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An unpublished study of observational records from 1967 to 2003 showed the frequency of potential tornado days to be about nine per year for Perth. Experience suggests that this is probably an overestimate of actual tornado days, especially when considering the incidence in just the Perth metropolitan area. Based on reports from 1993 to 2003, the average number of tornadoes in the Perth area seems to be in the order of three per year. However, the theoretical number of nine potential tornado days may more closely match the number of frontal events that cause severe wind gusts in Perth. An unpublished study of cold fronts from 1995 to 2003 showed that there were an average 9.5 events per year causing severe wind gusts in Perth. Of these, two to three were considered major fronts accounting for widespread minor damage, four caused local damage, and three events had no known damage although severe wind gusts were recorded.
With respect to tornado alley reference, the southwest does have a frequency compared to the remainder of the country. Tornadoes have occurred right throughout the Southwest Land Division but are most frequent along the coastal strip south of Lancelin. The distribution of historical tornado reports is strongly biased towards the populated areas along this coastal strip. There is likely to be some north to south increase in tornadoes given that winds are stronger in the south (i.e. Bunbury is more at risk than Perth). There may be some localised increase in frequency in the Rockingham because of the coastline orientation (the coastline juts out and combined with Garden Is there may be some change in surface flow to increase low level shear assisting tornado-genesis), however there has not been a study into this.
Nebraska averages about 40 tornadoes a year. I live in a mini tornado hot spot in Nebraska. In fact the county just north of me, Hall County, has 4 times the tornado density than the average for the rest of the state.
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