Quick Take
Two tornado warnings over the past two weeks likely surprised many Santa Cruz County residents. No tornadoes touched down in the county this year, but one did in Scotts Valley at the end of 2024. So what prompts a tornado warning from the National Weather Service? And when, if ever, is tornado season locally?
The loud alarms and tornado warnings that popped up on phone screens around Santa Cruz County in recent weeks likely worried and confused local residents.
On Christmas Day, the National Weather Service issued a tornado warning after it observed a storm hovering over Monterey Bay that appeared to be rotating, which indicated to meteorologists that it had the potential to form a tornado. Then, this past Monday, the NWS issued another tornado warning for the far northwest part of the county bordering southern San Mateo County.
Both warnings came and went without a tornado touching down, but the question remains: How does the NWS decide when to issue a warning? NWS meteorologist Joe Merchant told Lookout that the agency is most commonly tipped off to favorable tornado conditions by its radar and the behavior of winds around its equipment.
“When we see the wind moving away from the radar and then toward the radar over a very small area, that implies that there’s rotation at that moment,” he said. “It’s the first indication to us that there’s ultimately potential for a tornado.”
Tornadoes, simply put, are columns of air that rapidly rotate and extend vertically from the surface of the Earth to the base of a storm cloud. A tornado warning means the federal weather agency has either spotted a tornado or radar has indicated one is forming, and there is imminent danger to life and property. This, however, is clearly not always the case, as no actual tornado formed after the two local tornado warnings over the past two weeks. At the end of the day, when conditions look favorable for a tornado, forecasters have to make quick decisions before it’s too late, Merchant said.
“We really can’t see the base of the clouds, which is what would really tell us whether or not there’s rotation and the possibility of a tornado,” he said, adding that radars in the Bay Area and along the Central Coast often look down on clouds, where there might be rotation, but where it’s difficult to see if the storm is near ground level.
Merchant said tornadoes in the Central Coast are “rare, but not unprecedented.” In other parts of the country, tornado season is the time of year with the most atmospheric moisture lingering in the region. That lines up with the December and January events locals have seen in recent years, the time of year when atmospheric rivers and long spells of wet weather drench Santa Cruz County in the middle of winter.
“For Kansas, that would be all the way from spring through the summer, probably maxing out during May,” said Jan Null, a Half Moon Bay-based meteorologist with Golden Gate Weather Services. “That’s because you have moisture coming up off the Gulf of Mexico and cold fronts coming out of Canada.”
So even though we are currently technically in “tornado season,” tornadoes are rather uncommon in Santa Cruz County and on the Central Coast.
Null’s data, which goes all the way back to 1950, shows eight tornadoes observed in Santa Cruz County, five in Monterey County and four in San Luis Obispo County. Additionally, all of the tornadoes observed in these regions have clocked in at the bottom of the Fujita or Enhanced Fujita scale, which rates tornado intensity based on damage, with F5 as strongest. Every funnel cloud that has hit the Central Coast has been in the F0 to F1 range.
Those data points come with interesting trends. The first Santa Cruz County tornado showed up in 1965, and only two others touched down prior to 2000. Since then, five have hit the county – including one in December 2024 in Scotts Valley. Is that because of climate change? “Maybe,” said Null, but added that there is much more technology in recent years, which allows meteorologists to detect possible tornadoes more often than they used to — and those might also lead to false positives.
“A problem we run into is having a radar on Mount Umunhum [in the Santa Cruz Mountains] at 3,500 feet,” he said. “Out in the plains, [radars] are at sea level. Here, a lot of the rotation you’re seeing is above the ground. I haven’t studied it, but that probably creates more false positives here, because you can get rotation on the profile, but it never touches down.”
Have something to say? Lookout welcomes letters to the editor, within our policies, from readers. Guidelines here.