Experts say December was 'ridiculously active' month for tornadoes in U.S.

Dec. 25 -- There's no question that December has been a month that's featured a host of anomalous weather, including a historic tornado outbreak and record-breaking temperatures, along with the first-ever December derecho.

There certainly has been no shortage of extreme weather.

"There is a high likelihood that December 2021 will end up as being the most prolific tornado-producing December on record once final damage surveys are completed," AccuWeather Forecaster Randy Adkins said.

While tornadoes aren't uncommon during December, the likelihood of a tornado occurring on a December day in the United States is 12-15%, which is significantly lower than the 90% chance of a tornado occurring on a day in July, according to NOAA. The three-year average number of tornadoes in December is 47.

The U.S. has experienced a handful of tornado outbreaks in December. The most tornadoes ever recorded for the month occurred in 1982 (100). On Christmas Day in 2012, 29 tornadoes struck from eastern Texas to Alabama, two of which were classified as EF3 storms. More recently in 2015, there were 93 tornado reports for the month after multiple severe weather events devastated the South.

The SPC listed 80 preliminary tornadoes through Dec. 14, and that was ahead of the second tornado outbreak of the month. In comparison, only 73 preliminary tornado reports were filed during April 2021. April is notoriously the second-most active month in terms of recorded tornadoes, with a three-year average of 224 tornadoes.

Adkins said that the difference has a lot to do with December being "ridiculously active" and April being "less active than normal."

Prior to December outbreaks, the number of tornadoes that touched down in the U.S. was pacing below the 10-year annual average of 1,251, according to SPC data. But after the unusual tornado outbreaks this month, the number of twisters has climbed closer to the annual average with a preliminary report of 1,375 through Dec. 21.

The abundance of severe weather this month is primarily due to the record-breaking warmth across the country. The Plains and Mississippi Valley recorded temperatures 8 to 12 degrees Fahrenheit above average while other areas shattered temperature records.

The West Coast was the only portion of the lower 48 states that recorded below-average temperatures through the first 21 days of the month, according to Adkins.

A stronger-than-normal jet stream has also increased the amount of severe weather across the country this month. This stronger jet stream is a result of La Nina this year, but Adkins said that it isn't a "casual relationship," adding that La Nina or El Nino don't independently guarantee a stronger or weaker jet stream.

Thus, the combination of higher-than-average temperatures, ample moisture from the Gulf of Mexico and a powerful jet stream over the center of the country has fueled these historic severe weather events.

The first of the two historic severe weather events this month occurred in the late evening hours of Dec. 10 and continued into the early hours of Dec. 11. According to preliminary reports from the National Weather Service, 66 tornadoes touched down that night.

AccuWeather Director of Forecast Operation Dan DePodwin referred to the outbreak as a "very rare situation," noting that severe weather during this time of year is "typically confined to the Gulf Coast region."

Tornado outbreaks of this caliber typically occur between February and June, but a powerful low-pressure area and unusually warm air are what drove this outbreak to occur.

Preliminary NWS damage surveys found that two of the more than 60 tornadoes that occurred that night will go down in history books as the two longest tornado paths ever recorded during December, with path lengths of 165.7 and 122.7 miles. The old record was a 121.7-mile path and was set back in 1956.

Officials say recovery will be a long road ahead for the tornado survivors who are just starting to pick up the pieces.

Days after the tornado outbreak, on Dec. 15, another historic severe weather event, which produced the first December derecho on record and was blamed for killing at least five, ravaged parts of the Midwest and upper Great Lakes regions.

This rare December derecho was classified as a "serial derecho," which is different from the progressive derecho that many people are familiar with.

Adkins said the "difference has to do with [the] organization" of the storm system.

Serial derechos have several damaging line segments within a massive squall line of thunderstorms that sometimes extend for hundreds of miles. Adkins said these are often associated with strong low-pressure centers and a vigorous jet stream overhead.

In contrast, progressive derechos are often characterized by one to two powerful lines of thunderstorms, much smaller in length that travel quickly along a stalled frontal boundary.

"At the end of the day, the distinction doesn't matter to anyone in the path of these phenomena," Adkins said.

As the storm continued to move eastward, the first-ever December tornado touched down in Minnesota. Dozens of other tornadoes were reported across Iowa and Nebraska, according to the NWS.

Another round of severe weather moved across southern Florida, where the NWS confirmed an EF1 tornado, on Tuesday, adding to the overall tornado count for the month.

All tornado reports included in this article are preliminary as crews are still assessing damage in many places. Comprehensive tornado surveys can take days, weeks or even months to conduct.

According to AccuWeather forecasters, there aren't any strong signals for widespread severe weather through the end of the year, but they warn that the weather pattern could become active after Christmas across the South.