Weather Watch

SLO's own tornado alley? What a recent funnel cloud tells us about this weather anomaly

A funnel cloud was spotted over Los Osos on Wednesday, March 14, 2018.
A funnel cloud was spotted over Los Osos on Wednesday, March 14, 2018.

A funnel cloud was spotted Wednesday traveling eastward from Los Osos to San Luis Obispo along the Los Osos Valley. Around the same time last year, March 22, Santa Maria saw a funnel cloud.

Funnel clouds are rotating columns of condensed water droplets. If wasn’t for the liquid water, you wouldn’t be able to see them. As soon as they hit the ground, they are classified as tornadoes. They’re called waterspouts when they form over the sea. Debris or dirt embedded in a funnel cloud indicates that it has already touched the ground and are categorized as a tornado.

So what causes funnel clouds?

To be honest, we understand the necessary conditions for development, but we don’t precisely know if a particular low-pressure system, front or trough will produce one.

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First of all, you need an unstable atmosphere with warm and/or humid air at the surface and cold air aloft.

If you remember from earlier columns, moist and warm air is less dense, and it naturally rises. As the air rises into the sky, it cools to the point that water vapor condenses to form visible clouds and releases tremendous amounts of latent heat. This condition keeps the air rising inside the cloud, producing updrafts that can reach speeds of more than 100 mph and can trigger thunderstorms. Clouds with vertical development are cumulus and cumulonimbus. “Nimbus” denotes precipitation, such as rain, hail or snow. The top of these clouds on rare occurrences can burst into the stratosphere at about 33,000 feet.

These updrafts can cause wind shear, the other ingredient needed for funnel clouds. Wind shear is when the air is traveling in a different direction and speed at dissimilar heights. The condition can cause a cylinder of air to rotate on a horizontal plane miles up in the atmosphere and eventually turn more vertical allowing it to reach the ground.

Even though the vast majority of tornadoes occur east of the Rocky Mountains, California is not immune to these violently rotating columns of air. There were 396 confirmed tornadoes from 1950 through 2012 in the state. In fact, on Jan. 6, 2016, an upper-level low-pressure system produced thunderstorms throughout Central and Southern California when a weak tornado touched down north of Hollister, becoming the first confirmed tornado in the United States in 2016.

Most of the state’s tornadoes occur south of Point Conception or in the San Joaquin and Sacramento valleys. Nearly all were rated weak on the Fujita scale. The California county with the highest number of confirmed tornadoes is Los Angeles with 43, followed by Orange at 31. San Bernardino, the largest county in the United States by area, has recorded 29 twisters, followed by San Diego at 26.

As you head northward along the California coast, the rate of tornado occurrences diminishes. Santa Barbara County has recorded six tornadoes, while San Luis Obispo County has only recorded two confirmed tornadoes.

I’ve been forecasting weather along the Central Coast for more than 26 years and have noticed a pattern of post-frontal thunderstorms occurring in the late winter/spring moving southeastward off the Pacific through the Los Osos and/or Chorro Valley toward San Luis Obispo that created tornadic activity. I’m not saying this is our own tornado alley, but it is an interesting anomaly.

On April 2, 2014, in the middle of the night, the Doppler weather radar at Vandenberg Air Force Base indicated a thick red line of severe weather over Cayucos moving southeast through the Chorro Valley toward San Luis Obispo. Later that morning, I received photographs and emails from numerous residents in Cayucos documenting 1-inch in diameter hailstones. This band of thunderstorms may have produced a weak tornado at Camp San Luis.

On April 7, 1926, a storm from the Pacific moved through the Los Osos Valley and produced lightning. The lightning struck large oil tanks along Tank Farm Road in San Luis Obispo. Altogether, more than 5 million gallons of oil burned over five days. Burning oil made it all the way to Avila Beach by way of San Luis Obispo Creek. Intense heat from these fires produced hundreds of fire whirls — many of them showed characteristics of actual tornadoes.

On May 5, 1998, another weak tornado hit San Luis Obispo. At the time, I was living on the corner of Kentucky and Fredericks streets in a neighborhood near Cal Poly where it touched down. At first, I thought it was a fast-moving train along California Boulevard. Tree branches were breaking, and then I saw debris rotating in a counterclockwise direction — clear evidence that a tornado was occurring. The National Weather Service came out to the site and confirmed that a low-level tornado had indeed occurred.

Here’s an interesting detail about this area near Cal Poly. Evidently, there may have been another whirlwind that occurred at this location back in the 1950s or ’60s. If you have any material or stories about this, I would love to hear from you. Please send me an email or letter to John Lindsey (PG&E) 4325 S. Higuera St., San Luis Obispo, CA 93401.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at or follow him on Twitter: @PGE_John.
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