Tornadoes in California? More than you think

California is known as the “Golden State” in part for its mild weather. But it turns out the sky can split open and suck your roof off here, too.

Three tornadoes have struck the Sacramento Valley so far this week, including one that ripped through the city of Roseville on Wednesday, levitating lawn furniture and barbecues and tearing up fences, windows and roofs. The others hit in Glenn County on Wednesday (60 almond trees uprooted), and Butte County on Tuesday (a few dead trees knocked over).

Amazingly, this is more tornadoes than any other state has experienced so far this year, according to the National Weather Service.

“We get them every year, but they’re usually pretty weak,” said George Cline, a forecaster at the National Weather Service in Sacramento. “They were really considered very rare in years past. But now, with our increased capacity with Doppler-style radar, more radars and more people, we are coming to appreciate that they are a lot more common than we thought.”

Since 1996, California has averaged 11 tornadoes a year, more than any other state west of the Rocky Mountains, according to National Weather Service data. As Cline said, most are weak. California has never seen a tornado rated stronger than 2 on the Enhanced Fujita scale, a 0-6 rating system that ranks tornado intensity based on wind speed and damage assessments.

The Butte County and Roseville tornadoes were both rated EF0, meaning light damage and wind speeds up to 72 mph. The Glenn County tornado was rated EF1, indicated by wind speeds up to 112 mph and surface damage to roofs.

No injuries were reported from any of these tornadoes. Injuries are rare in a California tornado, and apparently no one has ever been killed by a twister in the state.

“A death could occur, but it’s probably not going to be from a home being swept away,” said Tom Grazulis, who catalogs American tornado incidents dating back as far as 1680. “It’s the kind of thing where somebody could just be in the wrong place at the wrong time in regard to a plate of glass.”

A tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. Experts still don’t fully understand how tornadoes form.

The most destructive tornadoes occur from supercells, which are rotating thunderstorms with a well-defined circulation called a mesocyclone. A rotating updraft is a key to the development of a supercell, and eventually a tornado. One way a column of air can begin to rotate is from wind shear – when winds at two different levels above the ground blow at different speeds or in different directions

Tornadoes can form without supercells. These tornadoes form from a vertically spinning parcel of air already occurring near the ground, caused by wind shear from a warm, cold, or sea breeze front. When an updraft moves over the spinning air and stretches it, a tornado can form.

America’s worst tornadoes occur in the Midwest and southern Great Plains states. Last year, 35 people were killed and 230 injured by tornadoes nationwide. The worst was the tornado that hit Moore, Okla., on May 5, which killed 24 people. That twister was rated EF5, defined by wind speeds exceeding 207 mph, the strongest category ever recorded.

Some experts believe topography plays a role in tornado formation, and that the flat surfaces of the Midwest and Plains may help to form big tornadoes. But others aren’t so sure topography has any role.

“To develop a strong enough storm, you need a fairly homogeneous surface for heating the air,” Cline said. “We get all different terrain effects, and it kind of throws it off.”

One thing you definitely need is thunderstorms, and March and April are the busiest months for thunderstorms in the Sacramento region, according to the National Weather Service.

Grazulis, based in Vermont, is the author of “The Tornado: Nature’s Ultimate Windstorm” and director of tornadoproject.com, a repository of tornado history in America. His data indicate that no one has ever ever been killed by a tornado in California.

“There’s so many winter storms and so many different kinds of atmospheric setups (in California) that, sometimes, everything comes together right somehow,” Grazulis said. “But you don’t get the supercells.”

In total, California has seen 127 tornadoes since 1996, according to the National Weather Service. The worst incident Grazulis has recorded for California is a tornado that touched down in Sunnyvale on Jan. 11, 1951, which injured 30 people and damaged 150 homes. It was considered the third most damaging tornado in the U.S. that year.

The worst in Sacramento, according to his data, struck on Oct. 26, 1921, when a tornado injured five people and damaged about 35 homes.

The Los Angeles basin also seems to be a hotspot for tornadoes in California. Grazulis said this may be related to topography: The region, surrounded by mountains, is a kind of funnel that can draw moisture in from the ocean, where it then collides with warm, dry inland air to form twisters. This would seem to be backed up by National Weather Service data, which show that Riverside County has seen 14 tornadoes since 1996, more than any other California county. Its neighbor, San Bernardino County, ranks second with 13 tornadoes.

California has apparently never seen a tornado worse than EF2, and it has seen only three since 1996 that ranked that high: Sunnyvale (again) in 1988, March Air Force Base in Riverside County in 1988, and Thermalito in Butte County in 2011.

Wednesday’s tornado in Roseville scraped through an area about 300 yards long and 20 yards wide near the intersection of Pleasant Grove Boulevard and Monument Drive – a newer area of town. The twister stripped off roof tiles, tossed patio chairs into swimming pools, knocked over some fences and threw debris through a few windows. The Roseville Fire Department responded with ladder trucks to help people patch roofs, and the Utilities Department sent dumpsters to help people dispose of debris.

“The most damaged home had some windows that were broken, so the Fire Department helped them clean up and get secured so they could stay home that night,” said city spokesman Brian Jacobson. “We’re just happy nobody got hurt.”

The storm expected to hit California on Saturday is also expected to produce thunderstorms, and the National Weather Service has warned it could produce more tornadoes in the Sacramento Valley.