Statistical Models for Tornado Climatology: Long and Short-Term Views

Abstract

This paper estimates regional tornado risk from records of past events using statistical models. First, a spatial model is fit to the tornado counts aggregated in counties with terms that control for changes in observational practices over time. Results provide a long-term view of risk that delineates the main tornado corridors in the United States where the expected annual rate exceeds two tornadoes per 10,000 square km. A few counties in the Texas Panhandle and central Kansas have annual rates that exceed four tornadoes per 10,000 square km. Refitting the model after removing the least damaging tornadoes from the data (EF0) produces a similar map but with the greatest tornado risk shifted south and eastward. Second, a space-time model is fit to the counts aggregated in raster cells with terms that control for changes in climate factors. Results provide a short-term view of risk. The short-term view identifies a shift of tornado activity away from the Ohio Valley under El Niño conditions and away from the Southeast under positive North Atlantic oscillation conditions. The combined predictor effects on the local rates is quantified by fitting the model after leaving out the year to be predicted from the data. The models provide state-of-the-art views of tornado risk that can be used by government agencies, the insurance industry, and the general public.

An Analysis of Anomalous Winter and Spring Tornado Frequency by Phase of the El Niño/Southern Oscillation, the Global Wind Oscillation, and the Madden-Julian Oscillation

“…Seasonal tornado counts were generated for winter (December of previous year, January, and February (DJF)) and spring (March, April, and May (MAM)). DJF and MAM are the focus of this study because previous studies illustrate that tornado counts and ENSO are related in these seasons [12][13][14][17][18][19].…”

“…Cook and Schaefer [16] later reported that winter tornado outbreaks were stronger and more frequent during the neutral phase (N) of ENSO, followed by the LN then EN phases. e most recent studies report that tornado activity is heightened during the LN phase [12][13][14][17][18][19]. Recent studies also illustrate that the seasons with the most tornadoes tend to be classified as LN.…”

“…Due to the danger and variability of tornado activity, there is a need to improve seasonal outlook capabilities. One approach is to identify relationships between seasonal tornado count and climate oscillations, such as the El Niño/Southern Oscillation (ENSO) (e.g., [11][12][13][14][15][16][17][18][19]). …”

“…Elsner et al [18] developed a statistical model to evaluate variability in tornado activity in which ENSO was the most important predictor. eir model also indicated that tornado activity was heightened in the Midwest and Southeast regions of the US during LN and in the Great Plains region during EN.…”

“…Lee et al [19], for example, noted that an anomalously large number of tornadoes occurred in the 2015-2016 EN winter. Elsner et al [18] also noted the occurrence of anomalous seasons and attributed them to additional unknown factors. While previous studies have noted these anomalous seasons, none have provided detailed descriptions and analyses of them.…”