The burst of activity occurred at a time often resistant to vigorous thunderstorm development and is a conspicuous statistic in a year generally below average in tornadic activity.
It was a tornado-filled October
A preliminary count pegs the October tornado count at 119, according to the National Weather Service Storm Prediction Center. This is about double the long-term average for the month. If the number stands, this October will rank second-highest for tornado touchdowns during the month, behind the 123 in October 2018. It is certainly possible that the October 2021 number will rise after all of the month’s reports have been evaluated.
Oklahoma, often a tornado magnet, more than doubled its October tornado count in 2021.
Before the October onslaught, the year was atypically quiet across the Sooner State. Just 25 tornadoes had occurred. But 31 tornadoes occurred in four days during the month, more than doubling the yearly count and bringing the state close to its annual average.
In addition to the Southern Plains hot spot, tornadoes scoured the landscape across the northern Gulf Coast and Missouri, and as far north as North Dakota and Pennsylvania. Several tornadoes were strong or intense, rating 2 or 3 on the Enhanced Fujita (EF) scale for damage.
In an outbreak that would be unusual any time of year, a remarkable 19 tornadoes touched down near the border of Ohio and Pennsylvania on Oct. 22. The Weather Service office in Cleveland issued the most tornado warnings it had put out in one day since 2005.
An outbreak that struck Missouri on Oct. 24 was of considerable intensity, with two tornadoes rated EF3.
A twister that hit Mississippi on Oct. 27 killed one person, the state’s first October tornado fatality in seven years. But that was the lone tornado death nationally for the month, a remarkable outcome given the number of powerful twisters, some of which struck populated areas at night.
Autumn tornadoes and the ‘second season’
Tornadoes require a mixture of heat energy and atmospheric spin to form. As the sun’s warmth steadily wanes through autumn, so does this environmental energy, generally limiting the opportunity for twisters to spin up. This declining support leads to tornado counts typically lower than in any of the spring months.
Nonetheless, October can be considered part of what is casually known as the “second season” for severe weather. Outbreaks do happen but usually are of limited scope.
Much like the spring, the jet stream, the tempestuous river of air at high altitudes, acts as a catalyst for severe weather. As it returns to the Lower 48 states after retreating north for the summer, its strong winds drive atmospheric spin, which can incite tornadoes when other ingredients are present.
The sun’s energy in October is less than it is in spring. But when warm, humid air from the Gulf of Mexico is drawn northward and interacts with the roaring autumn jet stream, serious and sustained volleys of twisters can occur.
The pattern that drove October’s tornado tally
The prevailing October weather pattern was similar to prior active Octobers for tornadoes. It featured numerous storm systems crossing the country.
“Part of the reason for the active October was a favorable jet stream pattern combined with a lack of early season cold air outbreaks,” wrote Matt Elliott, warning coordination meteorologist for the Weather Service’s Storm Prediction Center, in an email. It “helped to ‘set the table’ for several days where severe thunderstorms developed.”
Major drivers included a significant and persistent zone of low pressure near the Gulf of Alaska that extended into the Pacific Northwest. It forced the jet stream to drive south toward the Pacific Northwest, ejecting waves across the Lower 48 states for much of October.
At the same time, the circulation around high pressure anchored over eastern North America and the western Atlantic helped draw warm, humid air northward over the eastern two-thirds of the country.
“The lack of any significant cold air intrusion kept near surface temperatures warmer … contributing to a more favorable tornado environment,” Elliott wrote.
Where 2021 stands after a record month
Despite the historic October tornado activity, the national tornado count for 2021 remains on track to be below average.
That outlook is largely due to an April, May and June that featured far-below-average numbers. These months, which typically accumulate a significant majority of tornado incidents in a given year, were marked by persistent weather patterns that were unfavorable for widespread and vigorous thunderstorms.
2021 has also featured a scarcity of deadly violent (EF4 or higher) tornadoes. October, despite the high number of tornadoes, brought none this strong; only one has occurred this year nationally. This year’s relatively low tornado death count of 14 in the United States is due in large part to this lack of violence, in addition to the advance warnings from the Weather Service and its partners.
The year is on track for a total of violent tornadoes that is lower than almost all years in the modern record, aside from 2005 and 2018, which had one and zero, respectively.
Top-tier EF5 tornadoes, meanwhile, have been absent, with a record-long eight-year gap since the last U.S. tornado of that intensity was observed.
Elliot said the active October for tornadoes probably won’t have any bearing on the outlook for the months ahead.
“Although October 2021 has been relatively active for severe weather, it is hard to predict how active the remainder of fall and winter will be,” he wrote.
Jacob Feuerstein is a college junior studying meteorology at Cornell University.