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Yesterday's savage tornado caught weather forecasters by surprise, even though they had been prepared for thunderstorms - a necessary condition for twisters.
MetService forecaster Andy Downs said lower-atmosphere winds detected by weather balloons sent up from Whenuapai before the tornado did not appear to be rotating enough for such an event to develop.
"It's a hard one to call because we normally need to have a bit more rotation in the winds."
He could only assume that hilly terrain between Whangaparaoa Peninsula and Albany had given the air enough rotation to produce a strong updraft of moist, warm air to collide with cooler conditions from the low-pressure front bringing in thunderstorms.
"Any thunderstorm can maybe produce a smaller tornado, but this one was on the higher side - normally we would need a bit more wind shear."
"This was bigger, and we need to have a look back at it," Mr Downs said.
"It tended to lack the conditions we would expect, which is why we can only assume the key component is the terrain.
"It is warm moist air that helps to generate a thunderstorm, but the updrafts have to be fairly strong and we also need rotation."
He said Whangaparaoa Peninsula stuck out in the Hauraki Gulf in such a way that air flow over its hills could create low-level swirling winds, which combined with strong updrafts, could create tornado conditions.
Auckland climate scientist Jim Salinger and WeatherWatch analyst Philip Duncan estimated from video images of the tornado and the damage it left that it would have been twisting at 200km/h or more.
That put it at a force of 2 on zero-to-5-point scale.
Mr Duncan said that was consistent with reports of cars and roofs being lifted and trees being uprooted.
His website had received a report from a Beach Haven resident of pink building insulation raining down on her neighbourhood.
Dr Salinger described tornadoes as violently rotating columns of air funnelling down from the base of low-pressure thunder clouds.
They were usually about 10m to 30m wide, and their velocity increased as they tightened.
He said they were usually isolated events in New Zealand, exceptions being the swarms of tornadoes which hit Taranaki over two days in July of 2007, forcing people from their homes and causing about $7 million worth of damage.
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