Steven Weinstein
Dr. Jana Houser
Recently, several studies have sought to answer the question of whether or not there has been an eastward shift seen in tornado activity from the traditional “tornado alley” states to portions of the Southeast and Midwestern U.S (e.g. Agee et al, 2016; Gensini and Brooks, 2018). While these studies have provided basic evidence for this eastward shift in tornado activity and frequency, the studies have not fully explored the synoptic drivers causing this shift. Indeed, some studies such as (Agee et al, 2016) have linked the shift in tornado activity to changing atmospheric thermodynamics by analyzing climate models. However, while this same study had found increasing CAPE across the Southeastern U.S. by climate model simulations, this was negated by contradictory counter evidence suggesting decreasing low level shear across this same region by climate models. While thermodynamics and climate change could have potential links to the shift, there are many factors and limitations to consider with these parameters that goes beyond the scope of this study.
This study specifically examines all tornado outbreaks (defined as 10 or more (E)F1 or stronger tornadoes following Galway (1977), over a geographically contiguous region) east of the Rocky Mountains during two time periods (1978-1988 and 2007-2017) and compares them to first provide evidence of this eastward geographic shift most recently seen over the time scales specified. In addition to proving this geographic shift, this study also seeks to answer the question of whether this shift can be correlated to a change in kinematic atmospheric patterns. These kinematic patterns are most often the main drivers of storms and severe storms and include variables such as upper level trough and surface low placements. The average trough and surface low coordinates (latitude/longitude) of these outbreak events are compared between the two time periods, which allows us to see if kinematic atmospheric patterns have been changing with time in correlation with the geographic shift in tornado activity. The recent increase in tornado reports over the observational record is sufficiently accounted for by acquiring a single representative latitude and longitude point for each outbreak, rather than looking at every tornado report over the two periods. Furthermore, the statistical significance of any changes in locations between the two periods is determined through the use of T-tests.
Agee, E., J. Larson, S. Childs, and A. Marmo, 2016: Spatial Redistribution of U.S. Tornado Activity between 1954 and 2013. J. Appl. Meteor. Climatol., 55, 1681–1697, https://doi.org/10.1175/JAMC-D-15-0342.1.
Gensini, V. A., & Brooks, H. E. (2018, October 17). Spatial trends in United States tornado frequency. Retrieved from https://www.nature.com/articles/s41612-018-0048-2.
Galway, J.G., 1977: Some Climatological Aspects of Tornado Outbreaks. Mon. Wea. Rev., 105, 477–484, https://doi.org/10.1175/1520-0493(1977)105<0477:SCAOTO>2.0.CO;2.
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