Four Odd Tornado Seasons in a Row for Illinois

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Click to enlarge. Source NOAA Storm Prediction Center.

So far the 2015 tornado season in Illinois and the rest of tornado alley is incredibly quiet. There are no tornadoes to report this year, except in the far Southeast and one in California. However, this quiet start is no reason to relax if the past few years are a guide.

Historically, the heart of the Illinois tornado season is March to June with two-thirds of our tornadoes occurring during those months. However, in the last few years, we have had more tornadoes occur outside of this period than inside.

And a lot of these tornadoes have been concentrated in just a few days of the year. In fact, 69 percent of the tornadoes in 2012-14 occurred on just 5 days. These high concentrations can put extra strain on forecasting, warning, and recovery operations.

Here is how 2012, 2013, and 2014 looked, compared to the historical averages.

2012

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Click to enlarge.

Drought was the big story in 2012. Typically, drought years tend to have lower tornado counts – fewer thunderstorms leads to fewer tornadoes. Out of a total of only 15 reports in 2012, 7 of those occurred on February 29 (a leap year). As the drought became widespread in March through August, the tornado count plummeted.

2013

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Click to enlarge.

This was a quiet year until the massive tornado outbreak of November 17. This is why a quiet tornado season can lead to a false sense of security. All it takes is one day with the right conditions for all that to change. In 2013, it was November 17. What’s especially odd about the November 17 outbreak is that it occurred in a colder-than-average November.

I have a whole series of blog posts on the November 17 outbreak.

2014

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Click to enlarge.

This was a moderate year with 35 tornado reports. However, 82 percent of those came on three days – February 20 (10), June 30 (10), and October 13 (8). As in the other years, two of these outbreaks occurred outside of the traditional tornado season.

2012 to 2014

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Click to enlarge.
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Historical distribution of tornadoes by month in Illinois. Click to enlarge.

If we combine 2012, 2013, and 2014, we find that the distribution of tornadoes by month does not match our idea of the tornado season. In fact, the busiest month by far was November with February in second place, and October in fourth place.

Is this a signal of climate change? It is hard to say at this point because it is too short of a record to draw conclusions. In addition, this analysis is just for Illinois. Changes in Illinois could be offset by changes elsewhere in the US. I plan on going to a talk this afternoon to hear Dr. Harold Brooks discuss this very topic. I will report back with what he says.

Even if the change is short-term, the need to stay vigilante for tornado watches and warnings, regardless of the time of year, is important.

Notes on the analysis: I used the EF1 to EF5 tornadoes in the 2012-14 analysis. EF-0 tornadoes are small and more elusive to track down. Also, I used the NOAA storm database at http://www.ncdc.noaa.gov/stormevents/ which may give slightly difference totals than NWS reports.

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