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The numbers of E(F)1+ tornadoes per year generally declined in all regions of the United States, except the Southeast, where they increased in frequency. The South Great Plains had the greatest median tornado frequency early in the study period whereas the Southeast had the greatest in the middle and late periods. The proportion of tornadoes occurring in the contiguous United States in summer are decreasing whereas the proportion occurring in fall are increasing. Annual tornado frequency in the (a) West, (b) North Great Plains, (c) South Great Plains, (d) Midwest, (e) Southeast, and (f) Northeast regions. The dashed line is the Theil–Sen slope estimate and the solid line is the local polynomial regression curve with a smoothing span of 0.66.

This review and analysis illustrates that the spatial distribution and temporal clustering of tornadoes in the United States are changing. Tornado activity is increasing throughout the Southeast and in the southern portion of the Midwest and is decreasing throughout the southern and northwestern portions of the Great Plains and in the northern Midwest. This eastward shift is seen in tornado density maps, regional and gridded trends, and in an eastward shift of the mean center of tornadoes at the annual scale and in summer. The mean centers of tornado activity in other seasons are rather insensitive to these changes. Tornadoes are also clustering on fewer days in the year, and days with few tornadoes are becoming less common whereas days with many tornadoes are becoming more common. These concurrent and opposing trends are subsequently altering the frequency distribution of tornado days. The seasonality of these big tornado days also appears to be changing, as their increase in frequency is greatest in the fall and winter. Lastly, the increasing frequency of these big tornado days, which are most common in the Southeast and southern Midwest, contributes to the documented eastward shift in tornado activity.

abstract. This study provides climatological descriptions of tornadoes associated with Gulf Coast‐landfalling hurricanes during the 1950–2005 period using gis and statistical methods. Specific climatological descriptions provided are hurricane‐tornado activity per hurricane and physical, temporal, and spatial characteristics of hurricane‐tornadoes. Our analysis indicates that, although hurricane‐tornadoes are a common phenomenon associated with hurricane landfalls along the Gulf coastline, their frequency is variable. Some hurricanes produce no tornadoes, whereas others produce more than 100. Relatively weak hurricane‐tornadoes and those with relatively short path lengths and narrow path widths are most common. Hurricane‐tornadoes occur most often in August and September, during afternoon hours, and after their associated hurricane makes landfall. Hurricane‐tornadoes have occurred throughout the Gulf Coast region, but most are located within 200 kilometers of the coastline. Lastly, hurricane‐tornadoes are most often situated to the right of their associated hurricane center, relative to directional heading.

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