O’FALLON, Mo. (AP) — A line of severe thunderstorms and tornadoes that swept across the north-central U.S. last week was the result of a serial derecho — the first on record in December, officials with the National Weather Service said Monday.

At least 45 tornadoes have been preliminarily confirmed in the Dec. 15 storms that crossed the Great Plains and Midwest amid unseasonably warm temperatures, said Bill Bunting, chief of forecast operations at the weather service’s Storm Prediction Center. Nebraska, Iowa and Minnesota took the brunt of the damage.

Bunting said at least 12 of the tornadoes were rated EF-2. Many of the thunderstorms also carried winds of up to 80 mph. Five deaths were blamed on the weather.

FILE- Wind fueled fires burns in a pasture which was part of a fire that burned and stretched across Ellis, Russell, Osborne and Rooks counties, Dec. 16, 2021, near Natoma, Kan. The National Weather Service has declared the series of thunderstorms and tornadoes that swept across the Great Plains and upper Midwest on Dec. 15 as a serial derecho, a rare event featuring a very lengthy and wide line of storms. The service said it was the first-ever serial derecho in December in the U.S. (AP Photo/Charlie Riedel)

Most of those deaths happened in Kansas as the storm produced high wind, blowing dust, and wildfires. Two men were killed in a crash caused by poor visibility and dust in Grant County. Two other men died from injuries suffered in wildfires. Richard Shimanek, 84, a farmer and rancher who lived near Leoti, died Thursday night at a hospital in Denver, Leoti Mayor and Fire Chief Charlie Hughes said. He was outside his home trying to fight the fire Wednesday when he fell and couldn’t get up, Hughes said. The Ellis County Sheriff’s Office said Friday that the remains of Derrick Kelley, 36, were found near his burned vehicle in a rural area of the county.

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.

“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.”

A derecho is often described as an inland hurricane. But, it has no eye and its winds come across in a line. The similarity is in the damage, which is likely to spread over a wide area, unlike a tornado where the damage is more spotty.

The weather service said a wind damage swath extending more than 240 miles with wind gusts of at least 58 mph along most of its length can be classified as a derecho.

A derecho was also declared in August 2020 when storms packing 100 mph 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 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.

The unprecedented December warm spell included temperatures that rose to 70 degrees Fahrenheit as far north as Wisconsin, creating evening temperatures that weather historian Chris Burt compared to that of a “warm July evening.”