Long term trends in weather patterns that favor tornado occurrence

What is it about?

This study considers atmospheric conditions, or weather patterns, that support tornado formation in four regions of the Central, Midwestern, and Southeastern United States. In it, we distinguish several weather patterns (or predictors) that are associated with tornado occurrence and tornado severity. Examples include vertical wind shear, atmospheric instability, a dry line, the position of the jet stream, and moist low levels. Additionally, we found that a coupled (i.e. two) jet stream pattern configuration was a good predictor in the southeastern and western midwest regions; such a link between a coupled jet stream pattern and midwestern US tornadoes is a new finding. Another novel finding is a stratospheric forcing influence, which is characterized by a warm, dry stratosphere and cold troposphere. This stratospheric forcing predictor has not been studied previously regarding tornado outbreaks. Lastly, we look at how these predictors have changed over time from 1980-2017, and how that would impact tornado risk in the four regions. For example, the stratospheric forcing mode has been decreasing in prevalence with time, which by itself, would mean fewer tornadoes. At the same time, the dry line mode has been increasing in prevalence in all four regions, which by itself would contribute to more tornadoes. We look at the changes in weather patterns in each region and how that would affect tornado risk there. The main summary is that we see no change in tornado risk in the central and southeastern US, a slight decrease in risk in western Midwest states, and an increase in risk eastern Midwest states (e.g., Illinois and Indiana). The increase in tornado risk in the eastern Midwest is related to an increasing prevalence of dry lines in that region while other factors remain steady or do not affect the tornado risk there. If this trend continues, then we would continue to see more tornadoes in the eastern Midwest region while other regions will see a similar risk or a slight decrease in risk.

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Photo by Lucy Chian on Unsplash

Photo by Lucy Chian on Unsplash

Why is it important?

Since we've identified the predictors or "forcings" that can contribute to tornado occurrence, we can continue to study how these forcings will change with time to understand how tornado risk will change in the future.