4:30 PM CDT | March 30, 2019
A debate began last year on whether or not tornado alley has shifted eastward into states like Tennessee, Mississippi, Alabama... There has been quite a bit of misinformation spreading on the internet regarding tornado alley. The term "tornado alley" was originally coined by the media. The boundaries are ill-defined since there are a number of ways to define where this alley is located. Is it where the most tornadoes occur annually? Is it where the most violent tornadoes occur? If it is where the most number of tornadoes occur, do we break it down by states? By counties?
Dependable tornado records only go back to the mid-1900s. The 1970s is when the satellite era began, so it is understandable that there would be an increase in the years to follow. The NEXRAD (Next Generation Radar) WSR-88D network began in 1988 and was not completed until 1997. In 2013, all 159 NEXRAD radars across the United States were modernized with dual-polarization technology. Each advancement in satellite and radar allows more possible tornadoes to be detected, even those well away from cities. Unless a tornado was seen as it was occurring, it is certainly conceivable that a number of weaker spin-ups went unreported. A weak tornado that occurred in a rural field may have been written off as just wind damage. The Enhanced Fujita-scale, which is the scale used in determing the strength of a tornado, is based on damage. You can see how the scale is based here.
It is only if more substantial things (buildings, trees, etc.) are impacted, that possible tornado damage can be assessed. Obviously, if a violent tornado tracks through a country field there will be obvious signs, but the intensity cannot be refined unless something more substantial is impacted. A recent example of this would be the El Reno, OK tornado back in 2013. Mobile radar detected more significant winds than what was officially determined. This was because there was a lack of supporting evidence on the ground since it touched down in a rural area. There were no substantial structures damaged to support the higher wind speeds detected on radar.
I have attached a picture of what most people think of when they hear "tornado alley". Below this graphic, you can see where Northern Illinois University has noted a shift in the tornado frequency over recent decades. I am not disputing their findings. In fact, they note in their article that even the tornado parameters have shifted eastward. This would imply that it is not primarily a result of increasing population centers and their infrastructure. The fact remains, however, that the range of tornado data is still somewhat limited in the grand scheme of things.
Lastly, NOAA mentioned the following in a September 2017 State of the Science publication:
"Emerging evidence suggests natural climate variations like El Niņo and La Niņa events, and in particular the transition from one to another in spring, have the potential to alter the environment necessary for tornado formation. Natural variations acting on decadal timescales (e.g., Atlantic Multidecadal and Pacific Decadal Oscillations) have also been similarly implicated. However, uncertainty about their impact is high because the tornado record is too short to verify the impact of potential decadal influences. The link between these interannual-to-decadal natural climate variability modes and variations in tornado activity is typically via shifts in the largescale upper and lower level jet streams across North America that act as focusing mechanisms for severe thunderstorm development. Shifts in the placement and strength of these jet streams will influence the locations of atmospheric instability and wind shear that promote enhanced or reduced tornado activity."
In short, larger scale weather patterns have the potential to shift our "tornado alley". Yes, I do agree that there has been an increase to our east. This, however, does not mean that tornado alley as we know has permanently shifted, as some news outlets are suggesting. The traditional tornado alley states remain the leader in tornadoes, despite the recent decrease. Who is to say that we will not see it shift westward again in the 2020s? Time will tell... Just verify what you read on the internet. Be wary of news outlets that do not provide examples or cite their sources of information. Often times, these outlets have their own agenda and purposely leave out information from the original sources.
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