On the Incidence of Tornadoes in California

Abstract

Climatological analyses of tornado occurrence in the state of California for the period 1950-1992 are presented. In constructing these analyses, the official historical record of California tornadoes was supplemented and corrected with tornado reports from other sources. In corroboration of the results of the few previous studies of California tornadoes, the distribution of tornadic events across the state is found to be very uneven; in particular, a relatively small area of south-coastal California has an incidence of tornadoes (per unit area per unit time) comparable to regions within the midwestern United States. Other subregions of the state with an enhanced incidence of tornadoes are also identified; these include a large portion of the Central Valley (which comprises the Sacramento and San Joaquin Valleys), the north-central coastal region (including the San Francisco and Monterey Bay areas), and a part of the vast southeast desert region. Annual and diurnal distributions of tornadoes in each of these areas are examined. Tornadoes in the southeast desert region are found to occur primarily during the warm season, while those in the other three identified subregions occur primarily during the cool season. Peak incidence generally occurs during the afternoon, though the diurnal distribution is complex in the two coastal regions. The average tornado in California is weaker and has a shorter path width and pathlength than the average tornado in the contiguous United States; however, the preferential occurrence of tornadoes in areas of California that are moderately-to-densely populated makes them a source of significant concern.