Why Was SF Tornado Warning But Not Scotts Valley Where Twister Hit?

San Francisco’s first ever tornado warning lasted less than half an hour before the National Weather Service canceled it. Although gusts of up to 80 mph were recorded around Golden Gate Park and the Mission District, there was no sign of a tornado — leaving some residents questioning the message, especially after a tsunami warning spurred a garbled response less than two weeks earlier.

In the small Santa Cruz County mountain town of Scotts Valley, on the other hand, a twister touched down early Saturday afternoon, causing hundreds of thousands of dollars in damage by overturning several cars and ripping traffic signals from concrete — yet there was no tornado warning.

“People were just out Christmas shopping and getting ready for the holidays,” said Mali LaGoe, city manager for Scotts Valley. “It was originally reported as a six-car pileup because no one could believe it was actually a tornado.”

With two tornadic moments in eight hours, weather experts questioned why the National Weather Service issued a tornado warning for San Francisco but not Scotts Valley. The weather service defended its message, saying national guidelines limit when it can trigger phone alerts: A severe low-level thunderstorm like the one forecast for Scotts Valley doesn’t qualify, but a tornado — when the signs are clearly in sight — do it.

John Monteverdi, professor emeritus of meteorology at San Francisco State University, believes the San Francisco warning was justified. Still, for Scotts Valley, he’s not sure why the weather service “didn’t issue a tornado warning because to me this was a stronger rotation than they saw for San Francisco. Tornadoes can occur without warning in severe thunderstorms.”

Although a tornado warning is very rare for the region, Monteverdi said tornadoes themselves are “rare in California, but not rare” because they are part of the state’s climatology and weather patterns. National Oceanic and Atmospheric Administration details more than 100 tornadoes across the state since 1950including a smattering of twisters across the Bay Area.

Tornadoes are rare in mountainous areas — like the Scotts Valley — but less so along the coast and Central Valley, Monteverdi said. But occasionally, when a storm is big enough and warm and cold air masses collide, a spiral of air can move upward and “be a precursor to a Wizard of Oz-type tornado that happened down in Scotts Valley.”

And tornadoes often originate from severe thunderstorms. Although there is no scientific consensus that human-induced climate change is increasing the frequency of tornadoes, there is evidence that the intensity of thunderstorms is likely to accelerate as the climate warms.

“It stands to reason that when conditions are otherwise favorable, a greater proportion of thunderstorms might occur in environments that are favorable for tornadoes,” said UCLA climate scientist Daniel Swain.

The National Weather Service plans to run simulations of last weekend’s storm to assess whether warnings should come sooner next time in both cities. Brian Garcia, warning coordination meteorologist for the agency’s Bay Area office, acknowledged that thunderstorms are becoming more intense and noted that the frequency of tornadoes may also increase as a result.

“We can’t wait for perfect data 10 to 40 years down the road,” he said. “We need to start acting now as if it’s caused by climate change so we can lean forward and hopefully protect more people.”

Still, Garcia defended how the agency handled its messages during the storm. A wireless emergency alert went out for San Francisco but not the Santa Cruz area because the weather service has a national threshold for warning of severe thunderstorms based on expected potential destruction, which Garcia said is defined by 80 mph winds and or baseball-sized hail .

The agency issued a severe thunderstorm warning for the Santa Cruz area, meaning media outlets and localities have a choice in how to notify the public, but cell phones are not automatically alerted.

Garcia argued that while the word “tornado” sounds more frightening, a severe thunderstorm carries just as much destructive power.

“No matter how you shake it, both a severe thunderstorm and tornado warning tell you to take shelter in an interior room of your house or the bottom floor of your home,” he said. “Trying to be one-size-fits-all is obviously a holy grail. But that’s not going to happen either. We’re not going to be able to talk to everyone in a way that they can take an appropriate response to that. “