Forty-three years later, this event still holds the record.
On this day in 1974, 148 tornadoes known together as the Super Outbreak wreaked havoc across 13 states. Three hundred thirty-five people died and more than 6,000 were injured, according to the National Weather Service. The storms destroyed or damaged thousands of homes.
Tornadoes are deadly, and they’re hard to predict even today, writes Brian Clark Howard for National Geographic. They form when a column of air gets caught in the space between a cloud and the ground, often in association with a thunderstorm. and begins spinning violently.
Scientists aren’t totally sure what causes a tornado to form, or what sets the column of air spinning, or even how to predict when it will end. They do know when conditions are right for it to happen, which is when they call a tornado watch.
A tornado outbreak is when the same weather system spawns multiple tornadoes. What happened in 1974 was a “super outbreak” because three different weather patterns collided, writes John Galvin for Popular Mechanics, causing an unprecedented number of tornadoes to happen near each other in a short period of time.
Although it was terrible, he writes, the super outbreak “brought about the modern tornado measurement system—and lots of cash for cyclone preparedness.”
In 1974, National Weather Service forecasters were still using 1950s-era equipment to detect potential extreme weather. Even with those, he writes, they knew something was up:
A sprawling mass of cold, dry air dropped down from Canada towards the Mississippi and Ohio River valleys, and an opposite mass of warm, moist air pushed northwards from the Gulf of Mexico. They were set to converge beneath an intense jet stream with 140-mph winds at an altitude of 40,000 ft.
The forecasters knew these conditions made for extreme storms, but they had no idea exactly how strong they would be, how widespread, or even precisely where they would erupt.
As three different weather patterns collided, tornadoes tore across the states, the worst one in the area of Xenia, Ohio. That tornado alone caused an estimated $100 million in damage and the loss of 33 lives, which was only a fraction of the total damage caused by the Super Outbreak.
But two important things happened because of the 1974 outbreak, research meteorologist Howard Brooks told Galvin. “First, the National Weather Service adopted the Fujita Scale. And second, support and money for tornado-intercept operations greatly increased.”
The Fujita scale created a standard language for the scientific community to talk about tornadoes, Galvin writes. Intercept operations, which send scientists out to actually chase tornadoes, have allowed them to observe what was happening firsthand, improving future warnings.
These innovations, combined with the money and political will to update detection gear, mean that the National Weather Service now has more weather stations and better forecasting technology, he writes. Research, more weather stations, and Doppler radar combined have increased the average tornado warning time from "about zero," as one meteorologist put it, to 12 to 14 minutes. "It doesn't seem like a lot," he told Galvin, "but when you need to take shelter every minute counts."
Even with all that, writes Howard, it’s not always possible to predict when—or where—a tornado will strike.