California Tornado Risk

By Ken Croswell

Published in Earth (April 1995, page 12)

Californians know they live in earthquake country, but most probably thought they were safe from another natural disaster: tornadoes. Now researchers have found that parts of the state experience nearly as many twisters per year per square mile as the Midwest. Worse yet, many of the newly identified tornado-prone regions are heavily populated.

According to meteorologist Warren Blier of the University of California at Los Angeles, these areas haven't shown up in official statistics because tornado records are not compiled for individual regions in California. Only the number of twisters for the entire state is recorded, so pockets of high risk just don't show up. That approach to record-keeping, he says, is appropriate for flat states like Kansas, where tornado risk is more or less uniform, but not for areas of varied topography like California.

In a study published in the journal Weather and Forecasting, Blier and his UCLA colleague Karen Batten used newspapers and other historic records to compile and analyze a database of 242 tornadoes that struck California from 1950 to 1992, the largest such record yet assembled.

Blier and Batten identified four regions with especially high risk. Risk area number one is the heavily populated south coast, which includes Los Angeles and San Diego. This region experienced up to 8.26 tornadoes per 10,000 square miles per year, a rate nearly equal to that of the Midwest. The other three danger zones are the Central Valley (including Sacramento), the north central coast (including San Francisco), and part of the sparsely populated southeast desert. Blier and Batten hope that their work will lead to an understanding of how and why California tornadoes form. "The ultimate goal is to be able to warn the public in situations where a tornado might occur," Blier says.

Official records state that no one has ever been killed by a tornado in California. This is in part because the storms are often weaker and shorter-lived than Midwestern ones. But it's also due to pure luck. "We have documented cases in which the outcome could have been far more serious had the tornado occurred either at a slightly different location or at a slightly different time," Blier says.

Unfortunately, when a tornado does hit, news reports don't raise awareness much, because they typically describe tornadoes as rare. "To my mind," Blier says, "these events are occurring too often for the adjective `rare' to be appropriate."

Ken Croswell is the author of Magnificent Universe and Ten Worlds.

"Magnificent Universe by Ken Croswell is elegant and eloquent."--Washington Post. See all reviews of Magnificent Universe here.

"On the basis of its striking design and photographs, this handsome, large-format volume is well worthy of praise. And astronomer Croswell's concise yet conversational, information-packed text wins it sky-high accolades in the narrative sphere as well."--Publishers Weekly, starred review. See all reviews of Ten Worlds here.