Tornadoes and fires and snow – oh my! Wild December closes out a 2021 of extreme weather
A December of violent storms and record snowfall put a final exclamation point on a wild weather year that was 2021 in the U.S.
The month produced the year’s two biggest tornado outbreaks and the most tornadoes on record for December. And there were twice as many severe weather reports issued than in any December since the National Weather Service started keeping track 21 years ago.
January also started in a record-setting fashion. Dozens of weather stations broke warm temperature records for Jan. 1, and heavy snowstorms have blanketed parts of the eastern U.S.
However, a look back at December illustrates how scientists say the warming climate influences long-term weather trends.
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It was "a relentless end to a relentless year for weather disasters in the United States," said meteorologist Steven Bowen, managing director of Catastrophe Insight at Aon, which provides analysis of natural perils, climate, socioeconomics and risk. "It will mark the costliest December on record for the U.S. insurance industry."
Chief among the events was a Dec. 10-11 tornado outbreak across eight states that claimed 90 lives, and a massive wildfire in Boulder County, Colorado, that damaged or destroyed nearly 1,000 homes.
At least 66 tornadoes were confirmed in that Dec. 10-11 outbreak, including a tornado that traveled 166 miles through western Kentucky.
An outbreak on Dec. 15 resulted in 100 tornadoes, including 43 in Iowa, the most ever recorded there in a single day. Minnesota had never had a recorded tornado in December. On the 15th, it had 20.
The weather service confirmed at least 200 tornadoes in December, double the previous record for the month – 97 in 2002. That was more tornadoes in a single month than ever recorded in the U.S. between October and December, dating back to 1950.
"Crossing the 200 tornado threshold is a big deal for any month, let alone December," Bowen wrote in an email. "The word 'unprecedented' gets thrown around too flippantly, but given what happened in December 2021, it’s entirely appropriate."
The tornadoes were well outside the typically busiest months of April through July. Tornado experts said it’s part of a trend in tornado and other convective thunderstorm activity expanding across the calendar. Tornadoes are occurring more frequently both earlier and later in the year.
Several states – including Tennessee, Kentucky, Nebraska, and Wisconsin – had more confirmed tornadoes last month than in all Decembers combined from 1950-2020, Bowen said.
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Between the tornadoes and other extreme weather, the Storm Prediction Center totaled 1,261 severe weather reports across the nation. Of those, 521 were issued on Dec. 15, the single greatest number on any day all year. High winds that day included 64 hurricane force wind gusts, breaking a record set in August 2020.
December also saw unseasonably warm weather across the nation. More than 1,900 record high and record warm low temperatures were set. Across the South, for example, new monthly records were set in Macon, Georgia; Jackson, Mississippi; and Houston and Austin, Texas.
Scientists put much of the blame for the stormier weather and extreme rainfall on the warmer water and air temperatures that feed more moisture into storms, producing heavier precipitation around the globe.
A La Nina in the Pacific Ocean also is in the mix this winter. The large-scale cooling in surface waters near the Equator alters atmospheric circulation patterns. It can set up cooler and stormier conditions in the Pacific Northwest, and warmer and drier conditions across the South.
In California, the Sierra Nevada saw its snowiest month on record in December. The University of California Berkeley Central Sierra Snow Lab near Lake Tahoe measured 214 inches of snow, breaking a record set in 1970.
A similar phenomenon occurred in Alaska, when record high pressures in the North Pacific pushed warmer than normal winds and record moisture into the state, said climatologist Brian Brettschneider.
Kodiak, Alaska, smashed its previous record warm temperature by 20 degrees on Dec. 26. At the airport, the 65 degrees was the highest temperature recorded for any day between September 21 and April 27, said Rick Thoman, a climate specialist at the International Arctic Research Center.
At the tide gauge in Kodiak's harbor, the temperature hit 67, the warmest temperature ever recorded anywhere in Alaska in December.
It was so warm in Southwest Alaska, he said, that ice in some rivers started breaking up. Because those rivers typically freeze over and become the main transportation networks in the winter, he said, when they become unsafe, "that’s a big impact for that region."
Meanwhile, many locations in western and northern Alaska experienced record December precipitation. In Fairbanks, December rainfall was 5.73 inches — 2.5 inches above the record and 10 times the normal for the month. "It was far more than the previous record," Thoman said. "Really quite remarkable.”
Denali National Park set a new record for its snowiest December – 78 inches – and highest snow depth, the park reported.
“Certainly, December kind of capped a dramatic year for Alaska in terms of extreme precipitation events,” Thoman said. “This is just another one of the extreme precipitation events, which is exactly what we expect in a warming environment.”
Ultimately, the warming climate also is thought to have played a role in the Colorado wildfire.
After a really wet spring, the state endured an extremely warm and dry period starting in June, said state climatologist Russ Schumacher. When dry periods occur, they're made worse by warmer temperatures, and prolonged periods of warmer than average temperatures are becoming more likely.
Grasses grew vigorously during the wet spring, then dried out through the summer and fall, he said. When an intense windstorm arrived on Dec. 30 in an area already prone to those type of storms at that time of year, conditions were primed.
So, the fire wasn't caused by climate change, he said, "but it is part of the story."