ENGLISH NYT1112AtornadoA
Twisters, Usually Loners, Came in a Swarm By ANDREW C. REVKIN Tornadoes are generally loners, arising as some particularly potent thunderstorm stirs the air in just the wrong way. But every once in awhile broad layers of air high in the atmosphere, moving in different directions at highway speed or faster, can spawn outbreaks of dozens of twisters over hundreds of miles. That is what happened over the weekend, federal meteorologists said, when cold and warm air collided. The potential for big trouble was evident just before midnight on Saturday, and the first advisory was issued at midnight, calling for caution across a broad stretch of the South and Midwest on Sunday. Meteorologists, using radar and other sensors that can chart the speed of different masses of air, looked at the air at around 18,000 feet over the midsection of the country and saw a powerful belt of frigid air pouring at 110 miles an hour from north Texas all the way to western New York. Beneath it, a broad, warm, moist air mass was shifting north at a relatively languid 50 miles an hour from the Gulf of Mexico to Ohio, said Dan McCarthy, a warning coordinator and meteorologist at the National Storm Prediction Center in Norman, Okla. Where two such layers meet, friction causes intense turbulence that can start cylinders of air spinning Enot unlike Play-Doh can be turned into worms between a child's rubbed palms. If those cylinders stand upright, they can reach toward the earth as raging tornadoes. This took place at least 66 times along the storm front, Mr. McCarthy said, with the three strongest storms apparently registering a 3 out of 5 on the Fujita scale of tornado intensity, with winds of 158 to 206 miles an hour. A final count would have to await more thorough inspections of damage and radar. Outbreaks tend to happen in the late fall and early spring, when clashes of cold and warm air are most likely, scientists said, but can occur in any season. The worst recent outbreak sent 70 tornadoes sweeping through Oklahoma and Kansas in May 1999, killing 44 people. But the most similar tornado swarm to the one that struck on Sunday and early Monday, federal weather officials said, occurred from Nov. 21 to 23, 1992, when 94 tornadoes ripped across 13 states, killing 26 people and causing about $300 million in damage. The worst tornado assault ever recorded in the United States struck April 3 and 4, 1974. In that "super outbreak," 148 twisters struck 13 states, killing 330 people and injuring 5,500 more, weather officials said.