NZPA , 5 July 2007, 11:24AM
NEW PLYMOUTH HAS HISTORY OF TORNADOS
Metservice severe weather forecaster John Crouch says the coastal areas around New Plymouth are a known "hotspot" for tornados.
The tornado which today left a trail of destruction through the city centre sprang from an unstable northwesterly flow onto the coast, which had been spawning bands of thunderstorms.
The western side of the North Island was the second most likely area to get tornados, after Westland. "Anywhere from Auckland, Waikato, right down to the Kapiti Coast," Mr Crouch said.
In the South Island the terrain brought a lot of rain and thunderstorms, and strong winds were "turned" by the Southern Alps, sometimes triggering tornados.
"In the North Island the trigger tends to be northeasterly surface winds, which get a bit of friction over land, while the high-altitude winds aloft tend to be northwesterly, causing a turning effect.
"It tends to occur particularly during winter, when thunderstorms spawned in the Tasman are moving onto the west coast of the North Island, and hitting this specific wind profile".
The average frequency of tornado occurrence in New Zealand has been estimated by the National Institute of Water and Atmospheric Research (Niwa) to be over 30 a year, including an average of four in the Taranaki region.
Most New Zealand tornados and waterspouts are relatively small and are typically very narrow -- with damage paths estimated to average 18m -- and have short tracks. Experts estimate only about a third of them occur near people and are reported.
In one cluster of tornado activity near New Plymouth, a dozen tornados were reported between 1961-1975, but Mr Crouch said today's twister was not as severe as the one three years ago at nearby Waitara, which demolished a house.
This big tornado, on August 15, 2004, hit 16km northeast of New Plymouth demolished all the main power lines feeding the area north and inland from Waitara, cutting off nearly 7000 consumers. It was particularly severe, with a wide destruction path, and debris spread over an area 400m long by 50m wide.
Rated at a similar intensity, New Zealand's worst tornado was in Frankton and a nearby part of Hamilton on August 25 1948, whichleft three people dead and 80 injured.
On March 10, 2005, a waterspout formed over the sea just west of Greymouth and made landfall in Blaketown, then carved through the town centre.
Like the Frankton tornado, it was rated at F2 on the "fujita scale" indicating windspeeds of 150-200kmh.
The Greymouth tornado caused almost $10 million in damage and seriously injured three people, but despite the magnitude of the tornado no-one was killed.