O’FALLON, Mo. – The O’A line of severe thunderstorms and tornadoes that swept through the north-central United States last week is the result of a serial derecho – the first recorded in December, National officials said on Monday Weather Service.

At least 45 tornadoes have been tentatively confirmed in the Storms of December 15 which has traversed the Great Plains and Midwest amid unusually warm temperatures for the season, said Bill Bunting, chief of forecast operations at the Weather Service’s Storm Prediction Center. Nebraska, Iowa and Minnesota were the hardest hit.

Bunting said at least 12 of the tornadoes were rated EF-2. Numerous thunderstorms also carried winds of up to 80 mph (129 km / h). Five deaths have been attributed to the weather.

Bunting said that normally in December, the air in the Gulf of Mexico has cooled and colder air is present in the upper Midwest – factors that reduce the risk of severe thunderstorms and tornadoes. But this year, “we haven’t had a lot of winter,” he said.

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“So you had a very powerful storm system with really somewhat unprecedented access to very hot, humid air flowing northward, and these are the ingredients you are looking for in severe thunderstorms,” he said. Bunting said. “The result was quite remarkable for December.”

A derecho is often described as a hurricane inland. But, he has no eye and his winds cross in line. The similarity is in the damage, which is likely to spread over a wide area, unlike a tornado where the damage is more uneven.

The weather service said a strip of wind damage spanning more than 240 miles (386 kilometers) with wind gusts of at least 58 mph (93 kilometers per hour) over most of its length can be classified as a derecho.

A derecho was also declared in August 2020 when 100 mph (161 km / h) storms lasted for several hours and caused damage in eastern Nebraska, across Iowa, and parts of Wisconsin and Illinois.

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There was a difference: The August 2020 storm was a gradual derecho, while last week’s was a serial derecho.

The weather service said a gradual derecho is fueled by a hot and humid environment with relatively strong winds aloft. Serial echoes are produced by storms with strong winds that tilt outward, the service said. They sweep an area both long and wide, driven by the presence of very strong winds in the atmosphere.

December’s unprecedented heat wave included temperatures that reached 70 degrees Fahrenheit (21 degrees Celsius) as far north as Wisconsin, creating nighttime temperatures that weather historian Chris Burt compared to those of a “hot July evening”.

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