The Bay Area does get tornadoes, and the National Weather Service does sometimes issue tornado warnings here. Bay Area tornadoes are rare, though, and most are weak and brief. The region sees about one or fewer in a typical year. Most are rated EF0 on the Enhanced Fujita Scale, the weakest level, with winds of roughly 65 to 85 mph. They usually touch down for a short distance, last only a few minutes and cause limited damage, such as downed trees, torn roof shingles and tossed patio furniture. If your phone sounds a tornado warning during a winter storm, take it seriously, but know that the risk here is very different from the Great Plains.
Can Tornadoes Actually Hit the Bay Area?
They can, and over the decades the region has recorded dozens of tornadoes and many more funnel clouds. Nearly all of them have been small and short-lived. They tend to form from compact, fast-moving thunderstorms and showers inside larger winter storms, rather than from the giant rotating supercells of Tornado Alley. Those Great Plains storms can produce tornadoes rated EF4 or EF5, far beyond anything recorded in the Bay Area.
Bay Area tornadoes usually come with strong winter storms, often as a cold, unstable area of low pressure moves overhead. Cold air high above the ground lets showers grow tall, and changing winds at different heights can set the air spinning. Flat, open ground such as the Delta, the Livermore Valley and the South Bay near San Jose sees many of the reported tornadoes. Funnel clouds have also been spotted over San Francisco and Oakland, though tornadoes that touch down in dense city neighborhoods are uncommon.
What Is the Difference Between a Tornado Watch and a Tornado Warning?
A tornado watch means the weather over a broad area could produce tornadoes. The Storm Prediction Center issues watches when wind patterns, instability and moisture line up in a way that favors tornadoes. A watch does not mean a tornado is coming. It means you should pay attention to the weather and be ready to take shelter if a warning follows. Tornado watches are rarely issued for the Bay Area.
A tornado warning is much more urgent. Your local Weather Service office issues one when a tornado has been spotted or when radar shows strong rotation in a storm that could produce one. Warnings cover a small area and usually last less than an hour. When a warning covers your location, move right away to an interior room on the lowest floor of a sturdy building, away from windows.
Tornado warnings set off Wireless Emergency Alerts on phones in the warned area, so many people first learn about them from a loud alarm. The alert names the area and the time the warning ends. Because Bay Area tornadoes are so rare, it is easy to dismiss the alert. Still, the few minutes it takes to move inside and away from windows are well spent.
In the Bay Area, tornado warnings come only now and then, usually during the strongest winter storms. Bay Area storms can spin up quickly and briefly, so a tornado may touch down before a warning goes out. If your phone sounds an emergency alert for a tornado warning during a January rainstorm, act on it. Even a weak tornado can cause injuries from flying debris and falling trees.
Notable Bay Area Tornado Events
The strongest Bay Area tornadoes on record have reached F2 strength on the older Fujita Scale. On January 11, 1951, a strong tornado struck Sunnyvale as part of a cluster of tornadoes across the region that day. Later studies of photos and eyewitness accounts suggest it may have been even stronger than its official rating.
On May 4, 1998, two tornadoes struck the South Bay during a spring storm. One tore through neighborhoods in Sunnyvale and was rated F2, damaging homes and businesses. The other hit Los Altos and was rated F1. These events showed that the Bay Area can, on rare occasions, see tornadoes strong enough to damage buildings.
Waterspouts, tornado-like spinning columns over water, are more common than tornadoes on land. They sometimes form over San Francisco Bay or just offshore during winter storms, when cold air passes over the warmer water. Most are weak and fade quickly if they reach shore. They can be striking to watch but rarely cause damage.
Funnel clouds are reported more often than tornadoes. A funnel cloud is a spinning, cone-shaped cloud that hangs below a storm but does not reach the ground. Ragged, low clouds called scud can look similar, but they drift without spinning. If you see a funnel that touches the ground, or debris swirling beneath one, it is a tornado. Reporting it to the National Weather Service, through social media or a local emergency line, helps forecasters warn others nearby.
Deep Dive: How Do Small Cool-Season Tornadoes Form?
California tornadoes form with much less storm fuel than those on the Great Plains. What they have instead is strong wind shear, a sharp change in wind speed or direction with height. In winter storms, that shear can set even modest showers and thunderstorms spinning.
What Does Wind Shear Do?
Sometimes the wind a few thousand feet up blows faster than the wind near the ground, or from another direction. The air between them then rolls like a pencil between your hands. A strong updraft can tilt that rolling air upward, turning it into a spinning column inside the storm. A storm with a long-lived rotating updraft is called a supercell.
On the Great Plains, supercells draw on large amounts of storm fuel, measured as CAPE. California's winter storms usually have far less. Forecasters describe these setups as high-shear, low-CAPE environments, where the shear does much of the work.
Shear is measured as the difference between the winds at two heights. By our calculation, a 15-mph southeast wind at the ground under a 45-mph southwest wind 3,300 feet up gives shear of about 47 mph across that layer. Strong winter storms can bring shear of that size ahead of their cold fronts.
What Has Research on California Tornadoes Found?
John Monteverdi of San Francisco State University has studied California tornadoes for decades. In a 2003 paper in Weather and Forecasting, he and co-authors Charles Doswell and G. S. Lipari examined 70 California thunderstorms, both tornadic and non-tornadic, in Northern and Central California.
They found that the stronger F1 and F2 tornadoes could be separated from non-tornadic storms by the wind shear in the lowest kilometer and the lowest 6 kilometers. Most, if not all, of those stronger events were probably supercells. The weakest F0 tornadoes looked much like ordinary storms in their shear, and most of them were probably not supercells at all.
A 2012 San Francisco State study by Stumpf and Monteverdi extended the work to 391 California tornadoes from 1951 to 2011. The research gives forecasters a practical clue. When strong low-level shear arrives with a cold, unstable winter storm, the odds of a brief tornado rise. That is when the Weather Service is most likely to issue a warning.
What Should You Do During a Tornado Warning in the Bay Area?
Respond to a tornado warning the same way you would anywhere. Go to an interior room on the lowest floor, away from windows and outside walls. Bathrooms, closets and hallways in the middle of a building are good choices. If you are in a car, do not try to outrun the storm. Instead, find a sturdy building if one is close, or pull over, keep your seatbelt on and duck below the windows. If you are outdoors with no shelter, lie flat in the lowest spot you can find and cover your head.
Tornadoes are a real but small risk compared with the Bay Area's other weather hazards. Flooding and landslides during strong atmospheric rivers have caused far more harm here than tornadoes ever have. Wildfire smoke, coastal flooding and heat waves are all bigger threats over time.
Still, a tornado warning is no time for complacency. Take shelter, wait for the warning to end and then turn your attention back to the rain and wind. In the storms that bring Bay Area tornadoes, flooding, falling trees and power outages are usually the bigger dangers.
