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Weather is constantly changing due to an evolving climate. Meteorological trends throughout the 21st century have pointed toward more variability in severe weather, specifically for tornadoes. For example, there has not been a year without a 32-tornado day since 2001. Thus, the frequency of big tornado days (a day where at least eight events occurred) has been increasing. In addition, a 2018 study published by npj Climate and Atmospheric Sciences explicitly provides data supporting this shift in frequency from Tornado Alley to Dixie Alley. That statistical analysis determined a significant downward trend in occurrences across the central and southern Great Plains and an upward trend in portions of the Southeast, Midwest, and Northeast.
Decades of Tornado Patterns
In a 2016 study published in the Journal of Geophysical Research: Atmospheres examined the U.S. tornado frequency and location variability for EF1+ events on a continental and regional scale using 1950-2013 data from the Storm Prediction Center (SPC). On a continental level, statistics show a slightly increasing trend in the frequency of tornado occurrences since the 1950s. On a regional level, about one-third of the continental United States demonstrated an increasing trend in the frequency of tornado occurrences. The other two-thirds showed a decreasing or flat trend. Regionally, most southeastern states displayed a consistent increase in event frequency. States in the West, Great Plains, and Midwest mainly exhibited no trends or decreasing trends, while states in the Northeast represented a fluctuating pattern.
The data points to an increasing trend in the frequency of tornadoes and an eastward shift in the location where a higher number of events are occurring.
In a similar study published in Climate Dynamics in 2014, the authors used SPC data to analyze the annual number of tornadoes occurring in the United States and the number of days at least one tornado occurred between 1950 and 2013 for EF1 and stronger events. Comparing trends in the number of tornado days sheds light on variability from a different perspective. The results depicted a downward trend in the number of tornado days (defined by at least four events) but an upward trend in the tornado days for those represented by at least 8, 16, and 32 events.
Often called “Tornado Alley,” the Great Plains is the U.S. region that has been widely known for having the most tornado occurrences. This assumption has been held for a long time, but scientific evidence like the ones mentioned above plus more recent studies suggests a new region is taking the lead. An area in the southeastern United States called “Dixie Alley” shows increasing occurrences of strong tornadoes. The data points to an increasing trend in the frequency of tornadoes and an eastward shift in the location where a higher number of events are occurring. However, more mapping like the one depicted in Figure 4 of the 2018 study is needed to see the ongoing migration of this threat.
Adapting to Climatological Changes
Understanding these climatological changes is critical for emergency planners when preparing for future threats. Communities may not be ready for the impact of tornadoes if their localities have not commonly experienced them before. Consider that all 10 of the top 10 costliest U.S. disasters involving tornadoes impacted at least one state outside Tornado Alley, and 7 of those involved a state outside Dixie Alley as well. The 34 states on this top-10 list highlight the importance of emergency planners preparing for these catastrophic events even if their communities are not within traditional high-risk areas. Risks and hazards can shift with or without warning.
Much evidence points to the shifting frequency of occurrences from Tornado Alley to Dixie Alley. As such, the states in and around the Dixie Alley region should reassess their state and local hazard mitigation plans to ensure that occurrence rates and probabilities for tornado risk and impacts are accurately addressed. Threat and Hazard Identification Risk Assessments (THIRA) would benefit by providing more specific thresholds of threat based on meteorological datasets. For example, if a state or locality had <1 tornado per month then it is low risk, 2-10 tornadoes per month is medium risk, and >10 tornadoes per month is high risk. This means a planner would need to physically access the data to determine their risk levels and coordinate with meteorologists to make this data more accessible.
A house that was heavily damaged by the EF2 tornado near Blum, Texas (Source: NWS Fort Worth, May 4, 2021).
Additionally, the evidence presented above implied that larger tornado outbreaks could occur in a single day. Emergency planners and responders may need to prepare for more extensive responses and recovery operations. Although there are no publicly available databases that specifically track shifts in tornado tracks, since tornado databases themselves are not widely available, the following resources can provide valuable tornado data to emergency planners:
- National Oceanic and Atmospheric Administration’s (NOAA) Annual Tornado Report
- National Weather Service (NWS), NOAA, and Mississippi State’s Tornado Database
- Federal Emergency Management Agency’s Resilience Analysis and Planning Tool (RAPT)
The importance for emergency planners located in and around the Dixie Alley region to evaluate their current hazard mitigation plans and accommodate the changing tornado climate is just one example of where change is needed. In any jurisdiction, environmental changes can lead to increased natural hazards (e.g., floods, fires) that may not have been included or updated on hazard mitigation plans. Overall, emergency planners must take the time to speak extensively with meteorologists and review the data for severe weather changes and patterns. It is time to keep pushing forward with change, which is necessary for saving lives in an evolving climate.