A SUPERCELL THUNDERSTORM WITH HOOK ECHO IN THE SAN-JOAQUIN VALLEY, CALIFORNIA

This study documents a damaging supercell thunderstorm that occurred i n 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 thunders torms with divergent paths developed from an initial echo that had for med just east of the San Francisco Bay region. The southern storm beca me severe as it ingested warmer, moister boundary layer air in the sou th-central San Joaquin Valley. A well-developed hook echo with a 63-dB Z core was observed by a privately owned 5-cm radar as the storm passe d through the Fresno area. Buoyancy parameters and hodograph character istics 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 th e Fresno area). Both the estimated and actual hodographs essentially w ere straight and suggested storm splitting. Although the actual CAFE w as similar to that which was estimated, the observed magnitude of the low-level shear was considerably greater than the estimate. The bulk R ichardson number for the observed conditions (BRN = 21) was well withi n the range observed for supercells elsewhere in the country. Both the estimated and observed 0-3-km storm-relative helicities were in the r ange observed for mesocyclone development. Forecasters familiar with t he relation of buoyancy, shear, and hodograph characteristics to the d evelopment of severe storms could have made useful inferences about th e potential for severe weather in the Central Valley on the basis of s imple modification of the morning OAK sounding.