You've probably heard that September 10 is the peak of Atlantic hurricane season. But tornado season also has a peak, according to a new analysis by AccuWeather.
Published Apr. 21, 2022 12:06 PM EDT | Updated Apr. 21, 2022 1:23 PM EDT
You have probably heard that Sept. 10 is the peak of Atlantic hurricane season. But what about tornado season in the United States? Analysis by AccuWeather’s data science team shows a particular day but also additional seasonal peaks.
Spring marks the beginning of warmth and changing conditions across the country, but it also means the danger starts to ramp up for severe weather and tornadoes.
The National Weather Service (NWS) has kept a database of tornado reports since 1950, which gives a 70-year view of when tornadoes occur in the United States.
Half the tornadoes occur in just 3 months
There are three months of the year in which more than half of all U.S. tornado reports have occurred. Why? Because the ingredients for powerful storms and large twisters more frequently come together during these months.
A tornado in Campo, Colorado (Getty Images)
Although there are significant daily spikes in April due to major tornado outbreaks that often occur during that month, the busiest month for tornado activity is May when the U.S. gets 22% of its yearly total. The month of May also features the seven-day average peak of tornado season. This peak occurs between May 19 and May 26. About 6% of all tornado reports on record have occurred during this seven-day period.
The single date with the most tornado reports is May 25
May 25 is the day with the highest number of reports, featuring 649 tornadoes between 1950 and 2020. That date could change in the future when newer data is added to the Storm Prediction Center's (SPC) database. At least one tornado has been reported on May 25 every year since 2004.
In addition, multiple EF5 tornadoes, the strongest on the Enhanced Fujita Scale, have occurred on this date, most recently in 2008 when an EF5 tornado tore a 43-mile-long path through Iowa, injuring 70 and killing six, according to the National Weather Service.
After June, tornado frequency begins to decrease through the beginning of August. This is a false comfort, however, because another peak is coming.
In the middle of September, a slight peak of tornadoes occurs
This is likely due to landfalling tropical storms and hurricanes, which are most common in September. Tropical storms and hurricanes can spin up their own twisters near and after landfall. Hurricane Frances and Hurricane Ivan dropped more than 200 tornadoes in a total of seven days in September 2004.
Frances hit the east coast of Florida but caused tornadoes all the way up the coast to Maryland. Ivan hit the Florida Panhandle, spawning tornadoes all the way up to Pennsylvania.
A larger peak of tornadoes in mid- to late November heralds the 'second' severe weather season
Meteorologists have long warned about this second severe weather season. The 1,499 tornadoes that have been recorded between Nov. 15 and 27 are almost as many as the entire month of December, which has produced 1,586 twisters. More specifically, Nov. 15 to 17 was a particularly bad three-day period in history, with 580 tornadoes. Major outbreaks in 1989, 2005 and 2015 contributed to these numbers.
AccuWeather Chief Meteorologist Jonathan Porter explained the reason for the uptick in fall tornadoes. "As the jet stream dips farther south -- and once again jet stream disturbances become stronger with increasing temperature variations across the country," he said, "the remaining late-summer warmth clashes with incoming chillier air from the north."
After a tornado the day before, family members and friends help salvage and clean up what they can Thursday, March 1, 2012, in Harrisburg, Ill. (AP Photo/Seth Perlman)
After the fall peak, the number of tornadoes per day stays low until the middle of December, when a smaller peak of twisters rises, with the worst three-day period being Dec. 23 to Christmas Day. Major tornado outbreaks in 1982 and 2015 made these days stand out.
Although last year's data has not been incorporated into the National Weather Service's database for this analysis, the Dec. 10 to 11 tornado outbreak, plus another on Dec. 15, which resulted in more than 160 tornadoes, including one of the longest in history. These numbers will likely cause the mid-December peak to start a few days earlier than this data shows.
Remarkably, at least 10 tornadoes were recorded on every date of the year
Tornadoes have happened on every date of the year. This even includes Feb. 29 -- a leap day that occurs only every four years. On Feb. 28-29, 2012, a tornado outbreak in the Midwest resulted in 35 tornadoes, including a deadly EF4 twister that killed eight people in Illinois.
