"The fact that it was a tornado [warning] came out of nowhere," said Carroll, executive director of San Francisco's Department of Emergency Management. "It took us a second because we didn't have messaging for tornadoes."
San Francisco's first-ever tornado warning lasted less than half an hour before the National Weather Service canceled it. Although straight-line winds of up to 80 mph were recorded around Golden Gate Park and the Mission District, there was no evidence of a tornado -- leaving some residents questioning the messaging, especially after a tsunami warning had spurred a scrambled 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 several hundred thousand dollars in damage by flipping several cars and wrenching 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 messaging, stating national guidelines limit when it can trigger phone alerts: A low-level severe thunderstorm like the one forecast for Scotts Valley doesn't qualify, but a tornado -- when the signs are clearly in view -- does.
John Monteverdi, emeritus professor of meteorology at San Francisco State University, thinks the San Francisco warning was justified. Still, for Scotts Valley, he isn't 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 "infrequent in California, but not rare" because they are part of the state's climatology and weather patterns. The National Oceanic and Atmospheric Administration details more than 100 tornadoes across the state since 1950, including a smattering of twisters across the Bay Area.
Tornadoes are rare in mountainous areas -- like Scotts Valley -- but less so along the coast and Central Valley, Monteverdi said. But occasionally, when a storm is large 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 of tornado, which happened down in Scotts Valley."
And tornadoes often stem from severe thunderstorms. While there's no scientific consensus that human-caused climate change is increasing the frequency of tornadoes, there is evidence that the intensity of thunderstorms is likely accelerating as the climate warms.
"It stands to reason that when the conditions are otherwise favorable, perhaps a greater fraction of thunderstorms could occur in environments 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, as a result, the frequency of tornadoes might also increase.
"We can't wait for perfect data 10 to 40 years down the road," he said. "We've got to start acting now like it is caused by climate change so we can lean forward and hopefully protect more people."
Still, Garcia defended how the agency handled its messaging 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 alerting on severe thunderstorms based on forecast 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, which means that media outlets and localities have a choice in how they message the public but cellphones are not automatically alerted.
Garcia argued that while the word "tornado" sounds scarier, a severe thunderstorm carries just as much destructive power.
"Either way you shake it, both a severe thunderstorm and tornado warning tell you to seek shelter in an interior room of your house or the lowest floor of your home," he said. "To try to have a one-size-fits-all all for everybody is obviously a holy grail. But it's also not going to happen. We're not going to be able to speak to everybody in the way that they can take an appropriate response to it."