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A new December tornado record was smashed in the United States from two outbreaks in parts of the Midwest, Plains and South.

The National Weather Service has confirmed 165 combined tornadoes from the outbreaks of Dec. 10-11 and Dec. 15.

The NWS confirmed 66 tornadoes in the deadly Dec. 10-11 tornado outbreak, including a pair of long-track EF4 tornadoes from northeast Arkansas and Missouri's Bootheel into northwest Tennessee and Kentucky.

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Just five days later, 99 more tornadoes were spawned in an unusual December derecho in the Plains and upper Midwest.

Including other tornadoes during the month, 232 tornadoes were confirmed in December, according to NOAA's Storm Prediction Center, more than doubling the previous December record of 99 in 2002 (NOAA's records date to 1950).

December is typically the second-least-active month for tornadoes in the U.S. An average of 36 December tornadoes have been documented over the last 20 years through 2019.

Incredibly, more tornadoes occurred in December 2021 than an average April or June, typically two of the three most active months behind May.

Looking back at the most tornadic December prior to 2021, notice that the majority of those tornadoes were in the Deep South in 2002.

That's where December tornadoes are most common, from southern Missouri to the northern Gulf Coast.

In the upper Midwest, not so much.

The Dec. 15 outbreak spawned at least 20 tornadoes in Minnesota, where no December tornadoes had previously been documented since at least 1950.

Iowa's 44 tornadoes set a state record for any day, which had been 35 tornadoes on Aug. 31, 2014, according to the National Weather Service in Des Moines. That's almost as many tornadoes as the Hawkeye State has averaged each year (48 tornadoes) since 1980.

Why Were There So Many Tornadoes in a Winter Month?

First of all, it was one of the warmest Decembers on record in much of the eastern two-thirds of the country.

Instead of this warm air being trapped in the South as usual in December, it was drawn unusually far north, fueling both outbreaks.

New state December record highs were set in Iowa (78 degrees) and Wisconsin (72 degrees) ahead of the Dec. 15 derecho.

The Gulf of Mexico was also unusually warm for mid-December, adding its warmth and moisture to the air mass ahead of each outbreak.

That unusually warm air was topped by powerful winds from just 1,000 feet above the ground to jet-stream level, providing the wind shear needed for severe thunderstorms and tornadoes.

Did Climate Change Have a Role in This?

As usual, a warming planet may not cause a particular weather event, such as a tornado outbreak. The link between global climate change and small-scale phenomena such as tornadoes is still not well understood.

But these outbreaks are consistent with some recent research, including a 2019 study that found tornadoes are increasing in late fall and winter.

The monthly counts of tornadoes also are becoming more variable, with wide swings high and low.

In a Climate Central webinar in 2019, tornado researcher Harold Brooks from the National Severe Storms Laboratory said almost all monthly records – both high and low – for EF1 or higher tornadoes have happened since 2000.

Editor's note: Monthly tornado counts were adjusted in the article after new information was released by NOAA's Storm Prediction Center.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.