"No day of the year is zero – which is why AccuWeather meteorologists always stress that with the right conditions, tornadoes are possible any day of the year almost anywhere in the country," Porter added.
A home sits destroyed by a tornado May 26, 2008, in Parkersburg, Iowa. Several storms throughout the Midwest produced tornadoes in Iowa and Minnesota on May 25, 2008. (Photo by Steve Pope/Getty Images)
"It is critically important for people and businesses to know how they are going to react to move to safe shelter if a tornado warning is issued, regardless of the day of the year. Often times tornadoes occurring outside of what people perceive as the typical season and places can result in more injuries or fatalities because people may be less prepared and ready to react."
Statistically speaking, the least likely date on which to see a tornado is Jan. 16
The lowest number of tornadoes falls on Jan. 16, but that doesn't mean you'll be safe from danger that day -- or that month. Only 11 tornadoes were reported on Jan. 16 during the 70-year period. However, less than a week later there is a two-day period with more tornadoes than any week of January. This is due mainly to major tornado outbreaks that occurred in the South in 1999 and 2017.
Until 2016, the strongest tornado to occur on Jan. 16 was an EF1 which carved a one-mile path through southeastern Washington in 2000. On Jan 17, 2016, two EF2 tornadoes touched down in western Florida, causing six injuries and two deaths. These are the only tornadoes to occur on this date to cause injuries or deaths.
A 5.5-inch-diameter hailstone was found in Salado, Texas, on the afternoon of April 12, 2022.
Tornadoes aren't the only severe weather threat that people need to be aware of
Looking at a wider definition of severe weather exposes another thunderstorm threat with a similar timeline. Hail reports also peak between April and June. About 58% of all hail reports in the United States occur during this three-month span, peaking in May.
The seven-day period with the most hail reports is May 24 to May 31, which is just five days later than the period most common for tornadoes. The peak for hail reports and tornado reports overlap three days: May 24, 25 and 26. With both tornado and hail reports peaking during this span, these three dates are historically the most dangerous days of the year for severe weather in the United States.
Since 1950, those three days have produced a total of 1,822 tornado reports with 1,809 total injuries and 173 fatalities, according to the Tornado Archive. Four EF5 tornadoes have occurred during this period, the most recent causing nearly 200 injuries in the Oklahoma City area on May 24, 2011.
Is climate change altering the tornado season?
Porter said that climate change impact is one factor driving tornadoes occurring outside of what people think of as the usual tornado season -- as evidenced by the tornadoes occurring in New York and southern New England in November 2021 and across the central and southern parts of the U.S. in December 2021 as well as Iowa in February 2022.
What about 'significant' tornadoes?
Before the widespread use of NEXRAD radar in the early 1990s, it is possible that many weak tornadoes went unreported, so it's possible the assumptions outlined above are biased toward tornadoes from the last 30 years. The SPC defines "significant tornadoes" as those rating EF2 to EF5 on the Enhanced Fujita Scale. In the same time period, only 19% of tornadoes ranked as significant.
Graphing only significant tornadoes, the same seasonal patterns are seen, but because there are fewer tornadoes overall, major tornado outbreaks stand out more, most notably April 3, due to the "Super Outbreak" in 1974, which ranks as the highest date.
January 16 is the only day of the year not to have a significant tornado reported. As with all tornadoes, a significant peak still occurs from late April into early June. Because tropical systems generally produce weak tornadoes, the peak in September is decreased.
"While the innovative analysis by AccuWeather’s data science team shows the peak of the tornado season in the U.S. to be May 25, as expected, it consistently shows another uptick in the fall, particularly noticeable when you look at the graph of significant tornadoes," Porter said. He added that this data analysis underscores a point he often finds himself reiterating: "It is critically important for people and businesses to know how they are going to react to move to safe shelter if a tornado warning is issued -- regardless of the day of the year."
Wind damage reports were not considered for this analysis because the SPC does not delineate between thunderstorm damage and non-thunderstorm damage or between wind damage and wind speed reports. Tornado reports before 1950 were not considered due to a bias for stronger tornadoes to have been reported.
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