Land surface heterogeneity and tornado occurrence: an analysis of Tornado Alley and Dixie Alley

Figures & data

Figure 1. Tornado counts in the contiguous United States for storms occurring between 1950 and 2015. Counts have been aggregated per county and symbolized showing standard deviation from the mean. Source: Author

Figure 2. Getis-Ord $G_i^{\mathrm{*}}$ hotspot results and z-scores for tornado occurrences at the county level. Tornado Alley and Dixie Alley are delineated based on 95% confidence bounds. Source: Author.

Table 1. Observation totals for severe storms in Tornado Alley and Dixie Alley (2001–2015).

	Total severe storms	Wind/hail (non-tornadic) storms (% total storms)	All tornadoes (% total storms)	Weakly tornadic (EF0-EF1) storms (% of tornadoes)	Significantly tornadic (EF2-EF5) storms (% of tornadoes)
Tornado Alley	100,421	95,213 (94.8%)	5208 (5.2%)	4711 (90.5%)	497 (9.5%)
Dixie Alley	28,906	26,620 (92.1%)	2286 (7.9%)	1975 (86.4%)	311 (13.6%)

Figure 3. Land surface heterogeneity variables for Tornado Alley and Dixie Alley: (a) elevation, (b) slope, (c) aspect, (d) distance to major rivers, and (e) land cover (Devel. = Developed). Source: Author

Table 2. Logistic regression results for weakly tornadic supercells (EF0-EF1) in Tornado Alley versus Dixie Alley.

	Tornado Alley			Dixie Alley
Variable	β Coef.	Odds Ratio	Std. Error	β Coef.	Odds Ratio	Std. Error
Intercept	–2.787	0.062***	0.044	–1.958	0.141***	0.073
Elevation	0.018	1.018***	0.004	–0.299	0.742***	0.038
Slope	0.016	1.016*	0.008	–0.006	0.994	0.015
Distance to River	–0.009	0.991†	0.005	0.035	1.035**	0.013
Aspect
Flat	0.194	1.214	0.279	0.532	1.703.	0.306
East	–0.048	0.953	0.040	–0.075	0.928	0.065
North	0.005	1.005	0.046	–0.006	0.994	0.070
South	–0.107	0.898*	0.045	–0.015	0.985	0.065
Land Cover
Water	–0.146	0.864	0.162	–0.029	0.971	0.179
Developed	–1.491	0.225***	0.056	–0.790	0.454***	0.068
Barren	0.295	1.344	0.303	0.238	1.268	0.408
Forest	–0.141	0.868*	0.059	–0.222	0.801***	0.064
Shrubland	–0.003	0.994	0.070	–0.197	0.821*	0.098
Grassland	–0.045	0.965	0.038	–0.075	0.927	0.164
Wetland	–0.121	0.886*	0.058	–0.264	0.768*	0.134

†Significant at $\alpha =0.1;$ *significant at $\alpha =0.05;$ **significant at $\alpha =0.01;$ ***significant at $\alpha =0.001.$

Table 3. Logistic regression results for significantly tornadic supercells (EF2-EF5).

	Tornado Alley			Dixie Alley
Variable	β Coef.	Odds Ratio	Std. Error	β Coef.	Odds Ratio	Std. Error
Intercept	–4.548	0.011***	0.135	–4.070	0.017***	0.184
Elevation	–0.105	0.900***	0.015	–0.088	0.916	0.085
Slope	0.047	1.048*	0.020	0.069	1.071**	0.027
Distance to River	–0.012	0.988	0.025	–0.221	0.802*	0.096
Aspect
Flat	0.911	2.488	0.717	–12.238	0.000	253.0
East	–0.080	0.923	0.122	–0.177	0.838	0.158
North	–0.079	0.924	0.139	0.152	1.164	0.160
South	–0.017	0.984	0.129	–0.140	0.869	0.161
Land Cover
Water	–0.607	0.545	0.556	0.498	1.645	0.405
Developed	–1.147	0.318***	0.157	–0.834	0.434***	0.178
Barren	0.185	1.203	1.009	–12.408	0.000	345.0
Forest	0.231	1.259	0.149	–0.128	0.880	0.157
Shrubland	0.120	1.127	0.228	0.021	1.021	0.222
Grassland	0.053	1.054	0.125	0.196	1.216	0.359
Wetland	0.395	1.485*	0.168	0.155	1.168	0.308

†Significant at $\alpha =0.1;$ *significant at $\alpha =0.05;$ **significant at $\alpha =0.01;$ ***significant at $\alpha =0.001.$

Figure 4. Tornado probability surfaces based on logistic regression coefficients. Please note scales are different between maps to allow visualization of variation. Source: Author