A line of strong storms swept through southern California on Friday morning, producing flash flooding and even spawning a weak tornado in southern Los Angeles. While not as common as we see out east, California has seen hundreds of tornadoes over the past six decades. Here's a deeper look at the state's tornado climatology.
On average, the United States averages a little more than 1,000 tornadoes every year, with almost all of them occurring east of the Rocky Mountains. The geography of this part of the country allows storm systems moving in from the west to interact with warm, moist air flowing north from the Gulf of Mexico. If the ingredients can come together in the right mixture, it can result in some of the worst tornado outbreaks nature can produce.
California, on the other hand, is in a unique spot when it comes to the weather. Bound by the Pacific to the west and mountains to its east, the state often finds itself in a meteorological limbo, leading to its famous and persistent calm, sunny weather. Given the cool, stable airmass that develops thanks to the ocean, the only chance that many of densely-populated coastal cities have at seeing active weather comes in the form of Pacific storms like the one we saw this week.
California's Central Valley—which is the vast stretch of low, flat land that dominates the center of the state—plays host to almost all of the state's tornadoes. This region of the state is susceptible to seeing convection like the rest of the country experiences, and when these storms coincide with atmospheric wind shear, they can rotate and produce some tornadoes.
The video at the top of this post is one of the most memorable instances of a tornado in California. The weak tornado touched down in a sparsely-populated agricultural community in Butte County, near Chico. A news helicopter from KCRA-TV in Sacramento chased the storm and caught the entire life cycle of the tornado on camera.
Between 1950 and 2013, there were 403 confirmed tornadoes in California, coming out to an average of around 6 or 7 tornadoes per year. The vast majority of them occurred in the Central Valley, but you can see a tight cluster of tornadoes down around Los Angeles. Most of the twisters are weak, with 66% of those surveyed by meteorologists rating either F0 or EF0 (we switched from the Fujita Scale to the Enhanced Fujita Scale in 2007). A handful of tornadoes recorded in the state have been significant, with 23 clocking in at F/EF2 and two achieving F3 status.
The two strongest tornadoes recorded in California happened five years and a few miles apart from one another. The first was recorded on August 16, 1973, down around Blythe in the Colorado Desert. The tornado was extremely short-lived—it was on the ground for a tenth of a mile and it was only ten yards wide—but it managed to produce between $5,000 and $50,000 damage that was severe enough for meteorologists to rate an F3. The other major tornado occurred on February 9, 1978 in Orange County; rated an F3, it injured 6 people and produced more than half a million dollars in damage.
These are just confirmed tornadoes. The number and location of tornado warnings in the state can show where the atmosphere is most likely to produce rotation that could result in a tornado.
Over the past 12 years, the greatest density of tornado warnings in California has been up in the Sacramento Valley, where severe thunderstorms are commonplace in the spring and summer months. Strong thunderstorms are also found in the Los Angeles area (when it rains, that is), with the occasional rotating thunderstorm or kink in a squall line triggering a tornado warning for the area. Waterspouts that make landfall also count as tornadoes, and many coastal communities have seen these rotating columns of air over the water come ashore and produce minor damage.
As for the Los Angeles area itself, the good folks over at U.S. Tornadoes have found that Los Angeles and Orange Counties in southern California are "hotspots" for tornado activity during the month of December:
Tornadoes in California are certainly rare, but they're not completely unheard of in this part of the country. As we saw with that guy's video from L.A. yesterday, they can be a shock to the system when you've had so little precipitation in recent years, let alone no tornadoes in a decade.