City's pounding an unusual event
A major thunderstorm that blasted Christchurch with near golf-ball-sized hail in September was a rare "supercell" storm.
The evening storm on September 4 was accompanied by spectacular lightning, ear-splitting thunder and heavy rain as it rolled in from the southwest.
Western and some northern suburbs also experienced a severe hailstorm, with hail up to 3 centimetres in diameter, as the system crawled across the city.
Thousands of insurance claims have been lodged for hail damage to buildings, including roofing and spouting, as well as damage to contents and vehicles.
A team of Australian hail-repair experts has been in Christchurch fixing dints in hundreds of cars.
MetService forecasters said the storm was unusual in its severity, its speed and location.
Spokesman Dan Corbett said supercell thunderstorms were infrequent across the Canterbury plains, perhaps every few years.
To get one above Christchurch was more unusual.
The storm had developed during the late afternoon, the optimum time for storms when heating across the plains was at its peak, he said.
There was also a sharp trough of low pressure at upper levels above Canterbury associated with the southerly change, which allowed the storm clouds to grow virtually unchecked.
Forecasters studying radar images found it was a good example of "storm splitting".
The right-hand end of the storm quickly weakened while the left-hand cell intensified into a severe storm.
The moisture supply was cut off to the right-hand cell, closer to Lyttelton Harbour, but the left-hand end sucked up the warmest, moistest air, allowing explosive development into a supercell, Corbett said.
"These are so big they take on their own life."
The large, damaging hailstones were a result of massive updraughts in the slow-moving storm, which allowed raindrops to rise and freeze, then fall and go through the same cycle several times before they got too heavy to be lifted any more by the up-currents, he said.
About 5000 insurance claims were lodged.
Turners Car Auctions Christchurch manager Ian Curry said about 100 cars with hail damage had since been auctioned.
"There's nothing mechanically wrong with these cars. It's more the cheaper cars, which was good for a lot of people who buy in the budget range."
Australian hail-repair specialists Smartfix Paintless Dent Removal has taken premises in Riccarton and is working for insurer AMI.
Workshop manager Paul Clarke said the company followed damaging hailstorms around the world.
They had repaired dints on the top and side-panels of about 500 cars and had about the same amount still to repair.
"We manage the dint back out, bring the metal back up to its original position," Clarke said.
"Metal has a memory, the same as if you crush a beer can and then push it back out - it knows where to go to."