During the past several years, researchers have uncovered a tremendous amount of information regarding severe thunderstorm structure and behavior. New theories regarding thunderstorm formation and tornado development have been presented. Storm-intercept teams have correlated these theories with observed visual features. Our current understanding of the thunderstorm is markedly more complete than it was just ten years ago. With this handbook and the Advanced Spotter Training Slide Set which was released a few years ago, the time has come to pass this new understanding on to you, the spotter. Only by providing fresh training material can the NWS expect to maintain what has become a very important group of observers.

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