Summer Thunderstorms Over Southern California

Abstract

Ten-day fire weather records from forestry lookouts in the southern California mountains are used to describe summer thunderstorm activity. The distribution of thunderstorms during a typical season closely resembles rainy-day frequencies for summer precipitation in Arizona, thereby supporting the concept that most southern California summer thunderstorms are caused by a westward extension of the same circulation that brings Arizona its “monsoon” rains.Thunderstorms are also induced by dissipating eastern North Pacific tropical cyclones that move up the west coast of Mexico. Tropical storm tracks, local National Weather Service reports, and fire weather records are used to show which occurrences of thunderstorms can be credited to this tropical cyclone activity and the attendant influx of upper level moisture.When the data from the summers of 1947–68 are divided into two 11-yr periods using 1957/58 as the break point, a significant increase in thunderstorms and tropical cyclones affecting southern California is noted during the latter period.