First-Ever December Serial Derecho Spawned 45 Tornadoes Across Great Plains, Midwest: NWS

National Weather Service (NWS) officials said Monday that the first-ever December serial derecho caused the severe thunderstorms spawning dozens of tornadoes in the North-Central U.S last week.

The NWS is calling the December 15 storms historic. Bill Bunting, chief of forecast operations for the NWS Storm Prediction Center, said the storms caused by a serial derecho produced 45 confirmed tornadoes. At least 12 tornadoes were EF-2.

A derecho is characterized as an inland hurricane with straight-line winds, no eye, creating widespread damage of more than 240 miles and wind gusts of at least 58 mph.

States in the Midwest reported unusually warm temperatures for the season the day of the storm. Wisconsin reported temperatures as high as 70 degrees Fahrenheit, more commonly found in the summer months.

"So you had a very strong storm system with really somewhat unprecedented access to very warm, humid air that flows northward, and those are the ingredients that you look for for severe thunderstorms," Bunting said. "The result was quite remarkable for December."

According to Bunting, Iowa, Nebraska and Minnesota saw the majority of the damage. Minnesota saw its first recorded tornadoes in December with a total of nine twisters, WCCO reported.

Several of the thunderstorms across the Midwest and Great Plains reportedly had winds up to 80 mph. At least five deaths have been attributed to the storm.

Rare Winter Tornado Iowa
The National Weather Service said that the series of thunderstorms and tornadoes that swept across the Great Plains and upper Midwest on December 15, 2021, was a serial derecho. The NWS said it was the first-ever serial derecho in December in the U.S. Above, a tornado approaches Interstate 80 near Atlantic, Iowa, on December 15, 2021. Bryon Houlgrave/The Des Moines Register/AP Photo

Bunting said that normally in December, air from 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 much of a winter," he said.

Weather historian Chris Burt compared Wisconsin's unusually warm December temperature the night of the storm as that of a "warm July evening."

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

A derecho is similar to a hurricane in its damage, which is likely to spread over a wide area, unlike a tornado where the damage is more spotty.

A derecho was also declared in August of 2020 when storms packing 100 mph (161 kph) winds lasted several hours and caused damage from eastern Nebraska, across Iowa and into parts of Wisconsin and Illinois.

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

The Associated Press contributed to this report

Winter Storms, Tornado, Iowa
Sheets of metal litter a field south of Jefferson, Iowa, on December 16, 2021, after a band of severe weather produced strong wind gusts and reports of tornadoes across much of the state Wednesday night. The storm caused property damage and downed power lines, leaving many Iowans without electricity. Bryon Houlgrave/The Des Moines Register/AP Photo