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A tornado strong enough to damage property and put lives at risk can be expected in Taranaki on average once every four years, according to research carried out by the National Institute of Water and Atmospheric Research (Niwa).
Principal climate scientist Dr Jim Salinger said there were 57 tornado "events" in the region from 1951 to 2006. Of those, 81 per cent did some damage and 21 per cent had inflicted major structural damage.
Dr Salinger said that on average about one potentially lethal tornado severe enough to damage property would hit Taranaki about once every four years.
Typical weather included the presence of low pressure and associated frontal activity over or west of Taranaki with winds from the north and west.
The tornadoes would often track from the coast inland.
In studying past tornadoes, Niwa found most had maximum wind speeds in the 116-180km/h range, with 10 per cent attaining wind speeds in excess of 180km/h.
Track or damage widths averaged 100m (range 15m to 500m) with a mean track length of 5km (range 1.5km to 16km).
Dr Salinger said 70 per cent of tornadoes in Taranaki were reported in the New Plymouth district, especially in or near the city. The highest risk of tornado development occurred over the sea surrounding the region.
"The many reports of Taranaki tornadoes suggest many of these spawn as waterspouts over the sea and come in across the north Taranaki coast from the northwest."
He said it appeared that Mt Taranaki had little effect on the spatial distribution.
Compared with other regions, Taranaki had a high rate of tornadoes, accounting for 12 per cent or more of the national occurrences, making it a relatively high-risk area, especially New Plymouth.
The city lay within an area of relatively high wind speeds and the study showed the district was more at risk than other parts of Taranaki because of its exposure to thunderstorms and unstable northwest air masses from the Tasman Sea. Most tornadoes had occurred in this district with the worst causing major structural damage and some loss of life.
"However, damaging tornadoes have also occurred in many other towns and rural areas throughout the Taranaki region."
Dr Salinger said case studies of two severe tornadoes had shown they originated offshore as waterspouts travelling towards the south or southeast. In both cases very strong cyclonic northwest airflow prevailed over Taranaki producing strong northerly-quarter winds, both causing damage to much property and one causing loss of life.
Dr Salinger said high winds occurred over Taranaki when vigorous fronts, troughs, deep depressions or cyclones caused strong northerly to westerly airflows or southeasterly airflows over the region.
Seasonally, the most tornadoes occurred in August, double the frequency of any other month, and the least in November, with none ever being reported in January. The most severe occurrences also occurred during August.
Dr Salinger said it was not known whether climate change would produce more tornadoes but gale- and storm-force winds from the west were likely to increase in Taranaki.
The broad pattern of expected changes out to 2100 for New Zealand included increases in westerly winds.
Global climate models suggested that for mid-range temperature change projections, the mean westerly wind component across New Zealand would increase by approximately 10 per cent of its current value by 2050.
The highest wind speed expected to occur once a year could increase by about 3 per cent by 2080. Over the sea or flat land the annual frequency of occurrence of winds of 120km/h or more might increase by about 40 per cent by 2030 and 100 per cent by 2080.
