NOTICE: The project that is the subject of this summary was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the group responsible for the planning of the workshop were chosen for their special competences and with regard for appropriate balance.

Support for this project was provided by the National Science Foundation and the National Aeronautics and Space Administration under Grant No. ATM-0135923, the U.S. Environmental Protection Agency under Grant No. X-82875501, and the National Oceanic and Atmospheric Administration under Contract No. 50-DGNA-1-90024. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect those of the sponsors or their subagencies.

Cover: Tornado near Verden, Oklahoma, as it progressed on to Oklahoma City, May 3, 1999. Copyright by Howard B.Bluestein.