December, a month typically known for cold weather and snow, was instead marked this year by a historic barrage of violent thunderstorms and tornadoes.
Here we summarize some of the key facts and figures.
December severe weather at a glance
While records are too numerous to list in one spot, these key points demonstrate how unusual the month’s severe weather was.
- 163 tornadoes have been confirmed, the most on record in December by far and the second most in any winter month. The tornado count would be above average for any month other than April, May and June. The two main outbreaks alone were responsible for at least 145 of these tornadoes. The Weather Service has confirmed 66 tornadoes from Dec. 10 to 11 and 79 tornadoes Dec. 15.
- There have been twice as many strong tornadoes this December as in April, May and June this year. December is on average the month with the fewest tornadoes, while April, May and June average the most.
- Ninety people have been killed by tornadoes in December, which now ranks as the 10th-deadliest month for tornadoes since the modern keeping of records began in 1950.
- The severe thunderstorm complex that struck the Upper Midwest on Dec. 15 was the first derecho on record in the month, according to the National Weather Service, and it produced the most reports of wind gusts greater than 75 miles per hour of any known event.
- There have been tornadoes on eight days of the (still ongoing) month, including one Tuesday near Fort Myers, Fla.
The exceptional Dec. 10 tornado outbreak
The long tracks of the tornadoes that slashed across the zone from Arkansas to western Kentucky on Dec. 10-11 are atypical for any time of year. There is simply no precedent for such enduring twisters in December.
Two of the long-track tornadoes were spawned by a single rotating thunderstorm or supercell that crossed five states, covering 300 miles.
The first of the two tornadoes associated with that supercell ripped through northeast Arkansas, southeast Missouri and northwest Tennessee, becoming a rare ‘tri-state’ tornado, with a track length totaling just over 80 miles. The same supercell produced a second twister that was on the ground for an astonishing 166 miles, devastating Kentucky communities such as Mayfield and Dawson Springs.
Just to the south, another long-track supercell produced a tornado that trekked over 122 miles through northwest Tennessee and southwest Kentucky.
Since 1950, only one other day has seen two tornadoes with tracks at least that long: April 27, 2011.
Here are some other remarkable facts and figures about these long-track storms and the tornadoes they spawned:
- The 166-mile tornado that tracked through Mayfield became the longest since 1975, the longest in December and in the top 10 longest on record for any month. It’s also the longest in the era of Doppler radar, which began in 1988, surpassing the 149-mile length of a 2010 tornado that struck Yazoo City, Miss.
- The five-state supercell that tracked from northeast Arkansas through western Kentucky produced two tornadoes rated as EF4 on the zero-to-5 Enhanced Fujita scale for tornado intensity, the first December supercell to do so since 1957.
- The EF4 tornado that tracked through Mayfield produced wind speeds estimated at 190 mph, one of only four rated that intense since 2013.
- The preliminary death toll of 90 from the Dec. 10 outbreak exceeds the toll of many other modern tornado outbreaks and is unprecedented for the month. Before 2021, the deadliest known December tornado outbreak killed 47 in 1953.
- The Dec. 10 death toll was high for any time of year: It was the 11th-highest fatality count of any tornado outbreak at any time of year since 1950. Since 1974, only two 24-hour periods have seen more people killed by tornadoes than Dec. 10: The 2011 ‘Super Outbreak’ in the southeast, and the Joplin, Mo., tornado of May 22 that same year.
An unusually prolific derecho on Dec. 15
Severe thunderstorm complexes capable of producing hundreds of significant wind gusts, known as derechos, can cause damage similar to a low-end tornado over dozens of counties. These storm clusters tend to require wide regions of warm, moist air and organizing wind shear (changing winds with altitude), making them few in summer and quite rare in winter.
Newsworthy derechos in recent years have included one that struck Iowa in August 2020 and another that affected the Mid-Atlantic in June 2012. Both featured great amounts of atmospheric instability and were responsible for widespread damage. Those thunderstorm complexes respectively produced 53 and 37 reports of wind gusts that exceeded 75 miles per hour.
The derecho of Dec. 15 produced 63 such reports in the Upper Midwest, smashing previous records for any time of year. That it happened outside of the warm months, especially so far north, is astonishing.
Here's a look at yesterday's historic #derecho from the perspective of radar sites from #KSwx to #WIwx.— Jack Sillin (@JackSillin) December 16, 2021
As per this morning's preliminary count, the storm produced nearly 600 reports of wind damage including 54 "significant" 75mph+ gusts over a roughly 720mi path. pic.twitter.com/Lbb4uET8Td
The tally of tornadoes the derecho spawned also was remarkable. The count is up to at least 79, surpassing the Dec. 10-11 event. It brought 15 tornadoes to Minnesota; before, there were none in the modern record in that state during the December. Weather.com wrote that “Iowa and Nebraska each quintupled their past December tornado tallies.” Numerous tornadoes touched down in central and western Iowa, where they hadn’t previously in December dating back to at least 1950.
Climate change and cool-season tornadoes
As the Lower 48 moves into winter, cold and relatively dry conditions, along other with other factors, inhibit the fuel needed for storms. At least in theory.
This year, large portions of the Eastern United States are in the midst of one of the warmest Decembers on record, a phenomenon driven by a persistent pattern of high pressure in the east and low pressure in Canada. Along with warmth has come unusually springlike air masses.
When anomalous warmth combines with a powerful jet stream, wicked things may happen — as seen this month. We can’t totally blame climate change for any one event, especially when it comes to the formation of small-scale phenomena such as tornadoes, but we can see the impact of climate change in a broader sense.
Studies have shown that the most likely time for increased tornado activity in a warming climate is during the colder months of the year. In that sense, it’s certainly possible December 2021 is a harbinger of our future winters.