Composite analysis of severe weather outbreaks

Severe weather outbreaks affect many parts of the United States, generally east of the Rocky Mountains. These outbreaks are classified into two main types, a tornadic severe weather outbreak and a non-tornadic severe weather outbreak. A set of 50 tornado outbreak cases and 50 non-tornadic severe weather outbreak cases was obtained. These cases were analyzed 24 hours, 48 hours, and 72 hours prior to the outbreak. The data used in the study were NCEP/NCAR reanalysis data valid over a domain that encompasses the entire United States. Five variables were considered, including height, temperature, the u-wind component, the v-wind component, and relative humidity. Convergence of longitude lines with increasing latitude led to an interpolation of these data onto a Fibonacci grid, which was equally spaced. This eliminated artificially higher correlation values at higher latitudes due to the distance between gridpoints. Composites of each outbreak type using a T-mode principal component analysis were created from the cases at all 17 vertical NCEP/NCAR reanalysis levels. A cluster analysis was performed on the resulting principal component loadings to determine different storm types. Two main storm types were revealed for each outbreak type at 48 hours and 72 hours, and for severe weather outbreaks at 24 hours. For tornado outbreaks at 24 hours, three main storm types are obtained. Many synoptic features expected in each outbreak type are seen from the different storm types.