The strong wind gusts which tore through Devonport last night
were caused by a sudden collapse of air from the sky, like
someone "pulling a plug'' on the storm, MetService says.
Winds of up to 110km/h hit the Auckland suburb shortly after
6pm, ripping up fences and sheds, toppling trees and
powerlines and leaving hundreds without electricity.
Initial reports suggested a tornado, but forecasters today
classified the wind as "strong, straight line wind gusts
caused by a downburst''.
MetService meteorologist Dan Corbett said a thin band of
thunderstorms, which moved in from the Tasman yesterday
afternoon, hit a front of cold air and collapsed, causing the
"It's almost like somebody pulled the plug,'' he said. "It
collapsed and there's a big rush down to the surface. As the
cold air hits the surface it spreads out, it's called a
downburst, you get a big rush of wind in all directions, and
we saw that in Devonport last night.''
The downbursts are "very localised", he said.
It was the same weather phenomenon that caused the deaths of
three workmen in a storm in Hobsonville last December.
Mr Corbett labelled thunderstorms "big wind machines", which
pick up warm, moist air as they grow. When they hit cold air
tornadoes and downbursts can occur.
In a thunderstorm which develops a tornado the storm clouds
have stronger "upper level dynamics'' which fight against the
cold air and rotate to form a twister.
"The thunderstorm yesterday didn't have the upper level
dynamics, or the oomph, to generate that,'' Mr Corbett said.
However, every thunderstorm has the potential to generate
such downbursts, given the right conditions.
"It does depend on the nature of the atmosphere, the nature
of the storm. Every storm is different,'' he said.
While forecasters can predict a storm and which direction it
might move in, it is difficult to predict the nuances in the
atmosphere which might cause a downburst, Mr Corbett said.
But he said there's "always the risk'' of damaging wind gusts
during a thunderstorm.
He advised people to check the MetService website for the
twice-daily thunderstorm outlook, which will also provide an
indicator of how severe a thunderstorm may be and whether there will
be a chance of high winds.
If a downburst occurs the advice is to stay inside and keep
away from windows.
The wind struck just after 6pm yesterday, with most damage
confined to the area east of the Waitemata Golf Club.
Debris littered Vauxhall and Wairoa Rds in Devonport, with
large sheets of corrugated iron hung on power lines like
washing hung out to dry.
A handful of properties remained without power this evening,
down from 600 at the height of the event.
Contractors were today continuing to clear loose debris while
tarpaulins were used to cover some properties damaged by the
Civil Defence controller Clive Manly said building inspectors
were assessing properties damaged by the squall.
Air New Zealand cancelled several flights to provincial
centres as the wild weather moved down the country.