Word: storm
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Dates: during 1990-1999
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...collaboration between radar developers and storm chasers was immensely productive. It led to the NEXRAD (or Next Generation Radar) system, which the National Weather Service is currently installing nationwide. Already NEXRAD has helped extend the lead time for tornado warnings from three to eight minutes, on average. Sometimes the warning comes even earlier. Last month weather forecasters in Little Rock, Arkansas, called a tornado warning for communities in the Ozark Mountains a full 35 minutes before the twister showed up, giving people who lived in trailer homes time to scurry to friends' basements for safety...
Good as it is, however, the NEXRAD system has not changed the rate of false alarms: 50% to 70% of the warnings forecasters issue are not followed by tornadoes. Why does one threatening-looking storm produce a tornado while others seemingly just like it do not? This question has dogged tornado experts for years, and the VORTEX project was launched to answer it. First in 1994 and again in 1995, VORTEX brought dozens of meteorologists to Oklahoma, Texas and Kansas during May and June--peak tornado season in that part of the country. Every few days for nearly 10 weeks...
...nozzle. To the eye, this is exactly what appears to be happening. But while scientists agree that the updraft is essential, many doubt that it provides the sole mechanism for tornado formation. Some scientists think the rapid sinking of colder, dryer air near the rear flank of the storm may be key. If this rush of air encounters wind shear on its way down, then it too will start to rotate. In this scenario, a tornado occurs as air from the downdraft nears the ground, swoops out horizontally and--attracted by the zone of low pressure created by the mesocyclone...
...another possibility, says UCLA meteorologist Roger Wakimoto, is that the tornadoes typical of supercell storms are formed by the same mechanism that creates the smaller, less destructive funnel clouds known as waterspouts, landspouts and dust devils. These twisters all build their vortexes not from the clouds down but from the ground up. They are triggered, Wakimoto says, by low-lying eddies of air that are perturbed by a fast-moving front or some other local disturbance. When a supercell storm passes over such an area, swirling air near the ground could easily be sucked into the updraft and spun...
Until VORTEX, the competing hypotheses about tornado formation could not be rigorously tested. The downdraft theory, for example, was bolstered by storm chasers' sightings. Observes Erik Rasmussen, field coordinator for VORTEX: "What storm chasers see first is a big dark cloud, then a bright spiral slicing into the base." The problem is that the flow of air within a big storm is so complex that what the eye sees cannot always be trusted. Hence the need for measurements...