Climatology of California Severe Weather: Population Bias or Geographic/Climatic Influences?

The present study looks at the climatology of severe convective weather events for the years 1974-1998. The authors use 25 years of Storm Data information to map the locations of severe weather events, including tornadoes, hail, waterspouts, funnel clouds, damaging winds, lightning and flash floods. Their frequencies and spatial and temporal distributions indicate that there are more reported events in populated regions of the state. There is also a marked drop in frequencies at night, when events could not be easily seen. These biases have also been seen in other parts of the country. However, certain patterns in severe weather distributions indicate that both geographic and synoptic factors also play a major part. During the cold season, deep upper-level lows move down from the Gulf of Alaska towards the California coast. These result in storm systems that bring heavy rains and severe weather phenomena in coastal plains and windward valleys. Topographic features, especially in orographic lifting, play an important role in severe weather genesis. During the warm season, warm, moist air, mainly from the eastern Pacific, via the Gulf of California, brings occasional severe thunderstorms into the southeastern California desert, sometimes reaching into the Central Valley.

Tornadoes and funnel clouds are most prevalent in the Los Angeles Basin and along the Central Valley. Hail reports center mainly around Fresno, in the San Joaquin Valley, the most intensively farmed region of the state. Waterspouts are mostly reported off the south coasts. The extent to which reported severe weather is biased by human factors is explored, while synoptic factors show both similarities and differences to those found in other U.S. regions.