ENGLISH NYT1114HtornadoH Rare Collision Caused a Swarm of Tornadoes By ANDREW C. REVKIN Federal storm scientists yesterday raised the count of tornadoes that struck Sunday and Monday to 88 from 69 and said the tally was likely to continue rising for several weeks as they examined local weather and damage reports. The scientists also mapped out the forces that created the twister outbreak, the worst in November in a decade, and saw in them a distinctive but rare recipe for widespread disaster: a collision between a big pre-winter gush of frigid air and an unusually broad reservoir of residual tropical warmth and moisture flowing from the Gulf of Mexico. Tornadoes are generally loners, arising as some particularly potent thunderstorm spins the air in just the wrong way. But every once in a while broad layers of air high in the atmosphere, moving rapidly in different directions, can lead to outbreaks of dozens of twisters spread over hundreds of miles. T hat is what happened in the latest severe-weather assault, federal meteorologists said. The violent weather unfolded well to the east of Tornado Alley, the swath from Texas and Louisiana to North Dakota where conditions are most frequently primed to produce twisters. Tornado outbreaks are less consistent geographically than individual tornadoes, mainly because the rare confluence of conditions that create them can occur anywhere. The potential for big trouble became evident late Saturday night, and the first federal advisory of 2002 stating that there was a high risk of an outbreak of many tornadoes was issued at midnight. The alert called for caution across a broad stretch of the South and Midwest on Sunday, and ? along with dozens of specific tornado warnings ? undoubtedly kept the death toll from climbing higher than it did, experts said. Meteorologists, using radar and other sensors that can chart the speed of different masses of air, noted a sharp southerly dip and then northerly twist in the jet stream, 18,000 feet up in the atmosphere, that sent a 110-mile-an-hour flow of frigid air from northern Texas all the way to western New York. Beneath that cold flow, warm moist air was shifting north at a relatively leisurely 50 m.p.h. from the gulf toward the Great Lakes, said Dan McCarthy, a warning coordinator and meteorologist at the National Storm Prediction Center in Norman, Okla. A cold front pushing east from the Rockies added power by building the temperature contrast between adjacent air layers. Where two such layers meet, potent thunderstorms can arise, causing huge updrafts that can spin off cylinders of spinning air ? not unlike the way a child's rubbed palms can turn a lump of modeling clay into a worm. Some of those cylinders can reach toward the earth as tornadoes. This took place at least 88 times along the latest storm front, Mr. McCarthy said, with the three strongest storms apparently registering a 3 out of 5 on the Fujita scale of intensity, with winds of 158 to 206 m.p.h. A final count, and a final estimate of the number of extremely damaging storms, must await more thorough inspections of damage and radar, he said. Most tornado swarms occur in the early spring and late fall, when clashes of cold and warm air are most likely, scientists said. But they can occur in any season. The worst tornado assault ever recorded in the United States struck April 3 and 4, 1974. In that outbreak, 148 twisters struck 13 states, killing 330 people and injuring 5,500 more, weather officials said. The tornado swarm most similar to the one that struck on Sunday and Monday, federal weather officials said, occurred Nov. 21 to 23, 1992, when 94 tornadoes ripped across 13 states, killing 26 people. The ability of federal experts to forecast such outbreaks has greatly improved, said Roger M. Wakimoto, a veteran tornado chaser and a professor of atmospheric sciences at the University of California at Los Angeles. Dr. Wakimoto said the specificity of the advanced warning last weekend undoubtedly helped prepare the right communities, while avoiding unnecessary fears elsewhere. "It's important to be right often enough that people pay attention," he said. "They clearly got this one right."