Scotts Valley did not receive a tornado warning, but San Francisco did. Why?

Residents of Scotts Valley were shocked when a tornado touched down in the middle of town Saturday afternoon without warning, hours after the first ever tornado warning for San Francisco turned out to be a false alarm.

Why did the National Weather Service issue a tornado warning for San Francisco but not Scotts Valley, where a twister actually touched down?

NWS meteorologist Rick Canepa said the weather system was being closely watched as strong winds, heavy rain and rapidly changing conditions battered the entire Bay Area.

“Things are moving really fast and evolving really fast in the atmosphere,” he said. “So it can be extraordinarily difficult to monitor, and we do our best to assess what we see off the radar.”

At 1:25 p.m. Saturday, meteorologists issued a severe thunderstorm warning for most of Santa Cruz County, predicting the possibility of nickel-sized hail and damaging wind gusts approaching 60 mph.

While their instruments detected rotating winds near Scotts Valley, Canepa said meteorologists weren’t sure those rotations would reach the ground — unlike earlier observations over San Francisco that triggered the warning there.

“(For Scotts Valley) there was uncertainty as to whether it would go down to the ground level,” he said. “It can run around quickly. The whole event also lasted about five minutes, so that gives context to how quickly these things can spin up and then spin down.”

Canepa said the vertical depth of a rotation of the wind and the height of it help NWS meteorologists decide whether to issue a tornado warning. “It’s a bit of guesswork because we’re looking at different slices through the atmosphere to see the rotation,” he said.

Although they missed the tornado, Canepa said NWS meteorologists observed enough in terms of weather activity to issue the severe thunderstorm warning, which the agency describes as potentially “imminent danger to life and property” and advises to “seek shelter in a substantial building” and “get out of mobile homes that can blow over in high winds.”

UC Santa Cruz environmental studies professor Michael Loik was closely monitoring the forecast Saturday from his home in Felton when he learned that a tornado had touched down in Scotts Valley.

“I said, ‘Holy something-you-can’t-release,'” said Loik, who studies how weather affects plants and ecosystems. “I’m fascinated by unusual weather, the heat waves and the cold events and all the other things. Saw a tornado in my neighborhood, it’s beautiful, beautiful, pretty interesting.”

Loik, who had been paying attention to weather forecasts, said he wasn’t too surprised to hear San Francisco was issued a tornado warning.

“I think I lose the ability to be surprised by a lot of things, but certainly in our weather,” he said. “The last few years have done that.”

As for how NWS meteorologists didn’t see the potential for a tornado or issue a warning for Scotts Valley, he said conditions over a wide area changed quickly and must have been difficult to predict.

“The weather was just so intense all over the Bay Area that it was probably hard to pick out a little bit of rotation, kind of a needle in a haystack, if they weren’t prepared to look for it,” he said.

Looking ahead, Loik said it’s hard to say for sure whether climate change and warming weather will bring more tornadoes to Santa Cruz County.

“From a mechanistic point of view, if you warm the atmosphere, you warm the ocean, you create more evaporation, you create more storminess,” he said. “From a statistical standpoint, it might lead some to predict more tornadoes, but there’s so much more that goes into it than that.”

In addition, he said, the technology to detect tornadoes has improved over time and likely made it appear that tornadoes are happening more often, when they may simply not have been as easy to spot in the past.

Several eyewitnesses to Saturday’s tornado in Scotts Valley said they never imagined such a weather event could happen in their community. But Canepa, the NWS meteorologist, said Santa Cruz County has actually seen eight tornadoes since 1950, with no evidence that they are occurring more frequently now than in the past.

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