorms formed in central Texas and moved southwestward towards Austin," he says, as a slow-moving cold front bulldozed its way into a mass of warm, moist air pumping up from the Gulf of Mexico.
Yet if the general weather pattern was unusual for tornados, once the storms took shape, they began to develop familiar signs that tornadoes could be in the offing, Mr. Imy says. As forecasters watched their radar screens and the storms developed, they saw echoes from wind-circulation patterns that can lead to tornado formation.
Typically at this time of year, thunderstorms would prompt severe thunderstorm watches for communities in the front's path. But on the basis of the radar returns, forecasters opted for tornado watches instead. As the circulation patterns strengthened, "they showed up so well that we could give up to 20 minutes' warning time," Imy says.
Understanding how tornado-spawning thunderstorms develop "will be the next big push" for researchers, says Morris Weisman, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colo. The idea, he says, is to forecast hours ahead where and when a potential tornadic storm will develop. "We'd like to be able to say: 'Within an hour, a storm will develop at this location with these characteristics,' " he adds.