The authors thank Dr. T. Hamill of NCAR and two anonymous referees for their insightful reviews. We also thank Dr. A. Simmons of ECMWF for his useful comments on an early version of the manuscript. M. Baldwin of the NOAA Storm Prediction Center provided access to the RFC data. M. Wandishin of the NOAA National Severe Storm Laboratory supplied the long-term climatic frequencies. The first author was supported by ATM9612487, ONR N00014-98-0551, and ONR N00014-99-1-0181.
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