On Upper Tropospheric Kinematics and Severe Weather Occurrence

Abstract

Historically, tornadoes have not been a meteorological concern to California. There has never been a tornado-related fatality recorded in the state, and those tornadoes that do occur are believed to be of the weak variety. Upon closer examination of the tornado data it is found that there is a small region in the highly populated Los Angeles area that has a tornado incidence disproportionately higher than the rest of Cali-fornia. This coastal zone is about 75 km long by 35 km wide. It is hypothesized that this region is a favorable location for tornadoes because the shape of the coastline and the inland location of the mountains enchance the low-level convergence field. The tornado occurrences all seem to have very similar synoptic patterns. Low centers extending from the surface to 500 mb are located off the coast of California northwest of Los Angeles. In all cases, a very strong upper-tropospheric jet is located along the California/ Mexico border. Many of the tornadoes occurred several hours after the passage of a cold front when the lower troposphere was destabilized and the low-level moisture increased as a result of the long over-water trajectory of the air mass. Several other tornado cases occurred at approximately the same time that cold fronts passed the area. A case study of the 9 November 1982 tornado outbreak is presented. During this particular episode, there were a record-setting seven tornadoes , all of which occurred in or near the tornado-prone zone.