A Supercell Thunderstorm with Hook Echo in the San Joaquin Valley, California

Abstract

This study documents a damaging supercell thunderstorm that occurred in California's San Joaquin Valley on 5 March 1994. The storm formed in a “cold sector” environment similar to that documented for several other recent Sacramento Valley severe thunderstorm events. Analyses of hourly subsynoptic surface and radar data suggested that two thunderstorms with divergent paths developed from an initial echo that had formed just east of the San Francisco Bay region. The southern storm became severe as it ingested warmer, moister boundary layer air in the south-central San Joaquin Valley. A well-developed hook echo with a 63-dBZ core was observed by a privately owned 5-cm radar as the storm passed through the Fresno area. Buoyancy parameters and hodograph characteristics were obtained both for estimated conditions for Fresno [on the basis of a modified morning Oakland (OAK) sounding] and for the actual storm environment (on the basis of a radiosonde launched from Lemoore Naval Air Station at about the time of the storm's passage through the Fresno area). Both the estimated and actual hodographs essentially were straight and suggested storm splitting. Although the actual CAPE was similar to that which was estimated, the observed magnitude of the low-level shear was considerably greater than the estimate. The bulk Richardson number for the observed conditions (BRN = 21) was well within the range observed for supercells elsewhere in the country. Both the estimated and observed 0–3-km storm-relative helicities were in the range observed for mesocyclone development. Forecasters familiar with the relation of buoyancy, shear, and hodograph characteristics to the development of severe storms could have made useful inferences about the potential for severe weather in the Central Valley on the basis of simple modification of the morning OAK sounding.