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U.S. has world’s highest tornado risk. Here’s why.

Most of Earth’s tornadoes occur in mid-latitudes, which puts the Lower 48 in the center of that not-so-sweet ordinate zone

A flag flies in Dawson Springs, Ky., on Saturday, after one or more tornadoes passed through the area the previous night. (Austin Anthony for The Washington Post)

Friday night’s cataclysmic quad-state tornado outbreak reminds us of the vulnerability of our country to tornadoes and how the threat of these destructive storms looms menacingly real over the eastern two-thirds of the United States during any season of the year.

We’re not alone, as tornadoes happen in other parts of the world, too. But by sheer numbers (about 1,200 on average per year), the United States has the dubious honor of residing at the top of the list.

The broad expanse of our country of course plays a significant role — more area equals more opportunity. But it is our geography and juxtaposition on the continent that provides the fuel for increased risk. What makes the United States so special?

Tornadoes tend to form where cold, dry air clashes with warm, humid air. These contrasts are maximized over the mid-latitudes, where the majority of Earth’s tornadoes occur. A good-size portion of the Lower 48 sits smack-dab in the center of that not-so-sweet ordinate zone. Add in the proximity to the warm waters of the Gulf of Mexico, mid-level dry air sloping down from the Rockies, plus unfettered access to cold air from northern environs (particularly notable due to the flat topography of the Great Plains), and you bring together a nearly perfect set of otherwise harmless ingredients that can suddenly become a volatile mix of atmospheric terror.

December tornadoes aren’t rare, but Friday’s outbreak was something totally different

According to the National Weather Service’s Storm Prediction Center online Tornado FAQ by Roger Edwards, there are more tornadoes in the United States than any other part of the world, though the author notes the data from other nations is problematic to assess since it’s comparatively sparse and collected differently by each country.

Edwards wrote “a blend of recorded and inferential study indicates that the U.S. remains tops in tornado production, with secondary tornado-prone areas including the Canadian Prairie Provinces, Bangladesh, Britain, northeastern Mexico, northern Argentina and southern Brazil, and portions of southwestern Russia.”

A quad-state tornado traveled 250 miles across Arkansas, Missouri, Tennessee and Kentucky on Dec. 10. Here's a look at the storm, by the numbers. (Video: Casey Silvestri/The Washington Post)

Further underscoring the worldwide risk, the National Severe Storms Laboratory of the National Oceanic and Atmospheric Administration notes that tornadoes occur in most of the world’s continents, from North and South America, to Africa and Australia, to Europe and Asia, with only the Arctic regions exempt.

In North America outside the United States, it’s illustrative to see that the most vulnerable areas for tornadoes occur relatively close to the northern border of Mexico and southern border of Canada.

In Mexico, they concentrate with greatest frequency over a relatively small part of its northeast region, where geography and topography most closely match that of the U.S. southern Plains. Along with similar proximity to the Gulf of Mexico, effects from dynamic fronts moving east of the mountains powered by colder air surging southward from the Great Plains bring a focus of tornado risk, where violent and deadly tornadoes can occur, just as in the United States, as evidenced by one that hit the border town of Ciudad Acuna in 2015.

To the north, Canada has a much more expansive territory of tornado risk, running across southern Alberta to Quebec. The geographical similarities of southern Canada to the U.S. Great Plains and Upper Midwest shape their contours of tornado prevalence.

The Northern Tornadoes Project, a major collaborative effort in Canada’s tornado data tracking and research, offers a telling look at the country’s recent tornado history. Their tracking dashboard reveals a couple hundred tornadoes over the past five years for all provinces, about a quarter of what the United States sees in an average year. Still, Canada places high in the world with its share of tornadoes, though those are rarely violent one, rated at EF4 or EF5 on the zero-to-5 scale for intensity. In fact. the first recorded top of the scale tornado was in 2007 near Winnipeg, Manitoba, as detailed by Environment and Climate Change Canada. By comparison, the United States averages several EF4 or stronger tornadoes every year, though EF5s are rare.

Within the United States, the boundaries of highest risk for tornadoes has traditionally been east of the Rockies, in the Plains, Midwest and South, but there is an expanding secondary threat area evident over the southeast northward through the Mid-Atlantic. Recent trends are showing that part of the United States as having increasing susceptibility to impacts from tornadoes.

Tornado Alley in the Plains is an outdated concept. The South is even more vulnerable, research shows.

No matter where they occur, though, it’s vital for each of us to understand the risk where we live, to know where and how to get alerts, and most importantly to have a plan on how to stay as safe as possible when this, the worst of weather, threatens.

Jim Duncan recently retired from his 40-year career as chief meteorologist with NBC12 WWBT-TV in Richmond. He runs a meteorological consulting firm, Jim Duncan, LLC, serving clients in media, education and other industries.

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