Quick Take
National Weather Service meteorologists issued a tornado warning for San Francisco on Saturday morning but ultimately, no twister appeared there. Lookout talked to a meteorologist to understand why Scotts Valley residents weren’t issued a tornado warning before one touched down there.
Scotts Valley residents were shocked when a tornado touched down in the middle of the city Saturday afternoon with no advanced notice, hours after a first-ever tornado warning for San Francisco proved 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 did indeed touch the ground?
NWS meteorologist Rick Canepa said the weather system was closely monitored as strong wind, heavy rain and rapidly changing conditions battered the entire Bay Area.
“Things are moving really quickly and evolving really quickly in the atmosphere,” he said. “So it could be extraordinarily tough to monitor and we do our best to estimate what we’re seeing 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 confident these rotations would reach the ground — unlike earlier observations over San Francisco that prompted the warning there.
“[For Scotts Valley], there was uncertainty as to whether or not it was going to be going down to the ground level,” he said. “It can spin up really quickly. The entire event lasted about five minutes, too, so that gives context as to how fast these things can spin up and then spin down.”
Canepa said the vertical depth of a rotation of wind and the elevation of it help NWS meteorologists determine whether to issue a tornado warning. “It’s a bit of guesswork, because we’re looking at various slices through the atmosphere to see the rotation,” he said.
Though they missed the tornado, Canepa said NWS meteorologists did observe enough concerning weather activity to issue the severe thunderstorm warning, which the agency describes as having the potential for “imminent danger to life and property” and advises to “take 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 following the forecasts 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-publish,'” said Loik, who researches how weather influences plants and ecosystems. “I am fascinated by unusual weather, the heat waves and the cold events and all those other things. So, a tornado in my neighborhood, it’s pretty, pretty, pretty interesting.”
Loik, who had been paying attention to weather forecasts, said he wasn’t that surprised to learn that San Francisco was issued a tornado warning.
“I think I’m losing the ability to be surprised by a lot of things, but certainly in our weather,” he said. “The past few years have done that.”
As for how the NWS meteorologists didn’t see the potential for a tornado or issue a warning for Scotts Valley, he said the conditions over a wide area were quickly changing and must have been difficult to predict.
“The weather was just so intense everywhere in the Bay Area that it was probably difficult to pick out one little bit of rotation, sort of a needle in a stack, if they weren’t prepared to be looking for it,” he said.
Looking ahead, Loik said it’s hard to say for certain if climate change and warming weather will bring more tornados to Santa Cruz County.
“From a mechanistic standpoint, if you warm up the atmosphere, you warm up the ocean, you create more evaporation, you create more storminess,” he said. “From a statistical standpoint, then that might lead some to predict more tornadoes but there’s so much more that goes into it than that.”
Additionally, he said, technology for detecting tornadoes has improved over time and likely made it appear that tornados are happening more often, when they might simply have not been as easy to detect 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 experienced eight tornados since 1950, with no evidence that they are occurring more frequently now than in the past.
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